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This study aimed to investigate the effect of soil–structure interaction (SSI) on the inelastic displacement ratio of bridge structures subjected to near-fault pulse-like earthquake ground motions. SSI was modeled using a discrete-time recursive filter approach and frequency-dependent foundation–soil impedance functions. A total of 105 near-fault earthquake ground motion records were used for the analysis. The study also compared the soil–foundation–superstructure (SFS) system with a fixed-base system to explore the effects of spectral period normalization, strength reduction factor, earthquake magnitude, site-to-source distance, and peak ground velocity on the inelastic displacement ratio. The findings indicate that both the SFS and fixed-base systems have significant inelastic displacement demand under near-fault earthquakes, with values that can be 2–3 times higher than those predicted by ASCE-41-13, particularly in the short period range. Furthermore, as the structural period approaches the earthquake pulse period, the SFS system generally shows larger inelastic displacement ratios compared to the fixed-base system. The analytical results provide insight into the factors affecting the inelastic displacement ratio for near-fault pulse-like earthquakes. [ABSTRACT FROM AUTHOR]

On 15 January 2023, a shallow, moderate earthquake with a magnitude (Mw) of 4.7 and a depth of one kilometer struck the northern part of Leyte Island in the central Philippines. Originating along the northern Leyte segment of the Philippine Fault, a well-established creeping fault, the earthquake caused significant geologic, structural, and socio-economic impacts despite its low magnitude. Probable surface rupture and landslides were reported, leading to a comprehensive field investigation. Our investigation revealed an ~8 km discontinuous surface rupture along the northern Leyte segment of the Philippine Fault, with a maximum left-lateral displacement of 2 cm. This was the first documented occurrence of such a phenomenon associated with an earthquake of a magnitude less than 6, particularly along a creeping fault segment. The maximum ground shaking felt was reported on the PHIVOLCS Earthquake Intensity Scale (PEIS) to be VI (very strong), equivalent to a Modified Mercalli Intensity (MMI) of VI along the fault strike. However, strong motion accelerographs recorded a peak ground acceleration (PGA) of 0.407 g, equivalent to PEIS VIII (very destructive), attributed to local site amplification influenced by subsurface geology. In the area where the local site amplification occurred, limited liquefaction was observed on marshlands with recent and alluvial deposits. Two landslides were observed in the mountainous area west of the fault. Structural damages were noted in areas with PEIS VI intensity and areas transected by the surface rupture. Despite the earthquake's low magnitude, the event documented significant impacts, including surface ruptures, liquefaction, landslides, and severe structural damage. The peculiarities of this event are attributed to the shallowness of the earthquake source, and local site conditions, including geology, geomorphology, and soil properties, contributed to the severity of the impacts. Moderate in size, this earthquake emphasizes the importance of documenting moderate-sized earthquakes as a tool and guide for medium- and long-term earthquake risk assessment and resiliency. [ABSTRACT FROM AUTHOR]

On September 17th and 18th, 2022, shallow earthquakes with magnitudes of 6.6 (ML) and 6.8 (ML) occurred in the eastern Taiwan Longitudinal Valley, which marks the collision zone between the Philippine Sea plate and the Eurasian plate, and led to noticeable surface deformation and ruptures within 70 km. This study primarily focuses on the southernmost section of the rupture zone—the Pingting Terrace. Surface rupture locations and behaviors correspond to changes in topography, providing mutual confirmation that the deformation behavior of Pingting Terrace is complex. Based on the distribution of surface ruptures and topography changes, this study roughly divides the Pingting Terrace into northern and southern segments, using the central concave feature as a boundary. The Riedel shear model analysis results show that the principal shear directions in the northern and southern segments are N–S trending and azimuth 20°, respectively. The maximum principal stress orientations are around 135° for the northern and 155° for the southern segments. These findings align with the fault mechanical investigation of the Lichi Mélange in the northern Muken River area of the Pingting Terrace. This suggests spatial changes in shear zone orientations within the Lichi Mélange, which contribute to developing pressure ridges due to transpressional forces. As a result, the Pingting Terrace experiences rapid uplift, causing the Luliao River to migrate southward into the Beinan River, while the eastern Beinan River turns to the eastward edge of the Pingting Terrace. Key points: Co-seismic surface rupture analysis using Riedel shear model. Flower structure of the Pingting Terrace. Relationship between the River channel change and the tectonic activity. [ABSTRACT FROM AUTHOR]

Displacement and seismic activities are critical elements in understanding the geodynamic behaviour of regions. Particularly in Banten, a region with significant irrigation infrastructure, these factors have implications for land use and infrastructure stability. From 2017–2020, Banten experienced varying displacement dynamics and seismic events, and DInSAR processing proved instrumental in achieving high-precision vertical displacement measurements. The challenge was to discern the pattern of these displacements in relation to significant earthquakes and the implications for specific high-density irrigation zones. Our study reveals a notable differentiation in displacement dynamics across different periods, especially in regions proximal to earthquake epicentres. For instance, the 2019 earthquake of magnitude 5 MW corresponded with uplift activities in certain irrigation areas, contrary to the subsidence observed in the preceding period. These findings emphasise the intricate relationship between seismic activities, vertical displacements, and their potential impact on key infrastructural elements in Banten. Moreover, understanding this interplay is not only crucial for regional planning but also offers insights into how such dynamics can influence areas with similar geophysical properties globally. Broadly, our results underscore the importance of consistent monitoring and analytical methodologies, like DInSAR, in regions susceptible to seismic shifts. Such knowledge can inform infrastructure development, resource allocation, and disaster preparedness strategies, ensuring the safety and sustainability of both natural and man-made environments. [ABSTRACT FROM AUTHOR]

Significant amounts of gases are emitted from the earth's crust into the atmosphere before, during, and after major earthquakes. To understand the relationship between gas emissions, earthquakes, and tectonics, we conducted a thorough investigation using satellite data from AQUA AIRS. We focused on three major earthquakes: the 12 May 2008 Wenchuan MW 7.9 earthquake in China's intra-continental plate, the 26 December 2004 Sumatra-Andaman MW 9.1 earthquake in Indonesia Island, and the 4 April 2010 Baja California MW 7.2 earthquake in Mexico's active plate margin. Anomalies in the total column (TotCO) and multiple layers (CO VMR) of carbon monoxide were observed along fault zones, with peak values at the epicenter areas. Furthermore, temporal anomalies of TotCO and CO VMR appeared in the month of the Wenchuan earthquake in the intra-continent, three months prior to the Sumatra-Andaman earthquake and one month before the Baja California earthquake in the active plate margins, respectively. Notably, the duration of CO anomalies before earthquakes in active plate margins was longer than that in the intra-continental region, and the intensity of the CO anomaly in active plate margins was higher than that in the intra-continental region. The results show a profound correlation with both seismic and tectonic activities, which was particularly evident in the earthquake's magnitude, rupture length, and the tectonic settings surrounding the epicenter. Furthermore, the type of the fault at which the earthquake occurred also played an important role in these CO anomaly variations. These findings support the identification of earthquake precursors and may help improve our understanding of earthquake forecasting and tectonics. [ABSTRACT FROM AUTHOR]

We conducted a study on the Aceh Fault, an active right-lateral strike-slip fault in northern Sumatra, Indonesia. Despite its seismic hazard potential, the slip characteristics of this fault are not well-understood. Using a combination of remote sensing and field methods, we mapped the distribution of displacement recorded by offset channels along the fault. Our goal is to identify evidence of past surface rupture, characterize their tectonic geomorphology setting, and assess their potentials as slip-rate and paleoseismic sites. The documented right-lateral displacements of channels and ridges from 35 observation points are mostly in the order of magnitude of hundreds of meters, with only a few sites exhibiting meter-scale measurable offsets. Our results provide important first-order data that can be used to locate sites suitable for paleoseismic trenching or offset investigations. This information will support the development of a maximum magnitude and earthquake recurrence model of the fault, which are crucial for seismic hazard analysis in the region. [ABSTRACT FROM AUTHOR]

On February 6, 2023, a magnitude 7.8 earthquake and subsequent strong seismic activity struck in Kahramanmaras region, causing over 50,000 deaths in Turkey and Syria. The earthquake resulted in a surface rupture of the East Anatolian Fault Zone (EAFZ) spanning approximately 300 km. To gain new insights into the neotectonic and current stress field in the region, we compiled 141 focal mechanism solutions (FMS) of earthquakes (with magnitude M ≥ 3) that occurred along the EAFZ between 05/01/2003 and 27/02/2023. Stress inversion, by using the Win-Tensor program, indicated a predominant strike-slip tectonic regime, with few normal and thrust events related to complex fault geometry along the principal displacement zone. The calculated pressure/tension axes in the first order stress field are mainly sub-horizontal, with a maximum horizontal compressive stress (SHmax) direction of N19 ± 9.9° E, which aligns well with the slip character of the EAFZ. To reconstruct the second and third order stress fields, the study area was subdivided into five zones based on their structures and geomorphological characteristics. Reduced stress tensors were obtained for each zone, indicating a small rotation of SHmax directions under a prevailing strike-slip faulting regime. The maximum (σ1) and minimum (σ3) stress axes are nearly horizontal; while the intermediate (σ2) stress axis is nearly vertical, consistent with a predominant strike-slip regime. The results show, also, that the Kahramanmaras earthquake was caused by the neotectonic reactivation of northeast-striking sinistral strike-slip fault, with a north–south-oriented maximum horizontal stress axis. The neotectonic activity along the EAFZ aligns well with the collision models between the Arabian and Eurasian plates. Finally, a detailed seismic hazard assessment is required for the EAFZ and nearby regions. [ABSTRACT FROM AUTHOR]

