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The Palu region has attracted attention due to significant seismic activity, including a destructive earthquake in 2018. This study aims to investigate postseismic deformation following the 2018 Palu-Donggala earthquake using the Interferometric Synthetic Aperture Radar (InSAR) technique. We utilized Global Navigation Satellite System (GNSS) data to obtain the viscoelastic mechanism decay time. Therefore, we subtracted the viscoelastic relaxation signal to obtain spatiotemporal afterslip distribution inferred from the 2.5D InSAR observation based on the Steepest Descent Method (SDM). Our results suggest the viscoelastic mechanism is indispensable, with an optimal decay time of about 2 years after the earthquake. Based on the chi-square statistical test, the spatiotemporal afterslip model can explain the observation with good qualification. We found anomalies indicating non-decreasing slip, which is likely due to several factors, such as the presence of Slow Slip Events (SSE). These findings provide valuable insights regarding the potential for future earthquakes and have significant implications for disaster risk assessment in the Palu region and its surroundings. [ABSTRACT FROM AUTHOR]

Slow earthquakes are the primary mechanism of slow energy release, and research on the focal mechanism has been inconclusive. Studies have primarily focused on the friction law based on physical mechanisms and have suggested that slow earthquakes are caused by brittle faults. However, the focal strength and structural characteristics of slow earthquakes in subduction zones provide evidence of plastic deformation. What is the role of plastic deformation in the focal mechanisms of slow earthquakes? Mechanochemical study have shown that mechanical forces can directly affect chemical bonds. In this study, we examine the storage and release of chemical energy during plastic deformation and consider a mechanochemical process in the focal mechanism of slow earthquakes. Combined with the Tersoff potential, molecular dynamics simulation on the shear deformation process of two α-quartz crystals show that the shear modulus of α-quartz is 18 GPa, and that the crystal model primarily exhibits atoms flowing and changing in the direction of chemical bonds during the steady-state flow stage. The molecular potential energy and stress vary in an oscillating up-and-down curve during shear, indicating that chemical energy can be stored and released during plastic deformation. This is consistent with the energy variation during slow earthquakes. Under the initial simple-shear loading condition, α-quartz crystals undergo general shear deformation instead of plane strain and the angle between the longest instantaneous stretching axis (ISA1) and the shearing direction is approximately 30°, not 45°. Both the deformation type and direction of ISA1 are contrary to basic deformation theory, which may provide clues for future research. This study reveals the process of quartz elastic-plastic shear deformation on an atomic scale. This information is useful for understanding focal mechanisms of slow earthquakes. This study is part of a series of investigations on tectonic stress chemistry. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive review on recent development and research conducted in domestic and international underground laboratories. We first introduce the differences in three environments—surface, mountain tunnel cavities and underground coal mine tunnels—by examining cosmic ray background, ambient noises related to gravity and seismic measurement, and electromagnetic noises in magnetic and magnetotelluric measurements. We highlight potential misuse of the term Underground Lab or Deep Underground Lab when describing observations in different physical fields. We introduce unique features of underground coal mine tunnels in China, such as large spaces, ultra-quiet conditions, and ultra-clean environments. When comparing with mountain tunnel cavities and borehole observations, coal mine tunnel observations have superior long-term stability and high precision. Through observations and comparisons of multi-physic fields at surface and the deep underground, we find that the higher SNR seismic observations conducted in deep underground tunnels in coal mines are beneficial to improve velocity tomography of the solid earth. The gravity observation with a Superconducting Quantum Interference Device (SQUID) makes it possibly to capture slow earthquake, which has not been observed previously in the Chinese mainland. SQUID magnetic observations can detect fluctuations as weak as femto-Tesla (fT), enabling us to explore the attenuation of Schumann Resonance down to the solid Earth. This opens opportunities to investigate the connections between the Earth's magnetic field and the interactions within the human brain and heart. To improve the precision of quantum measurement, we should consider the possible effects of weak magnetic disturbances in deep underground environments. Finally, we discuss the importance of deep underground laboratories, observing facilities and techniques deployed in these laboratories, and their possible connection with respect to "deep space" and "deep ocean" exploration, emphasizing the need for focused research on various scientific challenges. We hope to encourage greater attention to deep underground laboratory and high-precision scientific observation. [ABSTRACT FROM AUTHOR]

Seismic and aseismic slip events result from episodic slips on faults and are often chaotic due to stress heterogeneity. Their predictability in nature is a widely open question. In this study, we forecast extreme events in a numerical model. The model, which consists of a single fault governed by rate-and-state friction, produces realistic sequences of slow events with a wide range of magnitudes and interevent times. The complex dynamics of this system arise from partial ruptures. As the system self-organizes, the state of the system is confined to a chaotic attractor of a relatively small dimension. We identify the instability regions within this attractor where large events initiate. These regions correspond to the particular stress distributions that are favourable for near complete ruptures of the fault. We show that large events can be forecasted in time and space based on the determination of these instability regions in a low-dimensional space and the knowledge of the current slip rate on the fault. [ABSTRACT FROM AUTHOR]

A large portion of the stress release on seismic faults remains silent and undetected, requiring the development of novel observation techniques. Measuring traveltime perturbations from the correlation of ambient seismic noise at different stations is a well-known approach to assess temporal changes in seismic velocities, which can provide insights into hydrologic, tectonics and volcanic dynamic processes. In this work, we study the specific case of a P -wave phase retrieved from the correlation of freight train noise in Southern California and evaluate its potential to detect localized velocity changes along the San Jacinto Fault. We use a full waveform modelling approach to simulate this P -wave interference and further assess its sensitivity to the position of the train source, near-surface velocity changes and localized velocity changes in the fault zone. Our results show that the uncertainty in trains location can induce large traveltime biases which can be mitigated by averaging over many trains. Our results also highlight the weak sensitivity of these correlation P waves to near-surface velocity changes, while they show significant sensitivity to localized changes at depth. This modelling highlights the potential of monitoring traveltime perturbations of this ballistic P -wave interference to detect hidden slow-slip events on the San Jacinto Fault, particularly in identifying subtle velocity anomalies associated with fault zone changes that may otherwise go unnoticed by conventional seismic monitoring techniques. [ABSTRACT FROM AUTHOR]

The article delves into the intricate relationship between an earthquake swarm beneath the Noto Peninsula in Central Japan and the subsequent 2024 M 7.6 Noto Hanto earthquake sequence. It examines the various physical mechanisms at play, including pre-slip, cascade triggering, aseismic slip, and fluid migration, to understand the evolution process leading to the mainshock. By analyzing seismicity patterns, foreshocks, stress distribution, and rupture propagation, the study sheds light on the complexities of earthquake activity in the region. The research underscores the importance of in-depth investigations utilizing diverse data sources to unravel the connection between earthquake swarms and the initiation of major seismic events, making the Noto earthquake sequence a valuable case study for global earthquake research. [Extracted from the article]

Abstract: The westernmost Nankai Trough, southwest Japan, exhibits a rapid along-strike reduction in plate coupling in the proximity to the subducting Kyushu-Palau ridge. Yet how and to what extent the ridge subduction impacts physical properties at the megathrust have not been investigated. Here we present high-resolution seismic P-wave velocity models along the forearc wedge in the western Nankai Trough derived from full-waveform inversion analyses of seismic refraction data. The velocity models show that where the plate coupling is weak and the plate boundary presumably hosts slow earthquakes, the upper plate exhibits lower seismic velocities indicating higher degree of fracturing over a ~ 100 km length along trough. Intriguingly, the extent of the upper-plate low-velocity features is significantly larger than the surficial width of the Kyushu-Palau ridge, and this low-velocity zone is underthrust by the slab with increased crustal thickness by 2–4 km. Seismic reflection images consistently reveal that the thicker slab crust has appreciable basement roughness extending ~ 60 km from the eastern margin of the Kyushu-Palau ridge beneath the western Shikoku basin. We suggest that such a thicker and rugged slab crust, together with the main body of the Kyushu-Palau ridge, can cause significant fracture zones in the overriding plate, decrease the interplate coupling and produce preferable conditions for shallow slow earthquakes to occur when subducted. The results may also provide structural constraints on the western limit of future megathrust earthquakes in the Nankai Trough. Graphical Abstract

Abstract Recent advances in deep learning have enhanced our ability to analyze seismic waveforms. Here, we developed and evaluated a convolutional neural network (CNN) model to classify tectonic tremors, earthquakes, and noise in seismic waveform data recorded by a seismic array in the Nankai subduction zone. The trained CNN model achieved high accuracy, with both precision and recall exceeding 97%, and correctly detected 96% of distant earthquakes. The probability of tectonic tremor as a function of the signal-to-noise ratio (SNR) increased steeply from 10 to 90% at an SNR of 4. We highlighted tectonic tremor waveforms using the integrated gradients (IG) method for interpreting CNN models. IG filter averaging over the stations of an array outperforms bandpass filters and other interpretation methods for CNN models in locating tectonic tremors by semblance analysis, providing the largest number of tectonic tremor sources. As reported previously, located sources of tectonic tremor during episodic tremor and slip events migrate along the strike of the subducting plate. The source location error increases significantly at epicentral distances greater than 30 km because of low SNRs. The technique developed in this study equips CNN models with a high ability to distinguish tectonic tremors and earthquakes from noise and to locate tectonic tremors with sources that are not far from seismic stations. Graphical abstract

Slow earthquakes play a crucial role in understanding stress accumulation and release along plate interfaces in subduction zones. The northern Ryukyu Trench, where the Philippine Sea Plate subducts northwestward beneath the Eurasian Plate, experienced a major earthquake in 1911 and is currently regarded as a low-seismicity area (LSA). Understanding the seismic activity in this region, particularly the relationship between very-low-frequency earthquakes (VLFEs) and regular seismic events, is crucial for understanding subduction zone dynamics. We investigated the spatial and temporal distribution of VLFE activity in the northern Ryukyu Trench using broadband ocean-bottom seismometers deployed around Amami Island between September 2018 and June 2019. Our analysis, employing the envelope correlation method, revealed that VLFE activity is primarily concentrated northeast of Amami Island, an area characterized by low regular earthquake activity, with the distribution of VLFEs spatially segregated from that of regular earthquakes. Furthermore, we observed earthquake swarm activity at the edges of the LSA in the northern Ryukyu Trench following VLFE activity. In November 2018, intense VLFE activity northeast of Amami Island migrated northeastward, which was followed by a regular earthquake swarm at the edge of this LSA. Following VLFE activity in January 2019, additional seismic activity, including foreshocks, occurred at the edges of this LSA approximately 1 month later. The spatial segregation of VLFEs and regular earthquakes suggests that VLFE activity may be influenced by the migration of high-pressure fluids within the subducted slab. This migration appears to trigger related time-delayed seismic activity, similar to mechanisms observed in other subduction zones such as Hikurangi. Understanding these dynamics is essential for assessing the coupling state of subduction zones and associated fluid behaviors, which play a critical role in evaluating seismic hazards in LSAs. [ABSTRACT FROM AUTHOR]