Using different methods for processing SAR images from the Sentinel-1A satellite, the displacement fields were determined in the region of the East Anatolian Fault Zone (EAFZ) and the Sürgü-Çardak faults, as well as a small fault on the continuation of the East Hatay fault zone, which rupture initiated a series of catastrophic earthquakes in Turkey on February 6, 2023. DInSAR and offset methods were applied. The most detailed data on the displacements were obtained by the offset method using images from the descending orbit. When constructing the model from the available SAR data, the data with the maximum signal-to-noise ratio were selected. For the northern part of the region, above 37.4° N, the range displacements obtained by the offset method from a descending orbit were used. South of parallel 37.0° N, we used azimuth displacements from the same descending orbit. The model of the seismic rupture was constructed on the basis of solution of (Pollitz, 1996) of the problem of deformations at the surface of a layered spherical Earth caused by along dip and strike displacements on a rectangular fault located inside the planet. Pollitz (1996) demonstrated that ignoring the radial layering of the planet leads to errors up to 20%, with the largest errors occurring in the presence of a large strike-slip component. Ignoring sphericity also introduces an error when using the solution in the framework of the idealization of an elastic homogeneous half-space with a flat free surface (Okada, 1985) which was used when constructing USGS and (Barbot et al., 2023) models. In our model the surfaces of seismic rupture are approximated by 19 rectangular elements along the strike, divided into three levels along the dip. Another element approximated a rupture along the extension of the East Hatay Fault Zone. As in the models of other authors (USGS; Barbot et al., 2023), in our model in the southern part of the EAFZ, the displacements increase from south to north, and are mainly concentrated in the upper part of the Earth's crust to a depth of 10 km. At the southern end of this rupture, displacements in our model with an amplitude of up to 2 m are obtained at the lower levels of the model, and at its upper level, the displacements were only 0.11 m, and in this area on February 20, 2023 an earthquake of magnitude 6.3 occurred with a hypocenter depth of 11.5 km. The main displacements on the EAFZ are determined on its central segment. Here, the displacements go to a greater depth, their value reaches 10.2 m. On the Sürgü-Çardak fault, significant displacements occurred down to a depth of 20 km; displacements exceeded 10.2 m. In our model, at the northeast end of the seismic rupture along the EAFZ, a displacement area of 6.8 m overlaps with the southwest end of the seismic rupture model of the Doğanyol-Sivrice earthquake of January 24, 2020 with Mw 6.7, published on the USGS website. Therefore, our model does not confirm the hypothesis of the presence of a seismic gap here, which, according to (Barbot et al., 2023), is a zone of a possible nearest earthquake. [ABSTRACT FROM AUTHOR]

Joshimath has received much attention for its massive ground subsidence at the beginning of the year. Rapid urbanization and its unique geographical location may have been one of the factors contributing to the occurrence of this geological disaster. In high mountain valley areas, the complex occurrence mechanism and diverse disaster patterns of geological hazards highlight the inadequacy of manual monitoring. To address this problem, the inversion of deformation of the Joshimath surface in multiple directions can be achieved by multidimensional InSAR techniques. Therefore, in this paper, the multidimensional SBAS-InSAR technique was used to process the lift-track Sentinel-1 data from 2020 to 2023 to obtain the two-dimensional vertical and horizontal deformation rates and time series characteristics of the Joshimath ground surface. To discover the causes of deformation and its correlation with anthropogenic activities and natural disasters by analyzing the spatial and temporal evolution of surface deformation. The results show that the area with the largest cumulative deformation is located in the northeastern part of the town, with a maximum cumulative subsidence of 271.2 mm and a cumulative horizontal movement of 336.5 mm. The spatial distribution of surface deformation is based on the lower part of the hill and develops towards the upper part of the hill, showing a trend of expansion from the bottom to the top. The temporal evolution is divided into two phases: gentle to rapid, and it is tentatively concluded that the decisive factor that caused the significant change in the rate of surface deformation and the early onset of the geological subsidence hazard was triggered by the 4.7 magnitude earthquake that struck near the town on 11 September 2021. [ABSTRACT FROM AUTHOR]

Based on the results of the seismic hazard survey in Yunnan Province, four exceedance probabilities of peak ground acceleration were converted into earthquake magnitudes using the empirical relationship between epicentral intensity and magnitude. By using logistic regression, earthquake magnitude, source depth, cover layer thickness, and fault type were used as independent variables to calculate the probability of co-seismic displacement for different earthquake magnitudes. Using the magnitude-displacement empirical model in the southwestern region of China, horizontal and vertical displacements were obtained with a 95 % confidence level. The results show that all Holocene faults have the ability to produce surface displacement, but from the perspective of probabilistic hazard, the probability of surface rupture in the short term is low, while the probability of surface rupture for each fault significantly increases over a long time scale. Buildings with different service lives should adopt different avoidance strategies, and buildings with shorter service lives (<50a) can consider not avoiding active faults. [ABSTRACT FROM AUTHOR]

An active fault stretching for more than 100 km along the Kura River between the tailing part of the Mingeçevir Reservoir in the east and the Azerbaijan-Georgia border in the west - the socalled Kura fault - is described in brief based on the analysis of space images. This active reverse fault with distinct evidence of Late Quaternary movements crosses the waterfronts of the Shamkir and the Enikend reservoirs. Considering its expressiveness and location just at the foundations of the hydraulic structures it seems that hazard posed by this active fault could be underestimated and that more efforts should be undertaken to reveal its recent history. [ABSTRACT FROM AUTHOR]

The East Kunlun fault zone (EKFZ), located northeast of the Qinghai–Tibet Plateau, has experienced several strong earthquakes of magnitude seven or above since 1900. It is one of the most active fault systems and is characterized by left-lateral strike-slip. However, the Xidatan–Dongdatan segment (XDS) of the East Kunlun fault zone (EKFZ) has had no earthquakes for many years, and the Kunlun Mountains MS 8.1 earthquake has a stress loading effect on this segment, so it is widely regarded as a high-risk earthquake gap. To this end, we collected the Sentinel-1 data of the XDS of the EKFZ from July 2014 to July 2019 and obtained the high-precision interseismic deformation field by the Interferometric Synthetic Aperture Radar (InSAR) technique to obtain the slip rate and locking depth of the XDS of the EKFZ, and the seismic potential of the segment was analyzed. The results are as follows: (1) The LOS deformation field of the XDS of the EKFZ was obtained using Sentinel-1 data of ascending and descending orbits, which indicated that the XDS of the EKFZ is dominated by horizontal motion. Combined with the interference results, it is shown that the strike-slip rate dominates the deformation information of the XDS of the EKFZ. The deep strike-slip rate of the fault is about 6 mm/yr, the deep dip-slip rate is about 2 mm/yr, and the slip-deficit rate on the fault surface is about 6 mm/yr; (2) Combined with the spiral dislocation theory model, the slip rate of the XDS to Xiugou Basin of the EKFZ has a gradually increasing trend, with an average slip rate of 9.6 ± 2.3 mm/yr and a locking depth of 29 ± 5 m; (3) The stress accumulation is about 483 ± 92 years in the XDS of the EKFZ, indicating that the cumulative elastic strain energy of the XDS can produce an MW 7.29 ± 0.1 earthquake in the future. [ABSTRACT FROM AUTHOR]

Liquefaction occurs when the ground experiences intense shaking due to seismic activities, causing water-saturated soils or loosely packed sediments near the surface to weaken and behave like a liquid. The present paper has reviewed and investigated the liquefaction around tunnels due to any seismic event that may occur during the tunnel's lifetime. To effectively prevent soil liquefaction around the tunnel, it is crucial to analyze both the acceleration response of the tunnel and the soil motion that results from liquefaction. This paper examines the geotechnical engineering aspects of such liquefactions and discusses analytic techniques to mitigate their effects. Also, the response of tunnel displacement under different magnitudes of earthquakes has been discussed. Different mitigation methods are explored in response to the damages caused by the liquefaction of soil due to earthquakes. Case studies of this problem that occurred in different nations were analyzed as well, amidst which are tunnel disasters in Turkey, Syria, China, Italy, Japan, India, and the USA. These instances demonstrate the terrible repercussions of liquefaction, such as building collapse, road network destruction, and surface and subsurface structure deterioration. By reviewing these instances, major patterns and trends in liquefaction-induced damages emerge, giving valuable insights for future mitigation and engineering practices. This review serves as a valuable resource for engineers and researchers involved in tunnel design, construction, and maintenance. This review paper addresses the dangers associated with liquefaction in soils and highlights the need for robust design considerations and appropriate construction techniques to mitigate potential damages. [ABSTRACT FROM AUTHOR]