Recent advances in deep learning have enhanced our ability to analyze seismic waveforms. Here, we developed and evaluated a convolutional neural network (CNN) model to classify tectonic tremors, earthquakes, and noise in seismic waveform data recorded by a seismic array in the Nankai subduction zone. The trained CNN model achieved high accuracy, with both precision and recall exceeding 97%, and correctly detected 96% of distant earthquakes. The probability of tectonic tremor as a function of the signal-to-noise ratio (SNR) increased steeply from 10 to 90% at an SNR of 4. We highlighted tectonic tremor waveforms using the integrated gradients (IG) method for interpreting CNN models. IG filter averaging over the stations of an array outperforms bandpass filters and other interpretation methods for CNN models in locating tectonic tremors by semblance analysis, providing the largest number of tectonic tremor sources. As reported previously, located sources of tectonic tremor during episodic tremor and slip events migrate along the strike of the subducting plate. The source location error increases significantly at epicentral distances greater than 30 km because of low SNRs. The technique developed in this study equips CNN models with a high ability to distinguish tectonic tremors and earthquakes from noise and to locate tectonic tremors with sources that are not far from seismic stations. [ABSTRACT FROM AUTHOR]

The Nankai Trough region has a history of devastating earthquakes. The Hyuga-nada region is situated in the westernmost Nankai Trough and has not experienced an earthquake exceeding magnitude 8. It is expected that there is a difference in a coupling between the subducting Philippine Sea Plate and the overriding plate. The region is known for its active region of low-frequency tremors and very low-frequency earthquakes. Several long-term ocean bottom seismometer (LTOBS) networks were deployed to monitor seismic activities in the region to reveal the characteristics of seismicity in the study region. Seafloor seismic observations were conducted on the seafloor in the Hyuga-nada region with periods of 2015–2016, 2017–2018, and 2022. Initially, the hypocenters of regular earthquakes were determined by using a location program that integrated absolute travel times and a 1-D velocity structure. Station corrections for travel times were applied to compensate for structural heterogeneity just beneath individual LTOBS. Subsequently, events were relocated using a double-difference technique to enhance the accuracy of the location. Focal mechanisms were estimated using the polarity data of the first arrivals. Comparison with a plate model indicated that these earthquakes occurred within the subducting Philippine Sea Plate. From the spatial–temporal distribution, hypocenters of earthquakes were concentrated in a small region with a size of a few km and occurred within a few days. We could clearly distinguish swarm activities from regular activities. During the observation period, there were two swarm activities in the study area. Fault plane solutions of regular events except swarms, are dominant with a normal fault type mechanism. On the other hand, swarms had a small number of events with a normal fault type. The swarm activities started slightly later than the very low frequency earthquake (VLFE) activities. The regions of activity for the swarms and the VLFE seemed to generally overlap in consideration of the low spatial resolution of the VLFE locations. [ABSTRACT FROM AUTHOR]

Objectives: Slow slip events (SSEs) are slow dislocation that occur in weak zones in the crust, and they may cause surface deformations and lead to slow earthquakes. However, the mechanism of SSEs and whether they will trigger earthquakes are still in the stage of discussion and speculation for developing the new principles and methods. To further study the characteristics of SSEs and the relationship with earthquakes, we use global positioning system (GPS) coordinate time series to invert SSEs. The spatiotemporal characteristics and evolution laws of SSEs are analyzed, and the relationship between SSEs and earthquakes is discussed. Methods: The proposed method is executed based on standard linear trajectory model (SLTM) and network inversion filter (NIF). First, the slow slip coordinate time series are obtained by modeling GPS continuous coordinate time series using SLTM. The steps are repaired, and the gross errors, the constant, the steady state velocity, and the annual and semi-annual periodic season terms are removed. Then, NIF is used to construct the fault grid, and the elastic Green's function is introduced to describe the relationship between fault slip and surface displacement. Using the slow slip coordinate time series and NIF, the slip vectors on the fault grid and surface displacement vectors are inverted. Finally, the magnitude and frequency of the earthquakes before and after SSEs are calculated, and the possible relationship between SSEs and earthquakes is analyzed. Results: Taking the 2018 SSE in Boso Peninsula of Japan as an example, the results show that the active period of the slow slip ranges from the 156th to the 169th days. The maximum cumulative slip is about 11.3 cm, and the maximum slip rate is about 2.7 cm/d. The central area of the slow slip is located in the southeast of Boso Peninsula (140.2°E-140.9°E,34.8°N35.6°N). The SSE spreads southward slightly, and the depth of the slow slip changes from deep (about 23 km) to shallow (about 15 km). The frequency of regional earthquakes increases significantly during the SSE, and gradually recovers to normal in the following months. Therefore, the danger of approaching potential earthquake swarm can be identified from dynamic evolution of SSE in Boso Peninsula. Conclusions: GPS plays an important role in the detection of SSEs, but it is difficult to detect short period SSEs within a few days due to the influence of various long period tectonic movement information, and the uneven distribution of GPS stations on land and sea leads to the decrease of spatial resolution. Hence, in the future work, we will further study the physical mechanism of SSEs in combination with other monitoring means such as strain gauge, tilt-meter and submarine pressure gauge. [ABSTRACT FROM AUTHOR]

The Nankai subduction zone presents significant seismic and tsunami risks, given its historical earthquakes exceeding magnitude 8 and the expectations of similar future events. Slow earthquakes, common at the shallow and deep plate interface, result from different frictional properties linked to interplate slip deficit accumulation. This study estimates slip deficit rates at the Nankai subduction zone using land and ocean‐bottom geodetic data. Previous estimates encountered limitations, often smoothing slip deficits, omitting observational error differences between ocean‐floor and land data, and relying on homogeneous structure models. To address these issues, we employ a novel trans‐dimensional reversible jump Markov Chain Monte Carlo algorithm. This approach dynamically adjusts slip parameters, accommodating data resolution and producing a flexible slip distribution without predetermined spatial constraints. Additionally, it automatically weights data for observational errors and integrates elastic Green functions from a 3D structure of the Nankai region. Our results provide a finer, heterogeneous slip distribution, improving estimates in inland regions. However, limitations remain offshore in areas with sparse data. We revised the spatial distribution of Nankai slow earthquakes and confirmed a good agreement with intermediate slip deficit rates, identifying coupled and uncoupled regions. High slip deficit rates align with rupture areas of historic large earthquakes. Slow earthquakes occur at frictionally weak plate interfaces, and shallow slow earthquakes may result from subducting relief heterogeneities with important pore fluid pressure effects. We introduce an updated distribution of slip deficit rates for the Nankai subduction zone, considering observed slip deficit rates and the fast and slow earthquake occurrence. Plain Language Summary: The Nankai subduction zone poses a major risk of earthquakes and tsunamis due to its history of earthquakes exceeding magnitude 8 and the anticipation of similar events in the future. The area is also prone to slow earthquakes along both the shallow and deep plate interface, driven by variations in interplate slip deficit buildup. Our study focuses on estimating slip deficit rates along the Nankai subduction interface using data from both land and ocean‐bottom observations. Prior estimations of slip deficit rates had limitations, such as neglecting differences in observation errors or using simplified structure models. To overcome these challenges, we introduced an innovative algorithm that adapts to different observational errors, incorporates a detailed 3D structural model, and dynamically adjusts slip parameters without spatial constraints. This led to an accurate representation of slip deficit rates. Our findings not only align with past results but also contribute to a comprehensive understanding of the shallow and deep plate interface properties. Slow earthquakes tend to occur where the plate interface is weak, indicating a transition from coupled to uncoupled zones. Our slip deficit rate can propose regions at high risk of seismic activity in agreement with historical rupture areas in the Nankai subduction zone. Key Points: We introduced a new slip deficit rate estimation for the Nankai subduction zone, addressing previous limitationsWe applied a novel method to integrate ocean‐floor and land geodetic data with a 3D structural modelResults provide insights into the spatial distribution of slow and fast earthquakes and their implications for seismic hazard assessment [ABSTRACT FROM AUTHOR]

Many slow slip events (SSEs) occur beneath the ocean, and continuous ocean‐bottom pressure gauge (OBP) observations provide useful data. OBPs record both oceanic variations and crustal movements, so we developed a multi‐channel singular spectrum analysis method to remove oceanic variations and applied our method to OBPs and oceanic model data. Then components of the oceanic model with good correlations to the OBP data were subtracted from the observed data. This method compensates for the incompleteness of the oceanic model and removes oceanic variations better than use of the original model. We applied the method to OBP data for the 2018 Boso, Japan, SSE to estimate its slip distribution. Comparing slip distributions obtained with and without the OBP data, we found that the distribution obtained using OBP data extended further offshore, and the offshore estimation error was smaller. Our study shows that offshore observations using OBPs are important for characterizing SSEs. Plain Language Summary: Slow slip events (SSEs) occur when faults slip slowly without generating seismic waves. Because most SSEs occur under the ocean, continuous observations by ocean‐bottom pressure instruments (OBPs) provide useful data. OBPs record both oceanic changes and seafloor movements. To remove the oceanic changes and more clearly observe seafloor movements, we developed a signal processing method to extract similar components from multiple time series. We applied our method to OBP data and oceanic model data, then subtracted the components of the oceanic model that match the OBP data from the observed data. This method removed the oceanic changes from the OBP data better than use of the original oceanic model alone. We applied this method to OBP data for the 2018 Boso, Japan, SSE and used vertical motions in the OBP data to estimate the slip distribution. The distribution obtained with the OBP data extended further offshore and was more accurate than that obtained without the OBP data. This study shows that observations of the ocean bottom obtained with OBPs are important for characterizing SSEs. Key Points: We removed oceanic variations from ocean‐bottom pressure data using a reconstructed oceanic model, achieving ±4 mm observed accuracyThe reconstructed oceanic model includes only components with good correlations between observed data and the oceanic modelUse of the ocean‐bottom pressure data and the proposed method improved the accuracy of the slow slip distribution in offshore areas [ABSTRACT FROM AUTHOR]

Largescale volcanic eruptions and earthquakes are occurring frequently in the Philippines, and research has shown that slab metamorphism and diversity alter the impacts of subducted oceanic plates by changing water‒carbon productivity and interplate stability. Within the framework of the thermal evolution history of subducting slabs, the relationships between subduction zone seismicity characterized by both regular megathrust earthquakes and slow slip events of various magnitudes and long-term slab dehydration–decarbonation evolution in the Philippines remain poorly understood. Here, we constructed a comprehensive thermal model incorporating 3-D slab geometric data for the incoming plate and a 3-D subduction velocity field based on the MORVEL plate motion dataset for the Philippine subduction zone with high spatial and temporal resolutions. Our findings reveal that subduction seismicity and arc volcanism are prominent in belt-shaped regions with high thermal gradients (> 5 °C/km) and large-scale slab dehydration (> 0.05 wt%/km). Dehydration of serpentinite in ultramafic rocks in the subducting slab and decarbonation of carbonate minerals preferentially contribute to the generation and transport of fluids and carbonate melts, thus facilitating seismicity and carbon-rich magmatism. Our results suggest that slab geometry diversity-induced juxtaposed slab dehydration-decarbonation processes play a vital role in the generation of megathrust earthquakes below the forearc. [ABSTRACT FROM AUTHOR]