Stress accumulation and release reflected by acoustic emissions (AEs) during shearing of granular materials provide important information on failure mechanisms in seismic faults and landslides controlled by stick‐slip. Among many characteristics (amplitude, energy, counts, and frequency) of AE signals generated by stick‐slip, stress changes corresponding to various frequency AEs in different stages of the stick‐slip process are not clear, which limits our knowledge of the characteristics of precursory signals before stick‐slip failure. To better understand the physical mechanisms of granular stick‐slip, we monitored the mechanical and AE signals using high‐frequency (2 MHz) synchronous acquisition during constant‐speed shear of packs of uniform glass beads with different sizes at different normal stresses. The release rate of AE energy was found to accelerate with the dilatation of the sample volume, and the stress drop of stick‐slip was augmented with the increase of normal stress and particle size. Three characteristic events of single cycle stick‐slip were observed in this study: main slip, minor slip, and microslip. We analyzed the AE frequency spectra of these three event types. Both main slip and minor slip corresponded to stress drop and generated high‐frequency AEs (about several hundred kHz), while the AE frequencies generated by microslip were lower (about tens of kHz) and exhibited stress strengthening, which were not apparent in previous studies due to the low frequency of acquisition. We propose that the microslip is mainly due to sliding on grain contacts, while the main slip and minor slip resulted from breakage and reforming of force chains. Low‐frequency AEs from microslip may suggest a crucial precursor of seismic faults and landslides. Plain Language Summary: The occurrence of earthquakes of similar magnitude (quasi‐periodic earthquakes) in a fault zone every few decades is thought to be controlled by stick‐slip of fault gouges. The mechanical evolution of fault gouges prior to earthquake occurrence is extremely complex and difficult to predict. However, many studies have observed the presence of significant microslip prior to stick‐slip failure of seismic faults, which may provide precursor information for an impending earthquake. Laboratory study of stick‐slip nucleation and failure mechanisms in granular materials provides important understanding of the physical mechanisms of quasi‐periodic earthquakes. Our experimental results of high‐frequency acoustic emission (AE) and ring‐shear simultaneous acquisition showed that stick‐slip failure (corresponding to the occurrence of an earthquake) leads to a decrease in the shear resistance of a granular system, generating high‐frequency acoustic signals. Increments of microslip increased the shear resistance of the granular system, and were recorded as low‐frequency acoustic signals before failure occurred. We suggest that the stick‐slip nucleation and failure were controlled by grain friction and collision, respectively. The different AE frequencies of the two may shed light on the precursor monitoring of quasi‐periodic earthquakes. Key Points: Stick‐slip failure was associated with high‐frequency (∼105 Hz) acoustic emissions (AEs) and micro‐slip during stick‐slip nucleation was associated with low‐frequency (∼104 Hz) AEsHigh‐frequency AEs may be caused by particle collisions after force‐chain breakageLow‐frequency AEs may be caused by particle friction during force‐chain adjustment and may suggest important precursors of laboratory earthquakes [ABSTRACT FROM AUTHOR]

Ocean transform faults often generate characteristic earthquakes that repeatedly rupture the same fault patches. The westernmost Gofar transform fault quasi‐periodically hosts ∼M6 earthquakes every ∼5 years, and microseismicity suggests that the fault is segmented into five distinct zones, including a rupture barrier zone that may have modulated the rupture of adjacent M6 earthquakes. However, the relationship between the systematic slip behavior of the Gofar fault and the fault material properties is still poorly known. Specifically, the role of pore fluids in regulating the slip of the Gofar fault is unclear. Here, we use differential travel times between nearby earthquakes to estimate the in‐situ Vp/Vs of the fault‐zone materials. We apply this technique to the dataset collected by an ocean‐bottom‐seismometer network deployed around the Gofar fault in 2008, which recorded abundant microearthquakes, and find a moderate Vp/Vs of 1.75–1.80 in the rupture barrier zone and a low Vp/Vs of 1.61–1.69 in the down‐dip edge of the 2008 M6 rupture zone. This lateral variation in Vp/Vs may be caused by both pore fluids and chemical alteration. We also find a 5%–10% increase in Vp/Vs in the barrier zone during the 9 months before the mainshock. This increase may have been caused by fluid migrations or slip transients in the barrier zone. Plain Language Summary: Oceanic transform faults are ideal natural laboratories for studying earthquake physics due to their regular behaviors. The westernmost Gofar transform fault near the East Pacific Rise generates magnitude six earthquakes every 5–6 years on two rupture zones, respectively, which are separated by a barrier zone repeatedly stopping ruptures on the adjacent segments. The distinct behavior of the barrier zone might be due to differences in material properties from the rupture zones. To explore this hypothesis, we analyze thousands of small earthquakes in 2008 and find that the barrier zone has a higher ratio between P and S velocities than the rupture zone. This difference indicates that the barrier and rupture zones differ in their fluid content and chemical composition, which may have regulated their distinct slip behaviors. We also find an increase in the ratio between P and S velocities in the barrier zone during the 9 months before the magnitude six earthquake in 2008, which may reflect fluid flows or aseismic fault slips. Our findings suggest that pore fluids and the chemical composition of fault‐zone materials likely play a crucial role in regulating the slip behaviors of oceanic transform faults. Key Points: Rupture barrier zone has a moderate Vp/Vs of 1.75–1.80Down‐dip edge of the 2008 M6 mainshock has a low Vp/Vs of 1.61–1.69Vp/Vs in the rupture barrier zone increased in the 9 months before the mainshock [ABSTRACT FROM AUTHOR]

On 7 January 2022, a Mw 6.6 earthquake struck Menyuan County in the Qinghai province of China and the earthquake caused severe damage to infrastructures. In this study, the performance of the high-rate global navigation satellite system (GNSS) on real-time source modeling of the 2022 Mw 6.6 Menyuan earthquake was validated. We conducted the warning magnitude calculation, centroid moment tensor (CMT) inversion, and static fault slip distribution inversion using displacements collected from 14 1-Hz GNSS stations. Our results indicate that the warning magnitude derived from the peak ground displacement (PGD) first exceeds Mw 6.0 approximately 9 s after the earthquake and tends to be stable after about 45 s. The derived finally stable magnitude is Mw 6.5, which is near the USGS magnitude of Mw 6.6. Based on the inverted CMT and static fault slip distribution results, it can be determined that the 2022 Menyuan earthquake is a left-lateral strike-slip event after about 20 s of the earthquake. Although the fault slips, inverted with the 30-s smoothed coseismic offsets, are unstable after about 40 s, all the inverted slip models after that time present the obvious surface rupture and the most fault motions are concentrated between the depth of 0 km and 8 km. Compared with the results inverted with the 30-s smoothed coseismic offsets, the CMT and fault slips inverted with the 70-s smoothed coseismic offsets are more stable. The results obtained in this study indicate that the high-rate GNSS has the potential to be used for real-time source modeling for earthquakes with a magnitude less than 7; the stability of the inverted CMT and fault slips can be improved by using the coseismic offsets averaged by a relatively long-time sliding window. [ABSTRACT FROM AUTHOR]

Earthquake surface deformation provides key constraints on the geometry, kinematics, and displacements of fault rupture. However, deriving these characteristics from past earthquakes is complicated by insufficient knowledge of the pre‐event landscape and its post‐event modification. The 1987 Mw 6.5 Edgecumbe earthquake in the northern Taupō volcanic zone (TVZ) in New Zealand represents a moderate‐magnitude earthquake with distributed surface rupture that occurred before widespread high‐resolution topographic data were available. We use historical aerial photos to build pre‐ and post‐earthquake digital surface models (DSMs) using structure‐from‐motion techniques. We measured discrete and distributed deformation from differenced DSMs and compared the effectiveness of the technique to traditional field‐ and lidar‐based studies. We identified most fault traces recognized by 1987 field mapping, mapped newly identified traces, and made dense remote slip measurements with a vertical separation resolution of ∼0.3 m. Our maximum and average vertical separation measurements on the Edgecumbe fault trace (2.5 ± 0.3 and 1.2 m, respectively), are similar to field‐based values of 2.4 and 1.1 m, respectively. Importantly, this technique can discern between new and pre‐existing fault scarps better than field techniques or post‐earthquake lidar‐based measurements alone. Our surface displacement results are used to refine subsurface fault geometries and slip distributions at depth, which are further used to investigate potential magmatic‐tectonic stress interactions in the northern TVZ. Our results suggest the Edgecumbe fault dips more gently at depth than at the surface, hosted shallow slip in 1987, and may be advanced toward failure by interactions with nearby magma bodies. Plain Language Summary: Understanding earthquake behavior relies heavily on information about how past earthquakes changed the landscape. Detailed information about the surface topography before the earthquake is often limited, creating challenges for accurately measuring earthquake surface deformation. Aerial photos are widely available and can be used to create 3D digital surface models in places where other topographic information is lacking. We use historical aerial photos to make 3D datasets of the surface topography before and after the 1987 Edgecumbe earthquake in New Zealand. We calculated the difference between the two 3D datasets to identify and measure how the earthquake changed the landscape, and compared our results to previous measurements. We found that this method generally works as well as field methods for identifying and measuring fault movement, and has some advantages over other techniques. In particular, this approach can separate deformation from individual earthquakes, which had previously been a challenge. The results refine our understanding of the faults below the surface, provide insight into the earthquake's relationship to surrounding faults and volcanic systems, and allow us to better characterize seismic hazard both here and in other similar geologic settings. Key Points: Historical aerial photos can be used to generate pre‐ and post‐earthquake digital surface models for measuring displacementThis method performs at least as well as field surveys, and better captures single‐event displacements to constrain earthquake behaviorThe Edgecumbe earthquake is important for understanding low dip‐angle normal fault ruptures and possibly magma‐tectonic interactions [ABSTRACT FROM AUTHOR]