Kīlauea Volcano on Hawai'i Island is host to a complex volcanic and interwoven fault system. Over the last ∼120 years, a range of seismic events, including large earthquakes such as the 1975 Mw ${M}_{w}$7.7 Kalapana earthquake, creep, and slow slip events, have occurred along the décollement underlying Kilauea's south flank. We explore both the deformation and stress changes of Kīlauea from 1896 to 2018 by collating six geodetic data sets and creating an analytical model to determine the dominant deformation sources (i.e., fault planes, rifts, magma chambers) driving this system at different times. The 1975 Kalapana earthquake significantly altered the region's state of stress and deformation; we find the average slip along the décollement was reduced from 10 cm/yr prior, to 4 cm/yr after the rupture. Prior to 1975 no slip is resolved along the décollement where the earthquake nucleated, suggesting that this portion may have been locked leading up to the rupture. After 1975, décollement slip overall is smaller and more irregular, suggesting increased control by spatial variation of mechanical properties. We find increases in shear stress along the Kīlauea décollement and a decrease in normal compressive stress within the East Rift Zone prior to the Kalapana earthquake, creating favorable conditions for failure of the décollement and subsequent magmatic intrusion. Plain Language Summary: Kīlauea Volcano on Hawai'i Island encompasses a complex volcanic and interwoven fault system. The low‐angle fault underlying the volcano has consistently been moving it southward, and has a high hazard potential due to large magnitude earthquakes. The complexity of this region leads to questions about the evolution of deformation and its seismic cycle. The 1975 Mw ${M}_{w}$7.7 Kalapana décollement earthquake is of particular interest because large magnitude ruptures can dramatically alter the state of stress within a region. We therefore explore both the deformation and stress changes of Kīlauea's décollement from 1898 to 2018 by collating a wealth of surface displacement observations. We create a model to reproduce the observed displacements throughout time and to identify key structural features causing the deformation, such as fault planes, rifts, and magma chambers. Multiple deformation sources were needed to model the observations, with more of these sources required prior to the 1975 Kalapana earthquake than afterward. We also find varied patterns and magnitudes of deformation and stress both temporally and spatially within the region. Kīlauea's history has important implications for our understanding of the relationship between magmatic and earthquake cycle processes. Key Points: Collation of ∼120 years of geodetic data at Kīlauea reveals key displacement, stress, and deformation source patterns throughout timeGreater and more complex displacement occurred within Kīlauea's south flank prior to the 1975 MW ${M}_{W}$7.7 Kalapana earthquake than after itCompressive stress decreased in East Rift Zone before/after 1975; shear stress increased before and decreased after on Kīlauea décollement [ABSTRACT FROM AUTHOR]

Subduction interfaces exhibit various slip styles, including slow slip events (SSEs). We use a micromechanics‐based approach to calculate the effective rheology of a shear zone containing ellipsoidal amphibolite clasts deforming by dislocation creep within an interconnected linear‐viscous phyllosilicate‐dominated matrix. Frictional failure occurs if local stress exceeds Mohr‐Coulomb yield strength. At moderate fluid overpressure, mixed‐frictional‐viscous behavior emerges at ∼ ${\sim} $350–560° ${}^{\circ}$C, consistent with a broad zone of mixed fault slip behavior without requiring extreme fluid overpressures. Increasing stress in this transition zone promotes local frictional failure and raises bulk strain rate. If, however, the bulk strain rate increases by more than one order of magnitude, system‐wide frictional sliding becomes preferable. This strain rate increase is insufficient to explain the slip rates observed in geophysically detectable SSEs. Therefore, viscous matrix flow as modeled here cannot explain SSEs without either invoking dynamic weakening within a frictional‐viscous flow or a mechanism switch to dominantly frictional sliding. Plain Language Summary: Subduction plate boundaries are locked near the Earth's surface and will release the stored energy as earthquakes. Subduction zones creep steadily and viscously at deeper depths where temperatures and pressures are high. At the depth of the transition from earthquakes to steady creep, episodic aseismic slip is often observed. Rocks from this region are mixtures of strong, fractured clasts surrounded by a weak matrix. The observations of exhumed rocks suggest that the episodic, aseismic slip may nucleate when local frictional failure occurs in strong clasts, but the surrounding weak matrix stops this failure from generating major earthquakes. However, it is unclear how much the small‐scale rock behavior could be linked to the large‐scale slip. We use a numerical model to simulate the interplay between frictional and viscous creep and calculate the overall behavior of the subduction zone plate boundary. We explore how the slip style changes with depth and determine the transition zone's depth/temperature range. In the transition zone, a small increase in stress or decrease in strength can lead to a change from pure viscous flow to frictional sliding. This study overcomes the scale challenge between the small‐scale features preserved on outcrops and the large‐scale geophysical observations. Key Points: Frictional‐viscous flow in a two‐phase shear zone modeled by a multiscale approach occurs at ∼350–560°C with moderate fluid overpressureStress loading and/or fluid pressure weakening can cause a switch from steady viscous creep to transient frictional slipViscous creep modeled here can accommodate tectonic strain rates but not slow slip events without invoking a switch to frictional sliding [ABSTRACT FROM AUTHOR]

Tectonic tremors occur around the subducting seamount of the Kyushu Palau Ridge (KPR) in Hyuga-nada, Japan. We investigate the relationship between tremor activity and structural and physical characteristics using high-resolution reflection seismic imaging of the KR0114-8 line, encompassing areas with both high and low occurrences of tremors. The reflection data are reprocessed using broadband processing, reflection tomography and Kirchhoff pre-stack depth migration. The resulting image delineates complex deformation and lithological boundaries, such as the accretionary prism, underthrust sediments, décollement, and top of the seamount. The observed splay faults and seafloor uplift to the west of the KPR, coupled with frequent tremors, confirm the compressional stress regime on the leading side of the subducting seamount. A stress shadow over the seamount effectively suppresses tremors. Numerous faults indicate significant deformation of the overburden when positioned on the leading side of the seamount. The trailing side exhibits a compressional stress regime, rather than an extensional one, as evidenced by the development of in-sequence thrusts and frequent tremors. Local physical, mechanical, and structural factors critically influence the tremor activity. The increased frequency of tremors is correlated with the thickness of the underthrust sediments and presence of in-sequence thrusts, whereas it is inversely correlated with the reflectivity of the décollement. Several potential mechanisms for this phenomenon include elevated pore pressure above the décollement and/or within the underthrust sediments, as well as structural effects. High-resolution velocity imaging, scientific drilling, and precise tremor-depth estimation are essential for advancing our understanding of these mechanisms. [ABSTRACT FROM AUTHOR]

We develop a methodology to compile an objective tremor catalog by utilizing distinctive event features that differentiate tectonic tremors from non‐tremor events, and combining the envelope cross‐correlation method with clustering technique and neural network. This approach enables tremor extraction without subjective criteria, allowing for the detection of previously overlooked short‐duration tremors. The event features employed to distinguish tremors and non‐tremor events are depth, the mean amplitudes at high and low frequencies, the ratio of these two amplitudes, and event duration. The duration is defined as the minimum period that contains 50% of the seismic energy. The application of this method to western Japan detects 1.7 times more tremors than the previous studies, with the durations of 0.3–∼100 s. The events with short durations are considered low‐frequency earthquakes. The relationship between seismic moment and duration of the detected tremors is consistent with the scaling law of slow earthquakes. Plain Language Summary: Slow earthquakes are characterized by very slow underground deformation compared with regular (fast) earthquakes and are important for understanding the preparation period prior to large earthquakes. Tectonic tremors, which are a type of slow earthquakes, radiate tiny seismic waves with frequencies of several Hz, occur episodically and densely in space and time, and may last for long durations of up to several hundred seconds, which is much longer than the durations of fast earthquakes of equivalent magnitude. In this study, we detect and differentiate tectonic tremors from fast earthquakes and anthropogenic events. We do this using a set of event features, without relying on subjective criteria. The durations of the detected tremors range from 0.3 to ∼100 s, and they appear consistent with a previously proposed scaling relationship for slow earthquakes. This result suggests that fast earthquakes and slow earthquakes have different physical mechanisms. Key Points: We compile a more complete tectonic tremor catalog for western Japan using a clustering method based on event featuresEvent duration, newly defined using energy radiation, clearly separates tectonic tremors from fast earthquakesTectonic tremors, ranging in duration from 0.3 to 100 s, are consistent with the scaling law of slow earthquakes [ABSTRACT FROM AUTHOR]

The preface for the article collection "10 years after the 2011 Tohoku earthquake: a milestone of solid earth science" highlights the scientific discoveries made since the earthquake, thanks to near-field observations and existing knowledge about the northeastern Japan arc. The earthquake revealed dynamic frictional behaviors of the subduction interface and provided insights into subduction zone dynamics. The collection of research on the Tohoku earthquake is expected to benefit the understanding of subduction earthquakes worldwide. The preface also mentions various research papers included in the collection, covering topics such as coseismic stress drop, afterslip, seafloor observations, slow earthquake activity, tsunami deposits, paleoseismology, and numerical modeling of the earthquake cycle. Ongoing research on postseismic deformation and historical research on earthquake occurrence are also discussed. The authors express their appreciation to the reviewers and the editorial board for their contributions to the special issue. [Extracted from the article]

Interseismic coupling maps and, especially, estimates of the location of the fully coupled (locked) zone relative to the trench, coastline, and slow slip events are crucial for determining megathrust earthquake hazard at subduction zones. We present an interseismic coupling inversion that estimates the locations of the upper and lower boundaries of the locked zone, the lower boundary of the deep transition zone, and downdip gradient of creep rate in the transition from locked to freely creeping in the downdip transition zone. We show that the locked zone at Cascadia is west of the coastline and 10 km updip of the slow slip zone along much of the margin, widest (25–125 km, extending to ∼19 km depth) in northern Cascadia, narrowest (0–70 km) in central Cascadia, with moment accumulation rate equivalent to a Mw 8.71 and Mw 8.85 earthquake for 300‐ and 500‐year earthquake cycles. We find a steep gradient in creep immediately below the locked zone, indicative of propagating creep, along the entire margin. At Nankai, we find three distinct zones of locking (offshore Shikoku, offshore southeast Kii peninsula, and offshore Shima peninsula) with a total moment accumulation rate equivalent to a Mw 8.70 earthquake for a 150‐year earthquake cycle. The bottom of the locked zone is nearly under the coastline for all three locked regions at Nankai and is positioned 0–5 km updip of the slow slip zone. In contrast with Cascadia, creep rate gradients below the locked zone at Nankai are generally gradual, consistent with stationary locking. Plain Language Summary: Maps of where faults are not moving (the locked zone) can be used to assess future earthquake size and impacts on nearby communities due to ground shaking and tsunamis. Slow slip events, occurring below and around the locked zone, may transfer stress from deeper on the fault to the locked zone and increase earthquake potential. We use measurements of movement of the surface of the earth and models of how surface movements reflect to slip on a fault in order to locate the boundaries of the locked zone in relation to the coastline, the trench, and slow slip events at Cascadia and Nankai subduction zones. We find that a release of slip accumulated in the current Cascadia and Nankai locked zones would result in earthquakes of magnitude Mw 8.71–8.85 and Mw 8.70, respectively. We also find evidence that the depth to the bottom edge of the locked zone at Cascadia and in some areas of Nankai may have shallowed since the last earthquake. Our model provides better estimates and realistic ranges for the location of the boundaries of the locked zone which can inform earthquake rupture, ground motion, and tsunami models. Key Points: We developed a coupling zone boundary inversion with a forward model of shallow creep at constant stress and deep updip‐propagating creepThe locked zone accounts for 48% of the interseismic moment accumulation rate at Cascadia and 46% at NankaiWe infer steep creep rate gradients, indicative of updip‐propagating creep, at Cascadia and below Shikoku and Shima peninsula at Nankai [ABSTRACT FROM AUTHOR]