We present a two‐dimensional analytical solution for the static stress field around a rough interlocked interface under background stresses and its applications to faulting. The solution is derived using perturbation theory for the defined interface topography and background stresses. The failure‐ratio, which scales the likelihood to failure of the bulk material around the interface, is calculated adopting the Coulomb failure criterion. We compare model predictions with results of experiments performed on rough rock interfaces and find good agreement between the locations of off‐fault deformation zones and calculated high failure‐ratio values. We further study the effect of fault topography on stress distribution and failure around the San Andreas Fault and find a moderate correlation between Failure‐Ratio values and off‐fault seismic events. We conclude that our model is able to delineate off‐fault regions that are relatively prone to bulk failure due to local effects of fault topography. Plain Language Summary: An earthquake is triggered when the stress build‐up by tectonic motion exceeds the strength of the fault material. The factors that control the earthquake magnitude, location, and time are still largely unknown. The significance of fault roughness to earthquake physics has been recognized for many years; geometrical irregularities across fault zones drive stress heterogeneities that strongly affect the onset of seismic rapture. Different research groups are now attempting to quantify the effect of roughness on faulting, either experientially or theoretically. Particularly, it has long been attempted to develop an analytical solution for the local stress field and yield potential across interlocked interfaces as a proxy to natural faulting, albeit without much success. Here, we present an analytical model that solves the stress field around an interlocked fault segment for a given interface roughness and background stress field, and show how surface geometry affects rock failure around the interface. We argue that any dynamic sliding between surfaces is initiated by fracture of the intact rock material at immediate proximity to the interface. We use the stresses obtained by our analytical solution to assess the failure potential around seismically active faults, a novel contribution with specific implications to seismic risk assessment. Key Points: Analytical solution demonstrates how surface geometry and background stress affect the local stress field around rough interlocked faultsThe stress undulations induced by the rough interface and the background stress field, decay exponentially away from the faultThe likelihood for off‐fault failure is assessed using the Coulomb failure criterion and is demonstrated on laboratory and natural faults [ABSTRACT FROM AUTHOR]

A system of 22 km long surface rupture produced by paleoearthquakes has been mapped for the first time along the Kubadru Fault which delineates the Kokorya Basin in the north. The ruptures morphology record reverse and right-lateral strike-slip geometry of the Kubadru Fault. The ruptures pattern represent a combined effect of four paleoearthquakes, including three events in the past 1.4 kyr. The magnitudes of the earthquakes could range from 6.7 to 7.6. The revealed structure of the Kubadru Fault Zone consists of counter-dipping reverse faults. Motions on two reverse fault systems maintained thrusting of the Kurai Range on the basin sediments and caused the growth of a foreberg between the basin and the range. [ABSTRACT FROM AUTHOR]

The Sertengshan Range-front Fault (SRF) and Langshan Range-front Fault (LRF) constitute a normal fault system on the northern boundary of the Linhe Depression in its northwest, and an investigation of the LRF–SRF is significant for understanding the seismic activities of normal faults. We set four trenches at three study sites to reveal paleo-earthquakes in the western segment of the SRF (W-SRF) in this system, and established 5 surface rupture events along W-SRF since the Late Pleistocene. By integrating paleoseismic data from 27 trenches on multiple fault segments, we reconstructed the paleoseismic sequence for the LRF–SRF region since 15 ka, and identified 10 paleoseismic events with corresponding rupture segments and magnitudes. The slip rates at the three study sites decreased gradually from the west to east on the W-SRF, by comparing with previous studies, further evidencing that the tectonic activities of the two faults have gradually synchronized. Besides, based on the timing of the latest paleoseismic event (1.88−2.03 ka BP), earthquakes of > M 8 surface ruptures of >200 km can occur in the northwestern Ordos Block and its seismic behavior revealed the seismogenic possibility of normal faults triggering an M > 8 earthquake. • Identified at least 5 surface rupture events since the late Pleistocene on the W-SRF. • Reconstructed paleoseismic sequence of the SRF-LRF by integrating data of trenches. • The vertical slip rates decrease gradually from west to east on the W-SRF. • Confirmed the normal fault system can occur > M8 earthquake and cause surface ruptures >250 km. [ABSTRACT FROM AUTHOR]

The Nanbei River–Boli Fault is one of the largest NW-trending faults in Northeast China. However, limited data on the style and timing of recent earthquake activity on this fault greatly restrict the accuracy of seismic risk assessment. Here, we provide evidence of late Pleistocene to Holocene activity of the eastern segment of this fault for the first time. Specifically, the eastern segment of the Nanbei River–Boli Fault has experienced multiple earthquakes since the late Quaternary. Trench excavations indicate a recent strong earthquake at 7.28–2.11 ka, with approximately 1 m of co-seismic vertical displacement and preliminary estimates of the minimum vertical slip rates of 0.07–0.12 mm/a. According to a comprehensive analysis, we estimate that the eastern segment of the Nanbei River–Boli Fault can generate strong earthquakes of magnitude 7.0. This research provides important information for seismic risk assessments and seismic zoning, which can help seismic hazard assessment and mitigate their effects in Northeast China. • We analyze the Nanbei River–Boli Fault, southern margin of Boli Basin, Northeast China. • Radiocarbon and OSL dating provide the timing of recent strong earthquakes. • We estimate the maximum earthquake magnitude and minimum vertical slip rate. • The fault has experienced multiple surface rupture events in the late Quaternary. • This is the first evidence of late Pleistocene to Holocene activity along the fault. [ABSTRACT FROM AUTHOR]

The field monitoring data showed that the fracture of high-position thick and hard roof (THR) was the main reason for the frequent occurrence of high-energy mine earthquake events (magnitude > 2.0) in the no. 6 mining district of Dongtan coal mine. In order to study the fracture characteristics of high-level THR and seek a reasonable and effective control method, taking the 63up06 working face as the engineering background, the FLAC model was built to simulate the strata movement law before and after fracturing. The simulation results showed that the THR first breaking span was 340 m, the tensile failure runs through the whole THR, and the subsidence increased rapidly when THR reached the first breaking span. The THR breakage is related to the continuous upward transfer of horizontal compressive stress, which results in the concentration of horizontal compressive stress in THR. After ground fracturing, the first breaking span of THR decreased, and different fracturing positions have a great impact on the THR fracture form. When the fracturing position is close to the center of the first breaking span, there is no large cantilever breaking form in the THR, the THR breaks on both sides of the top and sinks along the hydraulic fracture in the middle, and the collapse degree is relatively uniform. Then, the fracturing test was carried out in the 63up06 working face, the results showed that during the first 400 m of the 63up06 working face mining after fracturing, 702 microseismic events occurred in total, including 12 high-energy vibration events which include 4 mine earthquakes of magnitude 2.0 and 8 mine earthquakes of magnitude 1.0-2.0. Compared with the adjacent working face, the quantity of high-energy mine earthquake events in the 63up06 working face has been significantly controlled. [ABSTRACT FROM AUTHOR]

A large-scale sand liquefaction producing typical and novel surface phenomena was found at the epicenter of Songyuan M5.7 earthquake occurring on May 28, 2018. Field survey and experimental test encompassing boring sampling, standard penetration test (SPT), cone penetration test (CPT), scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray fluorescence (XRF) were performed to ascertain the liquefaction damage and site characteristic. Cone penetration test is an excellent assay for the identification of liquefied sand layer and acquisition of physio-mechanical parameter. Moreover, the assay is applicable for in-situ post-earthquake investigation. Factors promoting the formation and controlling the distribution of the sand liquefaction were analyzed. The liquefaction impacted an 80 km2 area, and was primarily embodied as sand boil and water sprout on rice field, despite producing no significant structural damage. Due to the simple profile of local soil layer, ground motion, geomorphic condition, and groundwater level were the main factors governing the distribution of the liquefaction. Majority of the liquefied sand layer was discovered at the depth less than 10 m. However, deep layer liquefaction at the depth greater than 18 m was also discovered, which was demonstrated by the upward movement of liquefied sand towards the upper silty clay layer at the depth of 17 m. Most importantly, we have identified loess liquefaction, a phenomenon which had not been reported previously in Northeast China. Lastly, it is important to highlight the risk of significant liquefaction damage at Songyuan. Hence, investigating the liquefaction risk is potentially beneficial for augmenting planning on earthquake mitigation, engineering reconnaissance, and design project. [ABSTRACT FROM AUTHOR]

Rupture size is a fundamental earthquake source parameter that is challenging to infer independently from far‐field seismological observations. Here, we develop a novel observational constraint on source size based on the decay rate of wavefield coherence across a seismic array. For a given earthquake, waveform coherence decays with increasing interstation distance or, more precisely, with increasing projection difference defined as the difference between the takeoff vectors associated to the two stations projected along the rupture direction. We find that coherence generally falls off with projection difference faster for earthquakes of larger magnitudes. The magnitude dependence of the coherence decay rate can be explained by a finite source effect: larger source sizes cause larger differences of phase delays between waves arriving from different parts of the rupture at different stations, hence a stronger spatial variability of the wavefield, resulting in a breakdown of waveform coherence. Assuming a 1‐D Haskell's source model, the rupture size can be estimated from the coherence decay rate. We apply this method to USArray data of earthquakes in three subduction zones, the Sea of Okhotsk, South America and Japan. The source sizes inferred from the coherence decay patterns are consistent with scaling relations intermediate between width‐saturated L‐models and quasi equi‐dimensional rupture models. Our observation captures a unique pattern of array waveform coherence and demonstrates the potential of utilizing waveform coherence to study earthquake source parameters. Plain Language Summary: In this paper, we observe that for a given earthquake, the similarity of the waveform (coherence) recorded by stations in a far‐field array decays with increasing interstation distance. The waveform coherence generally falls off with interstation distance faster for earthquakes of larger magnitudes and therefore, larger rupture sizes. We develop a novel method using array waveform coherence to estimate the earthquake rupture size, and apply this method to USArray data of earthquakes in three geological regions: the Sea of Okhotsk, South America and Japan. The source sizes inferred from the coherence decay patterns are consistent with previous studies using different methods. According to our observation, there is a potential to infer earthquake rupture independently from far‐field observations by waveform coherence. Key Points: Waveform coherence decays as a function of interstation distance faster for large earthquakes (Mw > 7)The waveform coherence pattern is governed by earthquake source sizeRupture sizes inferred from waveform coherence satisfy empirical scaling relations [ABSTRACT FROM AUTHOR]

On May 5, 2014, an earthquake with a magnitude of Mw 6.1 (the largest earthquake in Thailand so far) occurred in Chiang Rai of the Golden Triangle area in northern Thailand. We had an opportunity to conduct field survey immediately after the earthquake. Serious damage to buildings and casualties of lives were observed, and the estimated Maximum Mercalli Intensity (MMI) of the earthquake is VIII (evaluated according to the MMI scale of the Chinese Standard). No long continuous ground ruptures were produced during the earthquake, but in the epicenter (commonly within MMI VIII extent), massive small linear ruptures (usually several tens of meters long) developed and displayed intriguing structural features, offsetting many roads several centimeters left laterally on NE trending cracks or offsetting right laterally on NW trending ones. The focal mechanism solution of earthquake shows that this is a pure strike‐slip event, and two nodal planes in NW and NE directions had the same motion senses respectively as those of breakage associated with the earthquake. The long axis of the isoseismals and aftershock distributions are in NE direction, which is consistent with the strike of Luang Namtha fault. The 230‐km‐long Luang Namtha fault which starts from the border of China and Laos, runs through northern Laos, and terminates at Chiang Rai of Thailand is predominated by left‐lateral strike‐slip and active in late Quaternary, and two earthquakes over Ms 6.0 occurred along the fault in 1925 and 2007 respectively. This Mw 6.1 earthquake occurred at the southwestern end of the fault. All related features such as evident structural rupturing, elongated orientation of MMI and aftershock distribution, as well as the location of the epicenter, suggest that the Luang Namtha fault may be responsible for the 2014 Northern Thailand earthquake. [ABSTRACT FROM AUTHOR]

To understand the seismotectonics and the seismic hazard of the study sector of the Northern Apennines (Italy), one of the most important earthquakes of magnitude M w = 6.5 which struck the Lunigiana and Garfagnana areas (Tuscany) on 7 September 1920 should be studied. Given the early instrumental epoch of the event, neither geometric and kinematic information on the fault-source nor its fault-plane solution were available. Both areas were candidates for hosting the source fault and there was uncertainty between a normal fault with Apenninic direction or an anti-Apenninic strike-slip. We retrieved 11 focal parameters (including the fault-plane solution) of the 1920 earthquake. Only macroseismic intensity information (from 499 inhabited centres) through the KF-NGA inversion technique was used. This technique uses a Kinematic model of the earthquake source and speeds up the calculation by a Genetic Algorithm with Niching. The result is a pure dip-slip focal solution. The intrinsic ambiguities of the KF-NGA method (±180° on the rake angle; choice of the fault plane between the two nodal planes) were solved with field and seismotectonic evidence. The earthquake was generated by a normal fault (rake angle = 265° ± 8°) with an Apennine direction (114° ± 5°) and dipping 38° ± 6° towards SW. The likely candidate for hosting the source-fault in 1920 is the Compione-Comano fault that borders the NE edge of the Lunigiana graben. The KF-NGA algorithm proved to be invaluable for studying the kinematics of early instrumental earthquakes and allowed us to uniquely individuate, for the first time ever, the seismogenic source of the 1920 earthquake. Our findings have implications in hazard computation and seismotectonic contexts. [ABSTRACT FROM AUTHOR]

Large magnitude earthquakes produce complex surface deformations, which are typically mapped by field geologists within the months following the mainshock. We present detailed maps of the surface deformation pattern produced by the M. Vettore Fault System during the October 2016 earthquakes in central Italy, derived from ALOS-2 SAR data, via DInSAR technique. On these maps, we trace a set of cross-sections to analyse the coseismic vertical displacement, essential to identify both surface fault ruptures and off-fault deformations. At a local scale, we identify a large number of surface ruptures, in agreement with those observed in the field. At a larger scale, the inferred coseismic deformation shows a typical long-wavelength convex curvature of the subsiding block, not directly recognizable in the field. The detection of deformation patterns from DInSAR technique can furnish important constraints on the activated fault segments, their spatial distribution and interaction soon after the seismic events. Thanks to the large availability of satellite SAR acquisitions, the proposed methodological approach can be potentially applied to worldwide earthquakes (according to the environmental characteristics of the sensed scene) to provide a wider and faster picture of surface ruptures. Thus, the derived information can be crucial for emergency management by civil protection and helpful to drive and support the geological field surveys during an ongoing seismic crisis. [ABSTRACT FROM AUTHOR]

The Dinaric Fault System in western Slovenia, consisting of NW–SE-trending, right-lateral strike-slip faults, accommodates the northward motion of Adria with respect to Eurasia. These active faults show a clear imprint in the morphology, and some of them hosted moderate instrumental earthquakes. However, it is largely unknown if the faults also had strong earthquakes in the late Quaternary. This hampers our understanding of the regional tectonics and the seismic hazard. Geological evidence of co-seismic surface ruptures only exists for one historical event, the 1511 Idrija earthquake with a magnitude of ∼ M 6.8, but the causative fault is still disputed. Here we use geomorphological data, near-surface geophysical surveys, and paleoseismological trenching to study two of these faults: the Predjama Fault and the Idrija Fault. In a paleoseismological trench across the Predjama Fault we found deformation features that may have been caused by an earthquake between 13–0.7 ka, very likely not earlier than 8.4 ka. At the Idrija Fault, a surface-rupturing earthquake happened around 2.5 ka. We show that instrumental and historical seismicity data do not capture the strongest events in this area. [ABSTRACT FROM AUTHOR]

The intensity of paleoearthquakes in the territory under study has been estimated by two methods: from the maximum displacement of Quaternary rocks induced by ruptures and from the parameters of clastic dikes of seismic genesis. Based on the use of archival materials from space surveys and Google Earth images, a method for searching for paleoseismic dislocations is substantiated. Searches for seismic manifestations within the Yukki—Toksovo fault zone have been carried out and forty seismogenic scarps have been discovered, some of which are opened by ditches. The study of the scarps makes it possible to estimate the intensity of earthquakes of the late and middle Holocene in this area, respectively, at 7.2 and 6.8 magnitudes. The deep structure of the area of concentration of paleoseismic dislocations has been studied by the spectral-seismic profiling method. [ABSTRACT FROM AUTHOR]

The various tremors coming from inside the Earth, i.e. the earthquake P and Ѕ waves, are registered on appropriate devices called seismometers. The records given by seismometers are called seismograms. The methods for analysis of analog and digitally registered seismic earthquake waves - seismograms and numerical processing of the results of those analyzes in modern seismological practice include the following elements: • Identification of different types of seismic waves on the seismogram, • Determining the exact time of registration of a certain type of seismic wave, • Numerical processing of the registered waves at a number of seismic stations, in order to determine the spatial parameter of the hypocenter of the earthquake and the moment of its occurrence, • Defining the mechanism for protection against earthquakes and dynamic parameters of protection, • Energy characteristics (magnitude) of the earthquake and analysis in the distribution of the macroseismic field, i.e. formation of a mathematical model for distribution of the surface effect of the earthquake, • Determining the structural elements of the Earth's crust based on the interpretation of surface and spatial waveforms, • Calculation of the empirical values of the amplification characteristics of the soil on a local scale and determination of the mathematical laws for those phenomena. [ABSTRACT FROM AUTHOR]