Geophysical and geological studies provide evidence for cyclic changes in fault‐zone pore fluid pressure that synchronize with or at least modulate slip events. A hypothesized explanation is fault valving arising from temporal changes in fault zone permeability. In our study, we investigate how the coupled dynamics of rate and state friction, along‐fault fluid flow, and permeability evolution can produce slow slip events. Permeability decreases with time, and increases with slip. Linear stability analysis shows that steady slip with constant fluid flow along the fault zone is unstable to perturbations, even for velocity‐strengthening friction with no state evolution, if the background flow is sufficiently high. We refer to this instability as the "fault valve instability." The propagation speed of the fluid pressure and slip pulse, which scales with permeability enhancement, can be much higher than expected from linear pressure diffusion. Two‐dimensional simulations with spatially uniform properties show that the fault valve instability develops into slow slip events, in the form of aseismic slip pulses that propagate in the direction of fluid flow. We also perform earthquake sequence simulations on a megathrust fault, taking into account depth‐dependent frictional and hydrological properties. The simulations produce quasi‐periodic slow slip events from the fault valve instability below the seismogenic zone, in both velocity‐weakening and velocity‐strengthening regions, for a wide range of effective normal stresses. A separation of slow slip events from the seismogenic zone, which is observed in some subduction zones, is reproduced when assuming a fluid sink around the mantle wedge corner. Plain Language Summary: Slow slip events are observed in subduction zones worldwide. Their mechanism is not well understood, but geophysical and geological research suggests a relation with recurring changes in fluid pressure within the fault zone. Here we explore the fault valve mechanism for slow slip events using mathematical and computational models that couple fluid flow through fault zones with frictional slip on faults. The fault valve mechanism (arising from cyclic changes in the permeability or resistance to fluid flow) produces pulses of high fluid pressure, accompanied by slow slip, that advance along the fault in the direction of fluid flow. We quantify the conditions under which this occurs as well as observable properties like the propagation speed and rate of occurrence of slow slip events. We also perform simulations of subduction zone slow slip events using fault zone and frictional properties that vary with depth in a realistic manner. The simulations show that the fault valve mechanism can produce slow slip events with approximately the observed rate of occurrence, while also highlighting some discrepancies with observations that must be addressed in future work. Key Points: We analyze the dynamics of fault slip with fault‐zone fluid flow and fault‐parallel permeability enhancement with slip and sealing with timeFault‐valve instability produces unidirectional aseismic slip and pore pressure pulses even with velocity‐strengthening frictionSubduction zone earthquake cycle simulations show that the fault‐valve instability can produce slow slip events below the seismogenic zone [ABSTRACT FROM AUTHOR]

Fluid overpressure from the water released by subducted sediments and oceanic crust is an important mechanism for generating earthquakes via brittle failure and frictional instability. If unstable, such fault materials may also host diverse fault reactivation mechanisms from slow slip events to moderate‐sized earthquakes in cold subduction zones. We examine this hypothesis for glaucophane gouge ‐ a key index minerals for blueschist facies ‐ at lower confining stresses where behavior is poorly understood. We measure friction and stability at temperatures of 100°C–500°C and effective normal stresses of 50–200 MPa, to explore the controls of temperature, stresses and excess pore fluid pressures on fault friction. The frictional coefficient of glaucophane at representative temperatures and stresses is ∼0.70, insensitive to temperature but with a slight increase at lower effective stresses. Elevating temperature promotes a transition from velocity‐strengthening to weak velocity‐weakening behavior, indicating the destabilizing effect of high temperature downdip in subduction zones. Reducing effective normal stress or concomitantly elevating pore fluid pressure further strengthens the velocity‐weakening response and would be manifest as moderate‐sized earthquakes. Our results support the potential for enhanced unstable sliding of glaucophane gouges at lower effective stresses and blueschist facies temperatures ‐ with weak to moderate velocity‐weakening responses upon fluid pressurization vital for understanding the abundance of slow slip to moderate‐sized earthquakes apparent in cold subduction zones. Plain Language Summary: Blueschist facies present in subduction zones with lower thermal gradient host abundant slow‐slip and dynamic earthquakes—such events requiring critical stress conditions. Such blueschist layers are characterized by abundant fluids that are released as the slab descends and heats—resulting in large excess fluid pressures that may trigger slip on faults at depth. We measure the frictional properties of simulated faults containing glaucophane powders representative of these blueschist products at temperatures, stresses and especially fluid overpressures representative of the descending slab at the plate boundary. Results indicate a temperature‐independent high frictional strength but a transition to unstable slip at both higher temperatures and increased fluid pressures. In the intermediate‐depth subduction zone, depending on the fluid pressure, it is possible to generate earthquakes in a diverse range of modes such as slow‐slip events through moderate‐sized earthquakes. This can explain documented observations in this region. Our results highlight the importance of fluid pressurization and effective stress reduction in controlling such slow‐slip and moderate‐sized events at higher temperatures and for understanding earthquakes in subduction zones. Key Points: Frictional stability of glaucophane gouge is P‐T dependent, with a change from v‐s to v‐w at ∼300°C with enhanced v‐w at lower stressesMinor v‐w of glaucophane gouge at critical stress conditions is favorable for slow slip events in subduction zonesFrictional stability of glaucophane gouge upon fluid overpressures supports the observation of abundant intermediate‐depth earthquakes [ABSTRACT FROM AUTHOR]

We apply a template matching method on GNSS data for stations located in Honshu, Japan, to detect slow slip events associated with the subducting Philippine Sea and Pacific plates during the period from 1997 to 2020. A measure of the minimum detectable moment magnitude is proposed, from which we infer that the method could potentially detect SSEs as small as Mw 5.2 on the westernmost part of the Philippine Sea plate and Mw 6 on the Pacific plate below Honshu eastern coastline. We find 12 slow slip events on the Philippine Sea plate, among which eight are located on the known Boso slow slip event asperity and the four others are located offshore north‐east relative to the Boso SSEs, at the transition with the Pacific plate. We find 9 SSEs on the Pacific plate, mainly on the northern section, offshore Iwate prefecture. A clear gap with no SSEs coincides with the main asperity that broke during the 2011 Tohoku earthquake. Most event locations correlate with low locking areas. We do not find any clear temporal pattern apart from the regular occurrence of the largest Boso SSEs. Plain Language Summary: Slow earthquakes are known to occur in northern Japan, especially off the Boso Peninsula. They are however harder to detect on the Pacific plate, likely due to their small sizes and/or their remoteness. We here apply a method that search for such slow ruptures, that process all the GNSS stations at once so to improve the signal‐to‐noise ratio. We find 21 such events, 13 of them being so far unrecognized. A clear lack of slow slip events is observed for the asperity that broke during the 2011 Tohoku‐Oki earthquake. The absence of any regular pattern, outside the well‐known Boso slow ruptures, is noticed, implying that the role of these events in the overall seismic cycle is still unclear for the Pacific plate subduction zone. Key Points: We search for slow slip events along the northern Japan subduction zones using a template matching approach on GNSS time series (1997–2020)We find 12 slow slip events on the Philippine Sea plate and 9 on the Pacific plate, coinciding with low locking areasA large gap devoid of any SSE correlates with the locked asperity of the 2011 M9 Tohoku‐Oki earthquake [ABSTRACT FROM AUTHOR]

Three decades of geodetic monitoring have established slow slip events (SSEs) as a common mode of fault slip, sometimes linked with earthquake swarms and in a few cases escalating to major seismic events. However, the connection between SSEs and earthquake hazard has been difficult to quantify and contextualize beyond regional studies. We aggregate a geodetic record of SSEs from subduction zones in the circum-Pacific region. In aggregate, earthquake rates increase up to threefold concurrent with and proximal to SSEs. The relative amplitude of this increase is correlated with the SSE size and, to a lesser extent, their depth and region. The subdued and coincident earthquake response to SSE stress transfer suggests a more limited role of static stress transfer and a very short relaxation timescale for the triggered seismicity. The observed range of behavior does not support a major connection between SSEs and earthquake hazard. [ABSTRACT FROM AUTHOR]

Various stress-releasing phenomena, such as episodic tremor and slip (ETS) and low-frequency earthquakes, occur at the downdip seismogenic zone in southwest Japan. However, it is unclear how much net stress and slip deficit accumulate at these depths during the interseismic phase. Here, we perform both elastic and viscoelastic earthquake-cycle forward models and reassess the locking state in Nankai from a synthesized perspective with the aid of geodetic modeling results. Our results suggest that the overestimation of the locking depth due to ignoring Earth's viscoelasticity is much smaller (less than 5 km) in this early interseismic subduction zone compared to that (~ 10 km) of late-interseismic margins. Considering viscoelastic modeling results and other physical arguments, the preferred steady-state viscosities for the continental and oceanic mantle are 5 × 1019 Pa s and 1020 Pa s, respectively. We find a clear trade-off between the full locking depth and the width of the transition zone when explaining both horizontal and vertical geodetic data, demanding other data to further resolve this inherent ambiguity. Unlike in Cascadia, partial megathrust locking in Nankai likely penetrates into the ETS zone, leaving no intervening gap between the shallow megathrust, where hosts large earthquakes, and the ETS zone. Assuming locking extends into the downdip of the ETS zone (i.e., 40 km), we propose a preferred viscoelastic locking model with a full locking depth of 18 km and a broad transition zone spanning a 22-km depth range. In this model, the downdip half portion of the transition zone corresponds to the ETS zone, which can accumulate certain slip deficit in a largely creeping and partially locked state. However, most of the accumulated slip deficit in the ETS zone may be accommodated aseismically simultaneously by stress-releasing phenomena, leaving limited to no budget to release during future megathrust earthquakes. We suggest that precise documentation of total slip during slow slip events, along with refinement of viscoelastic locking models, will provide new insights into the net slip budget available in the ETS zone. This will help assess the potential of future coseismic and/or postseismic slip penetrating into the ETS zone in Nankai, Cascadia and other subduction zones. [ABSTRACT FROM AUTHOR]

Slow earthquakes, including low-frequency earthquakes, tremor, and geodetically detected slow-slip events, have been widely detected, most commonly at depths of 40–60 km in active subduction zones around the Pacific Ocean Basin. Rocks exhumed from these depths allow us to search for structures that may initiate slow earthquakes. The evidence for high pore-fluid pressures in subduction zones suggests that they may be associated with hydraulic fractures (e.g., veins) and with metamorphic reactions that release or consume water. Loss of continuity and resulting slip at rates exceeding 10−4 m s–1 are required to produce the quasi-seismic signature of low-frequency earthquakes, but the subseismic displacement rates require that the slip rate is slowed by a viscous process, such as low permeability, limiting the rate at which fluid can access a propagating fracture. Displacements during individual low-frequency earthquakes are unlikely to exceed 1 mm, but they need to be more than 0.1 mm and act over an area of ~105 m² to produce a detectable effective seismic moment. This limits candidate structures to those that have lateral dimensions of ~300 m and move in increments of <1 mm. Possible candidates include arrays of sheeted shear veins showing crack-seal structures; dilational arcs in microfold hinges that form crenulation cleavages; brittle-ductile shear zones in which the viscous component of deformation can limit the displacement rate during slow-slip events; slip surfaces coated with materials, such as chlorite or serpentine, that exhibit a transition from velocity-weakening to velocity-strengthening behavior with increasing slip velocity; and block-in-matrix mélanges. [ABSTRACT FROM AUTHOR]

Large bursts of non‐volcanic tremor ("major" tremor episodes) correlated with geodetic deformation recur regularly in the Cascadia subduction zone and are often called episodic tremor and slip (ETS). Minor episodes of tremor between ETS are ubiquitous but have been understudied. This paper assesses time‐invariant characteristics of tremor episodes of all sizes within northern Cascadia. We derive a catalog of tremor episodes ranging in size from 10 to >13,000 tremor events using the results of 17 years of tremor monitoring. Minor episodes represent ∼96% of all 896 tremor episodes and their occurrence varies on 10‐km scales. Using estimates for the depth of the forearc Moho and subducting slab, we observe an association between the location of the forearc mantle corner (FMC) and tremor occurrence that leads to along‐dip modality. Bimodality, present in southern Washington and Vancouver Island, represents the segmentation of major and minor episodes up‐dip and down‐dip of the FMC, respectively. Unimodality, present in Puget Sound, results when the FMC is located near the down‐dip edge of the ETS zone and no segmentation occurs. We also use our extensive tremor episode catalog alongside three‐dimensional regional tomographic velocity models to reassess the relationship between tremor activity and low Vp/Vs signatures in the forearc. We do not find a correlation between tremor episode recurrence intervals and Vp/Vs, contrary to some previous work, suggesting that controls on silica precipitation in the forearc crust are not dominant controls of tremor episode recurrence, or that the association is not widely observable. Plain Language Summary: The Cascadia subduction zone hosts slow earthquakes that are observed as non‐volcanic tremor and surface deformation. Tremor events are typically clustered in time and space, and we define minor and major tremor episodes to be clusters consisting of small and large numbers of tremor events, respectively. Based on a 17‐year record of tremor in northern Cascadia, we use observations of a wide range of episode sizes (10 to >13,000 tremor events) to investigate their spatial characteristics and improve our understanding of the subduction zone. We find that the intersection of the upper‐plate crust/mantle boundary and the subducting plate, called the forearc mantle corner, controls the distribution of tremor along the dip‐direction of the subduction zone as well as the separation of major and minor episodes. From this, we infer that the forearc mantle corner creates a variety of physical conditions that result in specific spatial patterns of tremor behaviors. We also use our record of tremor episodes to reassess the relationship between upper‐plate composition and the recurrence of tremor episodes. Contrary to previous work, we do not find any link between the two, suggesting that processes controlling tremor episode recurrence do not significantly influence the composition of the continental crust. Key Points: Tremor episodes in northern Cascadia are defined from the longest tremor catalog spanning the years 2006–2023Spatial bimodality of tremor along dip results from major and minor episode segmentation controlled by the forearc mantle cornerEpisode recurrence intervals are not correlated with tomographic forearc Vp/Vs [ABSTRACT FROM AUTHOR]

Detecting crustal deformation during transient deformation events at offshore subduction zones remains challenging. The spatiotemporal evolution of slow slip events (SSEs) on the offshore Hikurangi subduction zone, New Zealand, during February–July 2019, is revealed through a time‐dependent inversion of onshore and offshore geodetic data that also accounts for spatially varying elastic crustal properties. Our model is constrained by seafloor pressure time series (as a proxy for vertical seafloor deformation), onshore continuous Global Navigation Satellite System (GNSS) data, and Interferometric Synthetic Aperture Radar displacements. Large GNSS displacements onshore and uplift of the seafloor (10–33 mm) require peak slip during the event of 150 to >200 mm at 6–12 km depth offshore Hawkes Bay and Gisborne, comparable to maximum slip observed during previous seafloor pressure deployments at north Hikurangi. The onshore and offshore data reveal a complex evolution of the SSE, over a period of months. Seafloor pressure data indicates the slow slip may have persisted longer near the trench than suggested by onshore GNSS stations in both the Gisborne and Hawkes Bay regions. Seafloor pressure data also reveal up‐dip migration of SSE slip beneath Hawke Bay occurred over a period of a few weeks. The SSE source region appears to coincide with locations of the March 1947 Mw 7.0–7.1 tsunami earthquake offshore Gisborne and estimated great earthquake rupture sources from paleoseismic investigations offshore Hawkes Bay, suggesting that the shallow megathrust at north and central Hikurangi is capable of both seismic and aseismic rupture. Plain Language Summary: Subduction zones, where one tectonic plate dives beneath another, generate the planet's largest earthquakes. They also host an important mode of fault slip called "slow slip events (SSEs)," which are essentially earthquakes in slow motion. The Hikurangi subduction zone, where the Pacific Plate subducts beneath New Zealand hosts large and frequent SSEs near the trench, where the plate boundary emerges at the seabed, requiring seafloor instrumentation to investigate them. Seafloor pressure measurements can track centimeter‐level up or down movement of the seafloor during slow slip, and reveal offshore displacement during a large 2019 SSE at the Hikurangi subduction zone. The 2019 event involved substantial migration, beginning at ∼15 km depth, and expanding to the trench over a period of several weeks. We also show that the same areas which have ruptured in previous seismic earthquakes (that involved faster slip) can also rupture slowly, in SSEs. This raises the possibility that regions where we currently observe SSEs could also produce seismic events. This result also demands that more work must be done to understand the physical processes that enable the same part of a fault to rupture both fast and slow at different times. Key Points: Central Hikurangi slow slip events (SSEs) propagate up‐dip over a period of weeks to monthsSeafloor geodetic data reveal that shallow SSEs may last longer than onshore Global Navigation Satellite System data suggestThe same portions of a shallow megathrust can host both large seismic and aseismic rupture [ABSTRACT FROM AUTHOR]

Recent numerical simulation studies suggest the existence of a seismic type that is distinct from regular earthquakes—the slow self‐arresting rupture (SSAR). Unlike regular earthquakes that propagate dynamically following the initiation, The SSARs automatically arrest within the nucleation zone without interference. Additionally, numerical simulations indicate that SSARs exhibit a significantly lower energy release compared to regular earthquakes, while also exhibiting a relatively long source duration. Given these distinctive properties, comprehending the source processes of SSARs assumes great strategic importance. However, our current understanding of SSARs, particularly regarding their response to different frictional conditions and their correlation with natural phenomena, remains limited in scope. To further explore the intricacies of SSARs, we employ a three‐dimensional fully dynamic source model to simulate SSARs under various slip‐weakening frictional conditions. The findings indicate that SSARs occur in frictional environments characterized by large normalized critical slip distances, with the seismic source process being primarily influenced by this parameter. Apart from displaying significantly smaller average slip and stress drop, which are two to three orders of magnitude lower than those of regular earthquakes of comparable magnitude, SSARs also showcase a decrease in duration, seismic moment, slip rate, and stress drop as the normalized critical slip distance increases. The moment‐duration scaling law of SSARs exhibits a linear pattern. Moreover, the observation of slow earthquakes offers further implications for the presence of SSARs, indicating their potential association with a wider range of intricate seismic phenomena. Plain Language Summary: The source process of earthquakes is an important issue that has been widely studied. Through numerical simulations, we have identified a distinct type of earthquake characterized by slow evolution process and relatively low moment release: slow self‐arresting rupture (SSAR). Here, we conduct simulations with different frictional conditions to gain insights into the unique source processes of SSARs and explore the influencing factors. Unlike regular earthquakes that dynamically propagate after the initiation of sliding, SSARs autonomously terminate within the same region of their initial occurrence, without encountering any barriers or obstructions. Additionally, SSARs occur on faults of different shapes and their occurrence is mainly influenced by frictional conditions. Unlike regular earthquakes, the source duration, slip, slip rate, stress drop, and seismic moment of SSARs decrease as the normalized critical slip distance increases. The similarity between the source characteristics of SSARs and observed slow earthquakes implies the possibility of detecting SSARs in real‐world scenarios and their association with intricate seismic phenomena. Key Points: The slow self‐arresting rupture releases less moment over a longer duration compared to regular earthquakesThe source process of slow self‐arresting rupture is predominantly governed by the normalized critical slip distanceThe slow self‐arresting rupture exhibits waveforms and spectra similar to low‐frequency earthquakes and very low‐frequency earthquakes [ABSTRACT FROM AUTHOR]

We investigate similarities and differences between natural and simulated slow earthquakes using nonlinear dynamical system tools. We use spatio‐temporal slip potency rate data derived from Global Navigation Satellite System (GNSS) position time series in the Cascadia subduction zone and numerical simulations intended to reproduce their pulse‐like behavior and scaling laws. We provide metrics to evaluate the accuracy of simulations in mimicking slow earthquake dynamics. We investigate the influence of spatio‐temporal coarsening as well as observational noise. Despite the use of many degrees of freedom, numerical simulations display a surprisingly low average dimension, akin to natural slow earthquakes. Instantaneous dynamical indices can reach large values (>10) instead, and differences persist between numerical simulations and natural observations. We propose to use the suggested metrics as an additional tool to narrow the divergence between slow earthquake observations and dynamical simulations. Plain Language Summary: Earthquakes are natural phenomena resulting from the Earth's crust cyclically loading and unloading. The unpredictability of seismic events, combined with the large energy they release during the co‐seismic phase, poses not only scientific challenges but also significant threats to numerous populated regions at risk. Numerical simulations of the seismic cycle are widely used to better understand the dynamics of this natural phenomenon. Nonetheless, a direct comparison of earthquake observations and numerical simulations of the seismic cycle is currently prevented by the lengthy recurrence time of large seismic events rupturing the same fault segment and the short observational record at our disposal. Slow earthquakes, exhibiting lower recurrence times, serve as a viable alternative for validating models against real‐world observations. We investigate similarities and differences between natural and simulated slow earthquakes through the lens of nonlinear dynamical system theory. We study the effects of observational noise and spatio‐temporal coarsening putting the simulations in conditions like real‐world observations. We find that observational noise does not suffice to explain the higher complexity retrieved for natural observations. By refining our understanding of these dynamical systems, this study contributes to advancements in seismic research, offering a picture of the complexities involved on active faults. Key Points: Natural observations and numerical simulations of slow earthquakes share common average dynamical propertiesNatural observations show higher complexity than numerical simulationsMatching instantaneous dynamical properties can help reducing the discrepancies between natural and simulated slow earthquakes [ABSTRACT FROM AUTHOR]