In this study, for feature extraction of seismic- and nonlinear trend terms of the nonlinear and nonstationary high-rate Global Navigation Satellite System (GNSS) seismic displacements, we present an adaptive complete ensemble empirical mode decomposition (CEEMD) method based on correlation coefficient. We also introduce an improved CEEMD (ICEEMD) based on multiway principal component analysis (MPCA), hereafter, ICEEMD-MPCA, denoising method for boosting the high-rate GNSS seismic displacements with/without nonlinear trend. The results show that the proposed method is suitable for extracting features of nonlinear and nonstationary high-rate GNSS seismic displacements. The ICEEMD-MPCA denoising method is able to obviously eliminate white noise at high frequencies, remove systematic errors, and effectively preserve the earthquake wave signals. The denoising method reflects that high-rate GNSS has millimeter-level accuracy in the vertical component and submillimeter-level accuracy in the horizontal component. To verify the performance characteristics of the feature extraction and denoising method, the focal mechanism inversion for the 2010 El Mayor–Cucapah earthquake is estimated using the high-rate GNSS data (5 Hz) and collocated strong motion data (50 and 200 Hz). The focal mechanism of the earthquake estimated using the denoised high-rate GNSS with/without nonlinear trend are more consistent with the teleseismic estimates than those estimated using the raw high-rate GNSS and strong motion data. For the GNSS observations, the inversion results are more affected by errors in low-frequency, which accumulate over time and display their signatures as trend in coseismic displacements. It can be concluded that with the adaptive CEEMD method and the ICEEMD-MPCA denoising method, high-rate GNSS at near-field stations only or combined with GNSS/strong-motion records can be applied to yield better information regarding the earthquake focal mechanism inversion, the lower bounds of earthquake magnitudes, and earthquake rupture parameters for small to moderate earthquakes. [ABSTRACT FROM AUTHOR]

The region around the town Albstadt, SW Germany, was struck by four damaging earthquakes with magnitudes greater than 5 during the last century. These earthquakes occurred along the Albstadt Shear Zone (ASZ), which is characterized by more or less continuous microseismicity. As there are no visible surface ruptures that may be connected to the fault zone, we study its characteristics by its seismicity distribution and faulting pattern. We use the earthquake data of the state earthquake service of Baden-Württemberg from 2011 to 2018 and complement it with additional phase picks beginning in 2016 at the AlpArray and StressTransfer seismic networks in the vicinity of the ASZ. This extended data set is used to determine new minimum 1-D seismic vp and vs velocity models and corresponding station delay times for earthquake relocation. Fault plane solutions are determined for selected events, and the principal stress directions are derived. The minimum 1-D seismic velocity models have a simple and stable layering with increasing velocity with depth in the upper crust. The corresponding station delay times can be explained well by the lateral depth variation of the crystalline basement. The relocated events align about north–south with most of the seismic activity between the towns of Tübingen and Albstadt, east of the 9 ∘ E meridian. The events can be separated into several subclusters that indicate a segmentation of the ASZ. The majority of the 25 determined fault plane solutions feature an NNE–SSW strike but NNW–SSE-striking fault planes are also observed. The main fault plane associated with the ASZ dips steeply, and the rake indicates mainly sinistral strike-slip, but we also find minor components of normal and reverse faulting. The determined direction of the maximum horizontal stress of 140–149 ∘ is in good agreement with prior studies. Down to ca. 7–8 km depth SHmax is bigger than SV ; below this depth, SV is the main stress component. The direction of SHmax indicates that the stress field in the area of the ASZ is mainly generated by the regional plate driving forces and the Alpine topography. [ABSTRACT FROM AUTHOR]

How earthquakes initiate is still a largely debated question in earthquake science. On the lab scale, rupture initiation is well studied, and detailed models of how rupture nucleation evolves have been developed. Contrarily, for real earthquakes, only a few high‐resolution observations of this process are available today, mostly limited to mainshock magnitudes > M5. Consequently, there is still no consensus on whether and how laboratory results can be transferred to real earthquakes. Here we show that rupture nucleation phenomena observed on the lab scale can also be imaged on the microearthquake‐scale with little instrumental effort. Our results highlight the potential of the applied analysis workflow to significantly improve the observation quality of seismicity patterns and immediate foreshock phenomena in microearthquake sequences. Our approach can help to narrow the existing observational gap to the lab scale and may contribute to a better understanding of the mechanisms of earthquake initiation in the future. Plain Language Summary: One of the key questions in seismology is how earthquakes start. Until now, many models of earthquake initiation have been established through laboratory experiments. But for real earthquakes, only a few detailed observations of rupture initiation exist because they are mainly restricted to large earthquakes (M > 5). Consequently, it is not known today if and how the results obtained in the laboratory can be transferred to real earthquakes. In this study, we show that rupture initiation can be studied with close‐to‐lab‐scale detail in foreshock sequences to small earthquakes (M > 2.5). Small earthquakes occur much more frequently than large ones ‐ generally 10 times more per unit decrease in earthquake size (i.e., magnitude). Therefore, our results indicate that the database of detailed rupture initiations in real earthquakes can be extended substantially. Our analysis method that generates decade‐long, high‐resolution, and consistent catalogs of sequences of very small earthquakes, helps to narrow the existing observational gap to the lab scale. In this way, it can contribute to a better understanding of earthquake initiation mechanisms in the future. Key Points: Observe precursory phenomena to ML ≤ 3.2 mainevents in a microseismic sequenceImage high‐resolution spatiotemporal evolution of the immediate foreshock zone of small earthquakesObserve lab‐scale‐like rupture initiation phenomena on the field scale with little instrumental effort similar to lab scale experiments [ABSTRACT FROM AUTHOR]

How earthquakes initiate is still a largely debated question in earthquake science. On the lab scale, rupture initiation is well studied, and detailed models of how rupture nucleation evolves have been developed. Contrarily, for real earthquakes, only a few high‐resolution observations of this process are available today, mostly limited to mainshock magnitudes > M5. Consequently, there is still no consensus on whether and how laboratory results can be transferred to real earthquakes. Here we show that rupture nucleation phenomena observed on the lab scale can also be imaged on the microearthquake‐scale with little instrumental effort. Our results highlight the potential of the applied analysis workflow to significantly improve the observation quality of seismicity patterns and immediate foreshock phenomena in microearthquake sequences. Our approach can help to narrow the existing observational gap to the lab scale and may contribute to a better understanding of the mechanisms of earthquake initiation in the future. Plain Language Summary: One of the key questions in seismology is how earthquakes start. Until now, many models of earthquake initiation have been established through laboratory experiments. But for real earthquakes, only a few detailed observations of rupture initiation exist because they are mainly restricted to large earthquakes (M > 5). Consequently, it is not known today if and how the results obtained in the laboratory can be transferred to real earthquakes. In this study, we show that rupture initiation can be studied with close‐to‐lab‐scale detail in foreshock sequences to small earthquakes (M > 2.5). Small earthquakes occur much more frequently than large ones ‐ generally 10 times more per unit decrease in earthquake size (i.e., magnitude). Therefore, our results indicate that the database of detailed rupture initiations in real earthquakes can be extended substantially. Our analysis method that generates decade‐long, high‐resolution, and consistent catalogs of sequences of very small earthquakes, helps to narrow the existing observational gap to the lab scale. In this way, it can contribute to a better understanding of earthquake initiation mechanisms in the future. Key Points: Observe precursory phenomena to ML ≤ 3.2 mainevents in a microseismic sequenceImage high‐resolution spatiotemporal evolution of the immediate foreshock zone of small earthquakesObserve lab‐scale‐like rupture initiation phenomena on the field scale with little instrumental effort similar to lab scale experiments [ABSTRACT FROM AUTHOR]

Remobilization and deformation of surficial subaqueous slope sediments create turbidites and soft sediment deformation structures, which are common features in many depositional records. Palaeoseismic studies have used seismically‐induced turbidites and soft sediment deformation structures preserved in sedimentary sequences to reconstruct recurrence patterns and – in some cases – allow quantifying rupture location and magnitude of past earthquakes. However, current understanding of earthquake‐triggered remobilization and deformation lacks studies targeting where these processes take place, the subaqueous slope and involving direct comparison of sedimentary fingerprint with well‐documented historical earthquakes. This study investigates the sedimentary imprint of six megathrust earthquakes with varying rupture characteristics in 17 slope sediment cores from two Chilean lakes, Riñihue and Calafquén, and evaluates how it links to seismic intensity, peak ground acceleration, bracketed duration and slope angle. Centimetre‐scale stratigraphic gaps ranging from ca 1 to 20 cm – caused by remobilization of surficial slope sediment – were identified using high‐resolution multi‐proxy core correlation of slope to basin cores, and six types of soft sediment deformation structures ranging from ca 1 to 25 cm thickness using high‐resolution three‐dimensional X‐ray computed tomography data. Stratigraphic gaps occur on slope angles of ≥2.3°, whereas deformation already occurs from slope angle 0.2°. The thickness of both stratigraphic gaps and soft sediment deformation structures increases with slope angle, suggesting that increased gravitational shear stress promotes both surficial remobilization and deformation. Seismic shaking is the dominant trigger for surficial remobilization and deformation at the studied lakes. Total remobilization depth correlates best with bracketed duration and is highest in both lakes for the strongest earthquakes (Mwca 9.5). In lake Riñihue, soft sediment deformation structure thickness and type correlate best with peak ground acceleration providing the first field‐based evidence of progressive soft sediment deformation structure development with increasing peak ground acceleration for soft sediment deformation structures caused by Kelvin‐Helmholtz instability. The authors propose that long duration and low frequency content of seismic shaking favours surficial remobilization, whereas ground motion amplitude controls Kelvin‐Helmholtz instability‐related soft sediment deformation structure development. [ABSTRACT FROM AUTHOR]