Earthquake cycle simulations based on the rate-and-state friction formulation are evolutions of nonlinear dynamical systems (NDS). The term "cycle" implies an overall stable structure that is a phase-space attractor naturally traced out by trajectories of NDS as it evolves. Quantitatively characterizing these attractors should be a basis for measuring complexities of the simulated earthquake cycles, i.e. to determine if and how regular or chaotic they are. I first revisit the textbook-standard quasi-dynamic spring-slider system from an NDS perspective, explicitly showing the attractors, their relationship with the parameters of the NDS, and how they can be characterized taken advantage of their low-dimensionality while aiming to extend the analysis to high-dimensionality. I evaluate two approaches, computing the Lyapunov exponents (LEs) and measuring correlation dimensions, with the simple spring-slider and earthquake-cycle examples whose phase-space attractors can be visually verified. I conclude LEs are too inconvenient and computationally expensive to use whereas measuring correlation dimensions is an easy and effective approach even with highly non-uniform time sampling present in all simulations. For earthquake-cycle simulations, an attractor reconstruction is performed based on Taken's theorem to corroborate my correlation-dimension results. The current method is limited in its ability to detect chaos in a dichotomous manner, which illuminates the direction for future study. An improving ability to quantitatively characterize earthquake-cycle simulations as an overall stable structure offers new opportunities to understand exotic seismic observations such as slow-slip events and enables more informative comparison with real data, particularly from paleoseismology, which could have far-reaching implications in earthquake forecasting. [ABSTRACT FROM AUTHOR]

Recently, integrated geophysical‐geological surveys in the Nankai subduction zone in Japan have revealed that slow earthquakes repeatedly occur beneath the outer wedge of the forearc. During December 2020 to February 2021, clustered slow earthquakes propagated around the frontal thrust of the accretionary wedge. The frontal thrust ramps up from the basal décollement and slips over trench‐filling sediment along the landward edge of the Nankai trough floor. Here, the Paleo‐Zenisu ridge has been subducted beneath the inner‐outer slope border. In addition, ocean floor topography and geologic structure revealed by seismic reflection surveys completed before 2022 document that the basement of the Philippine Sea Plate beneath the frontal thrust has a seamount and a horst‐like basement high. The northern edge of the basement high is located at the ramp‐up position of the frontal thrust. The 2020–2021 clustered slow earthquakes started at the Paleo‐Zenisu ridge and propagated to the topographic highs beneath the deformation front. Considering that the relative plate convergence between the upper Amurian Plate of the Nankai forearc and the subducting Philippine Sea Plate is ∼6.0 cm/year, the basement high at the deformation front has uplifted the frontal crest of the wedge at an average rate of 2.7–5.7 mm/year for several tens to hundred thousand years. These rates are among some of the highest rock uplift rates measured in the world. The slow earthquakes in the off‐Kumano Nankai Trough in 2020–2021 are a snapshot of a "living" Nankai frontal thrust during the megathrust interseismic period. Plain Language Summary: Slow earthquakes family: slow slips, very low frequency earthquakes, and tremors are significant events for understanding the interseismic dynamics between large earthquakes along the convergent plate boundary in the ocean. Slow earthquakes took place during December 2020 to February 2021 in the Nankai Trough in Japan and propagated along the frontal edge of the upper plate. Seismic reflection surveys revealed that seamount‐like and horst‐like basement highs exist beneath the front, collide with the upper plate, and uplift trench filling sediments by a ramp thrust. Even the incipient plate boundary of the décollement is initiated within the sediments beneath the Nankai Trough. Such a deformation feature around the front is a source candidate for the slow earthquakes. The uplift rate of the frontal crest of the upper plate presents the same order of a few to several mm/year as that of the foreland of the Himalayan Mountain belt. The slow earthquakes in the off‐Kumano Nankai Trough in 2020–2021 appear the first snapshot of a "living" Nankai frontal thrust during inter‐large seismic period. Key Points: Between 2020 and 2021, slow earthquakes propagated around the frontal thrust of the accretionary wedge in the Nankai Trough off KumanoSeamount and horst‐like basement highs beneath the frontal thrust cause uplift and are associated with the slow earthquake eventsThe average uplift rate of the frontal crest is 2.7–5.7 mm/year, approximately the same order as that of the Himalayan foreland [ABSTRACT FROM AUTHOR]

Abstract The westernmost Nankai Trough, southwest Japan, exhibits a rapid along-strike reduction in plate coupling in the proximity to the subducting Kyushu-Palau ridge. Yet how and to what extent the ridge subduction impacts physical properties at the megathrust have not been investigated. Here we present high-resolution seismic P-wave velocity models along the forearc wedge in the western Nankai Trough derived from full-waveform inversion analyses of seismic refraction data. The velocity models show that where the plate coupling is weak and the plate boundary presumably hosts slow earthquakes, the upper plate exhibits lower seismic velocities indicating higher degree of fracturing over a ~ 100 km length along trough. Intriguingly, the extent of the upper-plate low-velocity features is significantly larger than the surficial width of the Kyushu-Palau ridge, and this low-velocity zone is underthrust by the slab with increased crustal thickness by 2–4 km. Seismic reflection images consistently reveal that the thicker slab crust has appreciable basement roughness extending ~ 60 km from the eastern margin of the Kyushu-Palau ridge beneath the western Shikoku basin. We suggest that such a thicker and rugged slab crust, together with the main body of the Kyushu-Palau ridge, can cause significant fracture zones in the overriding plate, decrease the interplate coupling and produce preferable conditions for shallow slow earthquakes to occur when subducted. The results may also provide structural constraints on the western limit of future megathrust earthquakes in the Nankai Trough. Graphical Abstract

The shallowest regions of subduction megathrusts mainly deform aseismically, but they can sporadically host slow-slip events (SSEs) and tsunami earthquakes, thus representing a severe hazard. However, the mechanisms behind these remain enigmatic because the frictional properties of shallow subduction zones, usually rich in clay, do not allow earthquake slip according to standard friction theory. We present experimental data showing that clay-rich faults with bulk rate-strengthening behavior and null healing rate, typically associated with aseismic creep, can contemporaneously creep and nucleate SSE. Our experiments document slow ruptures occurring within thin shear zones, driven by structural and stress heterogeneities of the experimental faults. We propose that bulk rate-strengthening frictional behavior promotes long-term aseismic creep, whereas localized frictional shear allows slow rupture nucleation and quasi-dynamic propagation typical of rate-weakening behavior. Our results provide additional understanding of fault friction and illustrate the complex behavior of clay-rich faults, providing an alternative paradigm for interpretation of the spectrum of fault slip including SSEs and tsunami earthquakes. [ABSTRACT FROM AUTHOR]

The application of deep‐learning‐based seismic phase pickers has surged in recent years. However, the efficacy of these models when applied to monitoring volcano seismicity has yet to be fully evaluated. Here, we first compile a data set of seismic waveforms from various volcanoes globally. We then show that the performances of two widely used deep‐learning pickers deteriorate systematically as the earthquakes' frequency content decreases. Therefore, the performances are especially poor for long‐period earthquakes often associated with fluid/magma movement. Subsequently, we train new models which perform significantly better, including when tested on two data sets where no training data were used: volcanic earthquakes along the Cascadia subduction zone and tectonic low‐frequency earthquakes along the Nankai Trough. Our model/workflow can be applied to improve monitoring of volcano seismicity globally while our compiled data set can be used to benchmark future methods for characterizing volcano seismicity, especially long‐period earthquakes which are difficult to monitor. Plain Language Summary: Earthquake activity at volcanic regions is often monitored to indicate volcanic activity. Identifying the time when the energy radiated from an earthquake source arrives at a seismometer is essential for locating the earthquake, which can be difficult for volcanic earthquakes because of high noise levels, high event rates, and obscured onsets. Previous studies have demonstrated that deep learning can excel in picking the arrival times of regular earthquakes. However, it is unclear how sensitive these detectors are to earthquakes in volcanic regions. Here, we first compile a data set of earthquakes from various volcanoes globally. We then show that existing deep‐learning‐based detectors can miss a large fraction of these earthquakes, especially those without an abrupt change in signal amplitude. We then provide two new models which can better detect volcanic earthquakes than existing models. Our model/workflow can be applied to improve monitoring of volcanic earthquakes globally. Key Points: We compile a data set of seismic waveforms from various volcanic regions globallyWe show that existing deep‐learning phase pickers' performances deteriorate with decreasing earthquake frequency contentOur retrained models perform better and are more generalizable for monitoring volcano seismicity, especially long‐period earthquakes [ABSTRACT FROM AUTHOR]

The Superstition Hills Fault (SHF) exhibits a rich spectrum of slip modes, including M 6+ earthquakes, afterslip, quasi‐steady creep, and both triggered and spontaneous slow slip events (SSEs). Following 13 years of quiescence, creepmeters recorded 25 mm of slip during 16–19 May 2023. Additional sub‐events brought the total slip to 41 mm. The event nucleated on the northern SHF in early‐May and propagated bi‐laterally at rates on the order of kilometers per day. Surface offsets reveal a bi‐modal slip distribution, with slip on the northern section of the fault being less localized and lower amplitude compared to the southern section. Kinematic slip models confirm systematic variations in the slip distribution along‐strike and with depth and suggest that slip is largely confined to the shallow sedimentary layer. Observations and models of the 2023 SSE bear a strong similarity to previous slip episodes in 1999, 2006, and 2010, suggesting a characteristic behavior. Plain Language Summary: Studying the mechanical properties and behavior of faults is essential for understanding earthquake ruptures. In this study, we investigate a recent slip event on the Superstition Hills Fault (SHF), which has a well‐documented record of slip. A notable aspect of the SHF is that it periodically undergoes "slow slip events" (SSEs), where the fault slips and releases energy without any accompanied ground shaking. During May‐July 2023, the SHF experienced a major SSE for the first time in 13 years. Our analysis shows that it was the largest documented SSE on the SHF and released equivalent energy to a magnitude 4.5 earthquake. We also find that the spatial pattern of fault slip is very similar to several previous slip events in 1999, 2006, and 2010, suggesting that the SHF has a tendency to slip in a characteristic manner. Key Points: We document a recent spontaneous slow slip event (SSE) on the Superstition Hills Fault using creepmeter, Interferometric Synthetic Aperture Radar, Global Navigation Satellite System, and field measurementsOver 41 mm of slip occurred from mid‐May to mid‐July 2023, with moment release corresponding to a Mw 4.5 earthquakeThe kinematics of the 2023 event are remarkably similar to several previous SSEs, suggesting a characteristic rupture process [ABSTRACT FROM AUTHOR]