Three devastating earthquakes of MW ≥ 5.9 activated a complex system of high‐angle normal, antithetic, and sub‐horizontal detachment faults during the 2016–2017 central Italy seismic sequence. Waveform cross‐correlation based double‐difference location of nearly 400,000 aftershocks illuminate complex, fine‐scale structures of interacting fault zones. The Mt. Vettore–Mt. Bove (VB) normal fault exhibits wide and complex damage zones, including a system of bookshelf faults that intersects the detachment zone. In the Laga domain, a comparatively narrow, shallow dipping segment of the deep Mt. Gorzano fault progressively ruptures through the detachment zone in four subsequent MW ∼ 5.4 events. Reconstructed fault planes show that the detachment zone is fragmented in four sub‐horizontal, partly overlaying shear planes that correlated with the extent of the mainshock ruptures. We find a new, deep reaching seismic barrier that coincides with a bend in the VB fault and may play a role in controlling rupture evolution. Plain Language Summary: In 2016–2017, a sequence of three devastating earthquakes with magnitudes ∼6 near the towns of Amatrice, Visso, and Norcia in central Italy triggered hundreds of thousands of aftershocks. We compute high‐precision locations for these aftershocks that have the power to illuminate the fine‐scale structure of the complex system of intersecting and interacting faults that were activate during the sequence. The normal faults that ruptured during the three mainshocks are capped at ∼9 km depth by a sub‐horizontal detachment zone. The normal faults intersect the detachment at an acute angle, causing complex fault zone structures and fragmentation of the detachment where they intersect. We observe a deep reaching seismic barrier that we conclude may play a role in controlling rupture extent and evolution of large earthquakes along this section of the Apennines. In general, fault complexity here is higher than in similar sequences in 2009 near L'Aquila to the south and in 1997 near Colfiorito to the north. Key Points: Precise locations of 390,334 aftershocks of the 2016–2017 Central Italy sequence illuminate the complex normal fault systemA system of high‐angle bookshelf faults accommodates slip at the intersection between normal and detachment faultsA deep‐reaching seismic barrier is identified north of the Sibillini Thrust that may control the ruptures of large earthquakes [ABSTRACT FROM AUTHOR]

From California to British Columbia, the Pacific Northwest coast bears an omnipresent earthquake and tsunami hazard from the Cascadia subduction zone. Multiple lines of evidence suggests that magnitude eight and greater megathrust earthquakes have occurred ‐ the most recent being 321 years ago (i.e., 1700 A.D.). Outstanding questions for the next great megathrust event include where it will initiate, what conditions are favorable for rupture to span the convergent margin, and how much slip may be expected. We develop the first 3‐D fully dynamic rupture simulations for the Cascadia subduction zone that are driven by fault stress, strength and friction to address these questions. The initial dynamic stress drop distribution in our simulations is constrained by geodetic coupling models, with segment locations taken from geologic analyses. We document the sensitivity of nucleation location and stress drop to the final seismic moment and coseismic subsidence amplitudes. We find that the final earthquake size strongly depends on the amount of slip deficit in the central Cascadia region, which is inferred to be creeping interseismically, for a given initiation location in southern or northern Cascadia. Several simulations are also presented here that can closely approximate recorded coastal subsidence from the 1700 A.D. event without invoking localized high‐stress asperities along the down‐dip locked region of the megathrust. These results can be used to inform earthquake and tsunami hazards for not only Cascadia, but other subduction zones that have limited seismic observations but a wealth of geodetic inference. Plain Language Summary: The largest earthquakes on Earth occur along faults that develop between two tectonic plates that come into contact. Termed megathrust earthquakes, these catastrophic events are responsible for generating both strong ground‐shaking and tsunamis. The Cascadia megathrust fault straddles the Pacific coastline of North America and from evidence in both the United States and Japan, we know this fault last slipped in 1700 A.D. We have combined models of strain buildup (geodetic coupling models) with state‐of‐the‐art 3‐D computer simulations to understand the potential hazard of a future earthquake in Cascadia and show what factors might lead to the fault slipping its entire length. We compare our simulations to geologic measurements of permanent ground movement from 1700 A.D. Our results demonstrate that no matter where the earthquake is allowed to start, coupling models showing strain accumulation to the top of the fault easily leads to big earthquakes. We also look into what 1700 A.D. event may have looked like and show several scenarios that fit the geologic data very closely. This work represents the first set of 3‐D simulations that use the laws of physics to see what may control the size of future earthquakes in Cascadia. Key Points: We design the first fully dynamic 3‐D earthquake simulations based on geodetic coupling models for the Cascadia megathrustSegmentation in the stress drop is needed to produce subsidence amplitudes consistent with observed megathrust earthquakesDynamic rupture simulations demonstrate how fault friction and stress levels may control margin‐wide rupture [ABSTRACT FROM AUTHOR]

The Iranian Plateau has been subjected to destructive earthquakes throughout its history. Reliable assessment of the seismic hazard in this earthquake-prone region is therefore essential. Our study focuses on estimating the maximum earthquake magnitude as one of the main parameters of seismic hazard analysis. We implemented two quantitative approaches, namely, probabilistic and deterministic. The probabilistic method allows combining the historical (i.e. incomplete) and the instrumental parts of a catalogue with different levels of completeness and considers the uncertainties in earthquake magnitude determination. In this study, we used a unified, declustered, and complete catalogue of earthquakes in Iran, covering the period from the fourth century BC to 2019. We calculated the maximum possible magnitudes for hundreds of grid points by using the seismicity data in a 200-km radial region around each grid point. The maximum possible earthquake was observed to vary between 6.0 and 8.2, and the highest values were found in the Alborz-Azarbayejan seismotectonic province, Kopeh-Dagh, central east Iran, Makran, and the southeast Zagros. The lowest mmax values were found in the Persian Gulf, Arabian Platform, Esfahan-Sirjan region, and the Dasht-e-Kavir Desert in central Iran. As a second part to this study, we calculated the maximum credible earthquakes for 1103 identified major faults by using five empirical magnitude-scaling relationships. Our results were consistent with both the observed earthquakes and the seismic potential of the various seismogenic zones of Iran. The study results can be used in future seismic hazard analyses and have fundamental implications for mitigating seismic risk in Iran. [ABSTRACT FROM AUTHOR]

Active traces of the southern Fairweather fault were revealed by light detection and ranging (lidar) and show evidence for transpressional deformation between North America and the Yakutat block in southeast Alaska. We map the Holocene geomorphic expression of tectonic deformation along the southern 30 km of the Fairweather fault, which ruptured in the 1958 moment magnitude 7.8 earthquake. Digital maps of surficial geology, geomorphology, and active faults illustrate both strike-slip and dip-slip deformation styles within a 10°-30° double restraining bend where the southern Fairweather fault steps offshore to the Queen Charlotte fault. We measure offset landforms along the fault and calibrate legacy 14C data to reassess the rate of Holocene strike-slip motion (=49 mm/yr), which corroborates published estimates that place most of the plate boundary motion on the Fairweather fault. Our slip-rate estimates allow a component of oblique-reverse motion to be accommodated by contractional structures west of the Fairweather fault consistent with geodetic block models. Stratigraphic and structural relations in hand-dug excavations across two active fault strands provide an incomplete paleoseismic record including evidence for up to six surface ruptures in the past 5600 years, and at least two to four events in the past 810 years. The incomplete record suggests an earthquake recurrence interval of =270 years--much longer than intervals <100 years implied by published slip rates and expected earthquake displacements. Our paleoseismic observations and map of active traces of the southern Fairweather fault illustrate the complexity of transpressional deformation and seismic potential along one of Earth's fastest strike-slip plate boundaries. [ABSTRACT FROM AUTHOR]

River long profiles are widely used as tectonic event markers. Incision/uplift rate estimations based on the dating of river terrace or exposed scarp sediment give long-term averaged uplift rate values with the limitation of not giving the intermediate phases of highs and lows. On the other hand, GPS based estimates provide only decadal-scale uplift rate estimates. In our study, we adopted the long profile inversion methodology to obtain a temporal profile of uplift rate history (over the Late Pleistocene-Holocene time period) along an active fault (the Kuchchh Mainland Fault) in Kuchchh region, western India, which has experienced several large magnitude earthquakes in recent past. We also used long profiles analysis to quantify the transient response of Kuchchh landscape to uplift activity along the Kuchchh Mainland Fault. Steepness index and chi profiles were estimated to display the spatial variability in landscape disequilibrium. Integral method-based chi profiles were used to obtain the optimum value of basin area exponent of the Stream Power Incision Model (SPIM). Our estimates provide a mean uplift rate value of 1.1–1.4 mm/yr along the Kuchchh Mainland Fault during the Late Pleistocene-Holocene period, which matches with the average uplift rate derived from other independent observations in this region. Further, a chronology of major tectonic phases from the estimated uplift rate time series also corroborates with field-based estimates of major incision events in Kuchchh region. We also tested the robustness and sensitivity of our uplift rate estimates to model parameters. Such data can be complementary to the existing sparsely (both spatially and temporally) distributed uplift rate database, which is imperative to seismic hazard assessment. [ABSTRACT FROM AUTHOR]