Shallow creep events provide opportunities to understand the mechanical properties and behavior of faults. However, due to physical limitations observing creep events, the precise spatio‐temporal evolution of slip during creep events is not well understood. In 2023, the Superstition Hills and Imperial faults in California each experienced centimeter‐scale slip events that were captured in unprecedented detail by satellite radar, sub‐daily Global Navigation Satellite Systems, and creepmeters. In both cases, the slip propagated along the fault over 2–3 weeks. The Superstition Hills event propagated bilaterally away from its initiation point at average velocities of ∼9 km/day, but propagation velocities were locally much higher. The ruptures were consistent with slip from tens of meters to ∼2 km depths. These slowly propagating events reveal that the shallow crust of the Imperial Valley does not obey purely velocity‐strengthening or velocity‐weakening rate‐and‐state friction, but instead requires the consideration of fault heterogeneity or fault‐frictional behaviors such as dilatant strengthening. Plain Language Summary: Faults that slip in a slow, aseismic process called creep present an opportunity to understand the frictional behavior of fault systems. In the spring of 2023, two fault systems in southern California experienced large slip events that were recorded in high resolution by ground‐based and space‐based measurements from GPS and satellite radar. The slip began spontaneously on both the Superstition Hills and Imperial faults and slowly propagated to other parts of each fault. The average slip propagation speed ranged from 0.4 to 9 km per day. Interestingly, this velocity is very similar to propagation velocities observed in subduction zones around the world and is approximately the speed of a sloth or a snail. Future work may help us understand what physical properties, such as confining stress, frictional strength, fluid pressure, and fluid diffusivity, control the propagation velocity of a slow slip event. Key Points: The Superstition Hills and Imperial faults hosted centimeter‐scale transient slip events in spring 2023Interferometric Synthetic Aperture Radar, high‐rate Global Navigation Satellite Systems, and local creepmeters characterized each slip event in space and timeSlow slip propagated at 6–9 km/day along the Superstition Hills fault and 0.4 km/day along the Imperial fault, but locally faster or slower [ABSTRACT FROM AUTHOR]

Very low‐frequency earthquakes (VLFs) are characterized by longer source duration and smaller stress drop than regular earthquakes of similar magnitude. Recent studies have shown their frequent correlation with low‐frequency earthquakes (LFEs) on shared faults. The underlying source processes governing the occurrence of VLFs and their interaction with LFEs remain elusive. Here, we employ a slip‐weakening model for slow earthquakes. By comparing the source parameters of simulations and observations, it is suggested that VLFs are slow self‐arresting earthquakes that self‐terminate within the nucleation patch. Additionally, we adopt a composite model to reproduce the records of the simultaneous occurrences of a VLF and an LFE in the Nankai area. Our results present the possibility that VLFs, LFEs, and regular earthquakes can be distinguished using a unified dynamic framework. Plain Language Summary: Earthquakes can be divided into two groups: regular earthquakes and slow earthquakes. Regular earthquakes are the results of quick shear slip on faults, while slow earthquakes are generated from slow transient movements along the faults and radiate anomalous small energy. The mechanism that drives their different source characteristics remains elusive. Here, we employ an independent and composite dynamic source model to elucidate the unique attributes of very low‐frequency earthquakes (LFEs), a type of slow earthquakes. By comparing the source parameters and waveforms of observations with simulations. The outcomes indicate that very LFEs potentially represent a seismic type that self‐terminate within the region where the rupture is initially triggered. Key Points: Very low‐frequency earthquakes (LFEs) are characterized by slow self‐arresting processesOur model can simultaneously reproduce both very low‐frequency and LFEThe slip‐weakening frictional model generates both fast and slow earthquakes [ABSTRACT FROM AUTHOR]

Fault regions inferred to be slowly slipping are interpreted to accommodate much of tectonic plate motion aseismically and potentially serve as barriers to earthquake rupture. Here, we build on prior work using simulations of earthquake sequences with enhanced dynamic fault weakening to show how fault regions that exhibit decades of steady creep or transient slow‐slip events can be driven to dynamically fail by incoming earthquake ruptures. Following substantial earthquake slip, such regions can be under‐stressed and locked for centuries prior to slowly slipping again. Our simulations illustrate that slow fault slip indicates that a region is sufficiently loaded to be failing about its quasi‐static strength. Hence, if a fault region is susceptible to failing dynamically, then observations of slow slip could serve as an indication that the region is critically stressed and ready to fail in a future earthquake, posing a qualitatively different interpretation of slow slip for seismic hazard. Plain Language Summary: Earthquakes are thought to predominantly occur along sections of faults that appear stuck and actively accumulating strain under tectonic plate motion. Other fault regions observed to be slowly slipping are thought to release some of this strain without causing strong shaking, potentially limiting the location and amount of fault slip in earthquakes. Here we present numerical simulations of long‐term fault slip that add to a body of work showing how fault areas can host different styles of slow slip for several decades prior to failing destructively when pushed by an incoming earthquake rupture. Our models show how relatively short‐term observations of slow fault slip compared to the recurrence of large earthquakes over several centuries can mask fault regions that are capable of experiencing substantial slip in future earthquakes. Importantly, our simulations suggest that if a fault region is capable of failing during an earthquake, then observations of slow slip may indicate that the region is favorably stressed to fail in a future earthquake, representing a qualitatively different interpretation of slow slip for seismic hazard. Key Points: Short‐term inferences of fault coupling provide limited insight into which faults regions can undergo large slip in future earthquakesSteady and transient slow slip indicates that fault stress levels are loaded near quasi‐static failure conditionsIf a fault region is susceptible to failing dynamically, slow slip may suggest it is critically stressed to fail in a future earthquake [ABSTRACT FROM AUTHOR]

Lithologic heterogeneity and the presence of fluids have been linked to seamount subduction and collocated with slow earthquakes. However, the deformation mechanisms and fluid conditions associated with seamount subduction remain poorly understood. The exhumed Chichibu accretionary complex on Amami-Oshima Island preserves mélange shear zones composed of mudstone-dominated mélange and basalt–limestone mélange deformed under sub-greenschist facies metamorphism. The mudstone-dominated mélange contains sandstone, siliceous mudstone, and basalt lenses in an illitic matrix. The basalt–limestone mélange contains micritic limestone and basalt lenses in a chloritic matrix derived from the mixing of limestone and basalt at the foot of a seamount. The basalt–limestone mélange overlies the mudstone-dominated mélange, possibly representing a submarine landslide from the seamount onto trench-fill terrigenous sediments. The asymmetric S–C fabrics in both mélanges show top-to-SE shear consistent with megathrust-related shear. Quartz-filled shear and extension veins in the mudstone-dominated mélange indicate brittle failure at near-lithostatic fluid pressure and low differential stress. Microstructural observations show that deformation in the mudstone-dominated mélange was accommodated by dislocation creep of quartz and combined quartz pressure solution with frictional sliding of illite, whereas the basalt-limestone mélange was accommodated by frictional sliding of chlorite and dislocation creep of coarse-grained calcite, with possible pressure solution creep and diffusion creep of fine-grained calcite. The mélange shear zones formed in association with seamount subduction record temporal changes in deformation mechanisms, fluid pressure, and stress state during megathrust shear with brittle failure under elevated fluid pressure, potentially linking tremor generation near subducting seamounts. [ABSTRACT FROM AUTHOR]

Recent observations beneath Kanto, Japan have shown that seismic activity and seismic attenuation within the overlying continental plate change with time due to drainage caused by slow‐slip events (SSEs) along the upper boundary of the Philippine Sea plate. However, associated changes in rock properties have not been investigated. In this study, we estimate frequency‐dependent shear‐wave anisotropy to provide a detailed insight into the structural change associated with drainage. We perform shear‐wave splitting analysis in frequency ranges of 1–4, 2–6, and 4–8 Hz for 306 earthquakes that occur during September 2009–August 2021 and recorded at the Metropolitan Seismic Observation network. Obtained time differences between fast and slow S waves (delay time) range from almost zero to 0.16–0.18 s, exhibiting spatio‐temporal variation and frequency dependence. The fast S‐wave polarization directions are generally consistent with the direction of the maximum horizontal compressional axis in the study region, which suggests that the observed anisotropy is probably caused by the NE–SW‐oriented fractures developed under the regional stress field. The temporal variation in delay times is correlated with SSEs activity with a lag time of 0.0–0.1 year. Furthermore, comparisons between observed frequency‐dependent delay times and numerical calculation of fracture‐induced anisotropy suggest that the average fracture radius is almost constant (0.30–0.35 m) over time but fracture density temporally varies from 0.025 to 0.035. We infer that the fracture density is probably enhanced by opening of the NE–SW‐oriented fractures during the upward migration of fluids that are expelled from the plate interface. Plain Language Summary: Measurement of shear‐wave polarization anisotropy characterizes the intensity and orientation of fractures within the rocks beneath the Earth's surface. Furthermore, its frequency dependence provides important information for determining the fracture size and density. This study reveals that the strength of anisotropy shows temporal variations associated with the inferred periodic drainage from the subducting slab. We discuss a possible mechanism of the structural change focusing on the frequency dependence of the observed anisotropy and propose that the opening of fractures occurs immediately above the subducting slab following the periodic drainage. The obtained results will provide important constraints on fluid‐rock interaction above the subducting plate interface. Key Points: Shear‐wave splitting analysis in Kanto, Japan suggests that seismic anisotropy is orientated sub‐parallel to the regional stressAnisotropy above the plate interface is enhanced with a lag of 0.0–0.1 years from the occurrence of slow‐slip eventsThe observed frequency‐dependent anisotropy suggests that fractures open responding to drainage from plate interface [ABSTRACT FROM AUTHOR]

We investigated the effects of the propagation path and site amplification of shallow tremors along the Nankai Trough. Using far‐field S‐wave propagation from intraslab earthquake data, the amplification factors at the DONET stations were 5–40 times against an inland outcrop rock site. Thick (∼5 km) sedimentary layers with VS of 0.6–2 km/s beneath DONET stations have been confirmed by seismological studies. To investigate the effects of thick sedimentary layers, we synthesized seismograms of shallow tremors and intraslab earthquakes at seafloor stations. The ratios of the maximum amplitudes from the synthetic intraslab seismograms between models with and without thick sedimentary layers were 1–2. This means that thin lower‐velocity (<0.6 km/s) sediments just below the stations primarily control the estimated large amplifications. Conversely, at near‐source (≤20 km) distances, 1‐order amplifications of seismic energies for a shallow tremor source can occur due to thick sedimentary layers. Multiple S‐wave reflections between the seafloor and plate interface are contaminated in tremor envelopes; consequently, seismic energy and duration are overestimated. If a shallow tremor occurs within underthrust sediments, the overestimation becomes stronger because of the invalid rigidity assumptions around the source region. After 1‐order corrections of seismic energies of shallow tremors along the Nankai Trough, the scaled energies of seismic slow earthquakes were 10−10–10−9 irrespective of the region and source depth. Hence, the physical mechanisms governing seismic slow earthquakes can be the same, irrespective of the region and source depth. Plain Language Summary: The deployment of campaigns and permanent ocean bottom seismometers (OBSs) has enabled us to investigate the activity and physical properties of offshore seismic phenomena. Our knowledge of offshore subsurface structures is still limited; consequently, many studies have used conventional analysis methods with the simplest assumptions. Using observed and synthetic seismograms near the Nankai Trough, we found a limitation in the conventional analysis method applied to OBS data. Thick sedimentary layers, which have been confirmed by seismological studies along the Nankai Trough just below the OBSs, cause an approximately 1‐order overestimation of source parameters for seismic phenomena occurring around the shallow plate boundary. This overestimation may have occurred during the seismic energy estimation of shallow slow earthquakes in Hikurangi, Costa Rica, and Mexico. After correcting for the effects of thick sedimentary layers, we found that the scaled energies of seismic slow earthquakes were 10−10–10−9 irrespective of the region and source depth. This suggests that the physical mechanisms governing seismic slow earthquakes can be the same, regardless of region and source depth. Key Points: Effects of path and site on the seismic energy estimation of slow earthquakes at shallow plate boundaries were investigatedThe assumption of far‐field body waves without thick sediments causes an overestimation of seismic energies for shallow tremorsScaled energies of seismic slow earthquakes at both shallow and large depths range from 10−10 to 10−9 [ABSTRACT FROM AUTHOR]

Dilatant hardening is one proposed mechanism that causes slow earthquakes along faults. Previous experiments and models show that dilatant hardening can stabilize fault rupture and slip in several lithologies. However, few studies have systematically measured the mechanical behavior across the transition from dynamic to slow rupture or considered how the associated damage varies. To constrain the processes and scales of dilatant hardening, we conducted triaxial compression experiments on cores of Crab Orchard sandstone and structural analyses using micro‐computed tomography imaging and petrographic analysis. Experiments were conducted at an effective confining pressure of ∼10 MPa, while varying confining pressure (10–130 MPa) and pore fluid pressure (1–120 MPa). Above 15 MPa pore fluid pressure, dilatant hardening slows the rate of fault rupture and slip and deformation becomes more distributed amongst multiple faults as microfracturing increases. The resulting increase in fracture energy has the potential to control fault slip behavior. Plain Language Summary: When rocks are breaking, the pore spaces and developing fractures dilate, resulting in a decrease in pore fluid pressures. This decrease can strengthen the rock from ongoing deformation in a process known as dilatant hardening. We conducted experiments to better understand how this strengthening effect works, in particular looking at the ratio of pore fluid pressure to the external confining pressure (simulating rocks buried at depth), and also analyzed how the fractures that develop can vary from dilatant hardening. We found a threshold pressure at which the strengthening peaked, and increasing pore fluid pressure did not change how strong the rocks got from continuing deformation. We also observed a drastic increase in how damage was distributed due to this hardening effect at both a large (visible to the naked eye) and small scale (only visible in a high‐magnification microscope). These results indicate that dilatant hardening can increase how much energy must be expended to break the rock and to cause faults to slip when pore fluid pressures are high enough, and likely plays a role in stabilizing fault slip, causing earthquakes to slow down and be less dynamic. Key Points: We measured the transition between dynamic and stable rupture as a result of dilatant hardeningWe observed differences in microstructural development tied to the shift in rupture styleWe developed a model of fracture nucleation and propagation at different pore fluid pressures [ABSTRACT FROM AUTHOR]

We report the first catalog of low‐frequency earthquakes in the Hikurangi subduction zone, located beneath the Kaimanawa Range of the North Island at 50 km depth, downdip of regularly recurring (every 4–5 years) deep M7 slow slip events. To systematically detect low‐frequency earthquakes within the regional continuous seismic data, we utilized a matched‐filter approach with template waveforms derived from previous observations of tectonic tremor. We built our catalog of 36 low‐frequency earthquake sources, that produced almost 21,000 events over more than a decade, with two matched‐filter search iterations. In each iteration, the detections were gathered into families and their coherent waveforms processed and stacked to extract high‐quality waveforms, allowing us to pick seismic phase arrivals to locate the low‐frequency earthquakes. We highlight three characteristic features to validate that our detected events are indeed low‐frequency earthquakes: the eponymous deficit of high frequencies in their seismic waveforms, the episodic swarms of activity that define their activity through time, and their location at the plate boundary with a double‐couple source mechanism and geometry consistent with the subduction interface. Considering the observed low‐frequency earthquakes' relationship to neighboring slow slip, we observe the event swarms to occur much more frequently than the M7 slow slip events located just updip. Similar to other deep low‐frequency earthquakes in other subduction zones, we suggest that this characteristic clustering in time is driven by more frequent, smaller slow slip events that are not clearly observable at the surface. Plain Language Summary: Slow slip is episodic fault slip that lasts days, weeks or months, rather than the rapid ruptures of regular earthquakes. Geodetic observations of the surface displacement produced by slow slip suggest that their timing and location influence the seismic cycle of nearby faults and may even trigger large earthquakes. Although slow slip does not produce seismic radiation itself, slow slip is often accompanied by tiny repetitive seismic signals. These tiny seismic events, called low‐frequency earthquakes, can act as a powerful indicator of when and where slow slip is happening. In this study, we develop a new approach to detect low‐frequency earthquakes within continuous seismic waveforms, revealing the first observations of low‐frequency earthquakes in the Hikurangi subduction zone beneath the North Island of New Zealand. Our catalog of low‐frequency earthquakes suggests a complex pattern of slow fault slip at depth, with more frequent activity than geodetic data alone would suggest. The observed low‐frequency earthquake activity in the Hikurangi subduction zone thus represents a unique opportunity to study the slip history at depth beneath the North Island of New Zealand. Key Points: 36 low‐frequency earthquake sources are extracted from continuous waveforms through template matching, deblurring, and unsupervised learningLow‐frequency earthquake sources locate close to the plate boundary with source mechanisms consistent with the subduction interfaceDetected low‐frequency earthquakes are likely triggered by small, frequent, and deep slow slip not geodetically observable at the surface [ABSTRACT FROM AUTHOR]

The 2012 M5.6 Pernik earthquake in Bulgaria proceeded at slow slip rates and was accompanied with ground failure along the Meshtitsa fault scarp. Our investigation through paleoseismological trenching techniques and electrical resistivity tomography discovered a broad zone with multiple fault cores. In a trench, a 40-m-thick montmorillonite clay stratum is embedded in coarse-grained alluvial deposits along with two narrow gouge zones; together they demonstrate a frictional heterogeneity within the fault zone. The clayey deposits had experienced frictional stability which is recorded in intersecting shear bands interpreted to have formed at slow strain rates. A steep bedding of Oligocene alluvial deposits is interpreted as a result from an earlier phase of strike-slip motion. Since transitioning to normal dip-slip motion in the late Miocene, two gouge zones located at the periphery of the clayey deposits suggest strain localization during surface-rupturing earthquakes. In alluvial sediments deposited 3300 cal BP, localized slip on one of the faults and dispersed tensile cracks in the hangingwall of the other fault likely express failures at different strain rates. We infer that it is likely that the dispersed cracks in the trench, and similarly some of the 2012 ground cracks, resulted from afterslip, which followed ruptures at depth on relatively small seismically coupled fault areas. In contrast, we interpret the slip localized in the fault cores to have occurred when most of fault area was seismically coupled in larger earthquakes. This fault expresses a variability in earthquake sizes and seismic coupling in the past 3300 cal BP. [ABSTRACT FROM AUTHOR]

Interactions between fluids and deformation are widespread in the brittle crust. As experimentally shown, a high pore fluid pressure pf can fracture intact rocks or reactivate pre-existing fractures. The preference of reactivation over the formation of a new fracture depends on the orientation of the pre-existing fracture with respect to the stress axes and on pf. In nature, the predominant reactivation of misoriented pre-existing faults rather than the formation of new faults with more favorable orientations suggests that pressurized fluids are present in the brittle crust. There is a large body of evidence indicating that supra-hydrostatic pf contributes to the reactivation of lowangle thrust faults or normal faults. Conversely, supra-hydrostatic pf values are less common along vertical or steeply dipping plate boundary transform faults or intra-continental strike-slip faults. If these faults are severely misoriented with respect to the ambient stress field, their reactivation may not be due to supra-hydrostatic pf but to other mechanisms such as shear-enhanced compaction or thermal pressurization. Supra-hydrostatic pf also plays a role in the nucleation or propagation of seismic ruptures in the continental or oceanic crust, and in subducting slabs in convergent margins, as reported for aftershocks, swarms, slow earthquakes, and to a lesser extent for major earthquakes. Lastly, increase or decrease of pf in depth due to human activities such as hydrocarbon extraction, dam impoundment, gas storage or geothermal energy production result in many cases in the inception or enhancement of seismic activiy, adding clues in favor of a relationship between fluids and earthquakes. [ABSTRACT FROM AUTHOR]

Abstract Slow slip events (SSEs) lasting for approximately 1 year occur every 6–8 years around the Bungo Channel in the southwest Japan subduction zone. The slip time evolution of the latest Bungo Channel SSE that occurred in 2018–2019 has been studied; however, the detailed spatial and temporal relationship between the slip process and other nearby phenomena, such as tectonic tremors and SSEs, is not well understood. Moreover, the migration of such long-term SSEs from Hyuganada to Shikoku through the Bungo Channel has been suggested, but a slip process connecting the SSEs has not been observed. In this study, we utilized 21 continuous global navigation satellite system (GNSS) stations around the Bungo Channel and Hyuganada that have been installed by us since 2014 in addition to GNSS Earth Observation System (GEONET) stations to improve the spatial resolution of such interplate slip. Based on these data, we estimate the spatial and temporal slip evolutions of the major SSE in 2018–2019, which was accompanied by tremor activity in the deep episodic tremor and slip (ETS) zone, and a smaller SSE in 2015–2016 without tremor activity. We show that the slip area of the major SSE overlaps the ETS zone, whereas that of the smaller SSE does not. This strongly suggests that synchronized tremor activity with an SSE requires a slip close to or overlapping the ETS zone. We also show two distinct slip propagation paths from the Oita area during the 2018–2019 sequence: one is a southward propagation to the Miyazaki area, leading to an SSE around the Miyazaki Plain, and the other is an eastward propagation to an area close to Cape Ashizuri, where “invading slip” is proposed to propagate from the ETS zone to a shallower megathrust source area. These slip propagations may be two of fundamental slip modes that connect slow-slip patch-like areas around the Bungo Channel and Hyuganada. Graphic Abstract

Earthquake prediction remains a challenging and difficult task for scientists all over the world. The tidal triggering of earthquakes is being proven by an increasing number of investigations, most of which have shown that earthquakes are positively correlated with tides, and thus, tides provide a potential tool for earthquake prediction, especially for imminent earthquakes. In this study, publications concerning the tidal triggering of earthquakes were compiled and analyzed with regard to global earthquakes, which were classified into three main types: tectonic, volcanic, and slow earthquakes. The results reveal a high correlation between tectonic earthquakes and tides (mainly for semidiurnal and diurnal tides; 14-day tides) before and after the occurrence of significant earthquakes. For volcanic earthquakes, observations of volcanoes on the seafloor and land indicate that volcanic earthquakes in near-shore volcanic areas and mid-ocean ridges have a strong correlation with tidal forces, mostly those with semidiurnal and diurnal periods. For slow earthquakes, the periodicity of the tremor duration is highly correlated with semidiurnal and diurnal tides. In conclusion, the tidal triggering of these three types of earthquakes makes a positive contribution to earthquake preparation and understanding the triggering mechanism, and thus, the prediction of these types of earthquakes should be investigated. However, there are still several inadequacies on this topic that need to be resolved to gain a definitiveanswer regarding the tidal triggering of all earthquakes. The main inadequacies are discussed in this paper from our point of view.