The reverse fault known as Loma Negra Oriental is located on the orogenic front of the Central Precordillera, characterized by N-S trending thrusts with eastern vergence. This fault limits the eastern flank of the Loma Negra brachianticline, a fault propagation fold that deforms Neogene rocks and Quaternary deposits outcropping on the eastern piedmont of the Sierra de Talacasto. The faults related to this fold trend N-S and affect progressively younger deposits to the east, showing the advance of the Andean deformation in that direction. In the northern section of the Loma Negra Oriental Fault, a natural exposure shows deposits associated with single-event displacements, allowing us to determine its kinematics. The Loma Negra Oriental thrust shows a NNW trend (130°) and dips with low angle (22° SW) affecting recent alluvial deposits of early Holocene age (8,700 ± 1,370 years BP). To characterize the morphology of the scarps associated with this fault, we constructed detailed topographic profiles transverse to the analysed structure. Two scarps of 0.9 m and 3.1 m high were recognized, suggesting several seismic events. The fault seems to continue to the north and south as a blind (buried) structure that limits the entire eastern flank of the brachianticline, with a length of at least 16.5 km. Based on the scarps measurement, the surface and subsurface rupture extension, and the paleoseismic analysis of a natural exposure, with a total vertical slip of 0.9 ± 0.1 m (~ 0.4 m for the last event and ~ 0.5 m for the penultimate) and a dip-slip per event of 1.16 ± 0.15 m, we suggest the occurrence of two seismic events during the Late Pleistocene-Holocene. In this way, we consider this structure as a potential seismogenic source which could generate a maximum probable earthquake of magnitude Mw 6.5 or larger, with an estimated recurrence period of ~9,000 years, posing a threat to infrastructure and populated areas of the province of San Juan (>800,000 inhabitants). [ABSTRACT FROM AUTHOR]

The pre-Hispanic Mesoamerican cultures of the Mexican highlands, and specifically the P'urepecha and Aztec empires (Late Post-Classic Period, 1300 - 1500 AD), experienced great destructive earthquakes that left a mark on their society and determined both their spiritual rites and their urbanism. This paper studies several paleoearthquakes that the P'urepechas experienced in Lake Pátzcuaro, using paleosismology techniques, as well as the existence of large basaltic blocks associated with their religious rites appearing in the hanging wall of two large surface ruptures of coseismic origin (normal faults) that affected the geography of the lake itself. Paleoseoseismic studies suggest that several earthquakes of magnitude between M 6.5 and M 7 occurred with fault surface rupture and probably with a migration from the lake shore. In addition, these studies show that these earthquakes have been occurring since the Preclassic period (3000 BP). Later, the P'urepechas placed two large basalt blocks of more than 1 m3 in volume over the hanging wall of these faulting ruptures and carved their surface for spiritual purposes. In later times during the Spanish conquest, a large Maltese-style Christian cross was carved, which would correspond to the process of Christian evangelization throughout the 16th century in the area of Michoacán, thus reinforcing the spiritual significance of these blocks. In any case, we are facing one of the oldest human manifestations of the effects of earthquakes on ancestral cultures. [ABSTRACT FROM AUTHOR]

The article presents a study of the semibrittle seismic deformation related to a high-temperature mantle below the brittle lithosphere along the Romanche oceanic transform fault (OTF) in the equatorial Atlantic Ocean. Topics discussed include the inconsistency of the maximum depths of microseismicity along the OTF and a fixed temperature isotherm, influence of local dynamic processes such as hydrothermal circulation in microseismicity, and the common location of the microearthquakes.

Earthquakes, faulting, rheology, lithosphere However, the effect of the lithosphere thickness on the temperature structure is insufficient on its own to explain the depth of the earthquakes, and implies (by comparison to the oceanic lithosphere) that the continental lower crust in these regions is anhydrous [9]. (doi:10.1126/science.265.5168.82) 7 Jackson, J, McKenzie, D, Priestley, K, Emmerson, B. 2008 New views on the structure and rheology of the lithosphere. Keywords: earthquakes; faulting; rheology; lithosphere EN earthquakes faulting rheology lithosphere 1 5 5 02/25/21 20210322 NES 210322 Synthesizing geological and geophysical approaches to investigate the deformation of the lithosphere offers the potential for important new insights because of the range of timescales and lengthscales sampled by the different techniques, although successfully spanning this range represents one of the main challenges to be overcome when conducting such work. [Extracted from the article]

Earthquakes with magnitudes Mw = 6.4 and Mw = 7.1 occurred in July 2019, one after the other with an interval of 34 hours, several kilometers apart, near the town of Ridgecrest, Kern County, California, USA. There was a moderate shock with a magnitude of Mw = 5.8 in 1995 in the immediate vicinity of the epicenters of the future earthquakes. A strong earthquake with Mw = 7.4 occurred about a hundred kilometers to the north in 1872. To study the seismic deformation process in connection with these events, data from continuous GPS observations at 51 station were used, with a daily recording interval. The initial observation epoch was 13 years prior to the latest events (July 2006), and the final epoch was about a month (August 2019) after those. The study area has a diameter of about 250 km and includes the epicenters of modern and historical earthquakes. Digital models of the distribution of horizontal, total shear and dilatation deformations were obtained for each day of observations. Incorporation of the time sequence of the obtained models into an accelerated kinematic visualization of the seismic deformation process (https://www.researchgate.net/publication/343577013%5fRidgecrest%5ftotal%5fshear%5fstrain%5fand%5fseismicity%5fevolution) revealed some interesting features. The onset of shear deformation extremum was close to the time of the moderate earthquake (Mw = 5.2), which occurred 10 years before the events of 2019, about 70 km from it, and which presumably initiated the trigger mechanism of the future seismic rupture with the beginning of the development of a weakened elongated zone in the area of the Coso volcanic field. The area of anomalous shear spread in the direction of the future events and reached their epicenters at the time of the main shocks. An analysis of the space-time distribution of the length of the surface displacement vectors showed the possibility of determining the location of a future strong event based on long-term continuous GPS observations in seismic areas. [ABSTRACT FROM AUTHOR]

The earthquake Frequency Magnitude Distribution (FMD) or b values have been meticulously used in the last two decades as a deterministic earthquake precursor or indicator, in earthquake forecasting or probabilistic seismic hazard assessment studies, in a variety of tectonic and stress regimes. In the present study, temporal fluctuations of b values were analysed by means of 185 well located (Mw ≥ 5) earthquakes, in the region of Andaman and Nicobar Islands (N10° to 14°) for a time period of ∼100 years (1918 to 2018). Variation in b value rates before and after the 2004 great Sumatra-Andaman earthquake (Mw 9.3) was analysed, preceded by a comprehensive geodynamic and seismotectonic- crustal deformation overview along the Andaman trench section. The analysis suggest a precursory drop in b values from 1.05 ± 0.05 (1918 to 2004) to 0.90 ± 0.05 post 2004 earthquake (2004 to 2018) reflecting a greater slip deficit and strain localization, that exceeds considerably the pattern of energy release in this region. Further, the region of Andaman and Nicobar Islands have witnessed a poor slip distribution (on-fault reactivation) from the great earthquakes that nucleated near the Sumatra segment, and have ruptured few large earthquakes, with no accounts of great earthquake (>Mw 8) during the last century. This emphasizes that, strain accumulation has been concentrated over a long period of time, and can contribute a destructive deformation event in near future. [ABSTRACT FROM AUTHOR]

The Dinaric Fault System in western Slovenia, consisting of NW-SE trending, right-lateral strike-slip faults, accommodates the northward motion of Adria with respect to Eurasia. These active faults show a clear imprint in the morphology and some of them hosted moderate instrumental earthquakes. However, it is largely unknown if the faults also had strong earthquakes in the Late Quaternary. This hampers our understanding of the regional tectonics and the seismic hazard. Geological evidence of co-seismic surface ruptures only exists for one historical event, the 1511 Idrija Earthquake with a magnitude of ~M6.8, but the causative fault is still disputed. Here we use geomorphological data, near-surface geophysical surveys, and paleoseismological trenching to show that two of these faults, the Predjama Fault and the Idrija Fault ruptured in strong earthquakes in the Holocene. In a paleoseismological trench across the Predjama Fault we found at least one earthquake with a minimum magnitude of MW6.1 that occurred between 13-0.7 ka, very likely not earlier than 8.4 ka. At the Idrija Fault, a surface-rupturing earthquake with a magnitude of at least MW6.1 happened in the last ~2.1 ka. This event could correspond to the 1511 Idrija earthquake. Our results show that the faults rupture in rare, but strong earthquakes, which dominate the seismic moment release. We show that instrumental and historical seismicity data do not capture the strongest events in this area. The fact that many of the NW-SE trending, parallel faults are active implies that the deformation in western Slovenia is distributed, rather than focussed on one major structure. [ABSTRACT FROM AUTHOR]