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The reduction of effective normal stress during earthquake slip due to thermal pressurization of fault zone pore fluids is a significant fault weakening mechanism. Explicit incorporation of this process into frictional fault models involves solving the diffusion equations for fluid pressure and temperature outside the fault at each time step, which significantly increases the computational complexity. Here, we propose a proxy for thermal pressurization implemented through a modification of the rate-and-state friction law. This approach is designed to emulate the fault weakening and the relationship between breakdown energy and slip resulting from thermal pressurization and is appropriate for fully dynamic simulations of multiple earthquake cycles. It preserves the computational efficiency of conventional rate-and-state friction models, which in turn can enable systematic studies to advance our understanding of the effects of fault weakening on earthquake mechanics. In 2.5-D simulations of pulse-like ruptures on faults with finite seismogenic width, based on our thermal pressurization proxy, we find that the spatial distribution of slip velocity near the rupture front is consistent with the conventional square-root singularity, despite continued slip-weakening within the pulse, once the rupture has propagated a distance larger than the rupture width. An unconventional singularity appears only at shorter rupture distances. We further derive and verify numerically a theoretical estimate of the breakdown energy dissipated by our implementation of thermal pressurization. These results support the use of fracture mechanics theory to understand the propagation and arrest of very large earthquakes. [ABSTRACT FROM AUTHOR]

We successfully detected widely distributed ground displacements for the 2023 Kahramanmaraş, Turkey, earthquakes by conducting interferometric synthetic aperture radar (InSAR) analyses using a ScanSAR observation mode. Major deformation extended approximately 350 and 150 km along the southern and northern strands bifurcating in the west of the East Anatolian Fault, produced by the main shock and the largest aftershock. The deformation map reveals that the ruptures propagated on the Erkenek, Pazarcık and Amanos segments on the southern strand and the Çardak segment on the northern strand. The fault plane of the northern strand bends approximately 45° at both edges with Z-shaped crank geometry. The bending fault at the western edge runs just along the Çardak segment but does not reach the Savrun segment, while at the eastern edge it deviates from known active faults such as Sürgü, Malatya faults and Doğansehır fault zone. A 3-D displacement map demonstrates that almost pure left-lateral fault motions were distributed along the two strands, with little vertical deformation. The moment magnitudes estimated from the slip distribution model were 7.82 and 7.66 for the southern and northern strands, respectively, with the Erkenek and Çardak segments having the largest released seismic moments on each strand, corresponding to approximately 31 and 57 per cent of the total, respectively. The Coulomb Failure Function change values indicate that the main shock can promote the largest aftershock with a standard value of the effective friction coefficient. Additionally, the unclamping effect controlled by the frictional property of the rock was a key factor in pulling the trigger of the seismic event on the northern strand. The historically accumulated and released seismic energies were imbalanced for the Pazarcık and Erkenek segments, suggesting that the 2023 event does not support a simple characteristic earthquake model; rather, it may be consistent with a supercycle model, in which the slip remnants from the characteristic earthquakes have been historically accumulated as coupling on a fault and released as huge earthquakes at longer intervals. [ABSTRACT FROM AUTHOR]

Atlantic Canada encompasses geological evidence of the orogenic and rifting episodes that inspired the development of the theory of plate tectonics and the fundamental concept of the Wilson cycle. To provide a regional crustal-scale view that can complement surface mapping studies and sparse seismological investigations, an onshore–offshore 3-D constrained gravity inversion methodology is proposed involving incorporation of topography and an inversion mesh that is laterally variable in terms of its maximum depth extent. A 3-D density anomaly model for the entirety of Atlantic Canada and its environs is generated, with the inverted density distribution structure and extracted isodensity surfaces showing excellent correspondence with independent and co-located controlled source and passive seismic constraints. The full density model and crustal thicknesses from this work are made freely available so that they may be used for further study, for instance as inputs for deformable plate reconstruction modelling. [ABSTRACT FROM AUTHOR]

Fault slip induced by fluid perturbation in shale formations may only lead to a sparse seismicity. However, fault slip may strongly impact the integrity of shale formations that serve as caprocks for geological reservoirs holding buoyant fluids such as CO2, natural gas or hydrogen. A better understanding of the fluid reactivation processes of fault and the seismic triggering process is therefore critical for reservoir monitoring and fault stability. Here we analyse the seismic responses of a shale fault exposed to fluid pressurization during an in situ field-scale injection experiment at ∼340 m depth in the Mont Terri underground research laboratory (Switzerland). Two main types of seismic signals are observed as the fault was activated and started to slowly slip. After an aseismic phase, we observed tremor signatures and an increase in noise amplitude, which were directly associated with the slowly propagating fault slip in response to fluid injection. These signatures were later followed by micro-earthquakes that seem to occur further away from the fluid-pressurized area. We interpret these micro-earthquakes to be triggered by stress perturbations from the main slip growth. These two classes of seismic responses therefore highlight two different processes. Tremors seem to be a more direct observation for the fluid-induced slip propagation than micro-earthquakes. Even hidden in the noise, they precede earthquake failures, thus providing a useful tool for monitoring fluid leakage activated by slow deformation on low permeable shale faults, with applications for sealing integrity of caprocks. [ABSTRACT FROM AUTHOR]

We present the results from a fully unconstrained moment tensor inversion of induced seismic events in a complex and high seismic hazard region (Val d'Agri basin, Southern Italy). The study area hosts two well-documented cases of induced microseismicity linked to (i) a wastewater injection well of a giant oilfield (the largest in onshore Europe), and (ii) severe seasonal level changes of an artificial lake. In order to gather information on the non-double-couple components of the source and to better understand the rupture mechanisms, we analyse seismic events recorded during daily injection tests in the disposal well. The computed moment tensors have significant non-double-couple components that correlate with the well-head injection pressure. The injection parameters strongly influence the rupture mechanism that can be interpreted as due to the opening/closing of a fracture network inside a fault zone of a pre-existing thrust fault. For the case of the reservoir-induced seismicity, no direct correlations are observed with the loading/unloading of the reservoir. [ABSTRACT FROM AUTHOR]

SUMMARY: Large continental earthquakes are infrequent, but they are important for understanding active tectonics. Many pre-1990, magnitude greater than 7 earthquakes have not been well documented due to a lack of data. The declassification of KeyHole (KH) imagery provides opportunities for investigating those historical events. In this study, we present new geodetic observations of surface deformation from the 1976 November 24 Ms 7.3 Chaldiran earthquake in eastern Turkey. We use various historical KH imagery and modern Pleiades stereo imagery to determine the coseismic deformation, providing new constraints on the 1976 rupture. Based on the image measurements, we resolve the coseismic slip at depth. The inverted slip from a simple elastic dislocation model, ∼4.0 m at shallow depth of the Hidirmentes segment, is larger than the surface displacement, around 3 m from previous field surveys, suggesting that the coseismic deformation is a combination of slip on the primary fault with 30 ± 16 per cent off-fault deformation. We also measure cumulative offsets at two sites with well-preserved features, and find a constant ratio of horizontal to vertical offsets throughout the past few earthquake cycles. Both the observed constant slip ratio in this study and offset clusters from previous field measurements support that the Chaldiran fault exhibits a characteristic slip behaviour in the late Quaternary. By combining all the geodetic and geological observations in the broader region, we also present a brief analysis of the slip rate of the Chaldiran fault. [ABSTRACT FROM AUTHOR]

SUMMARY: Fault zone head waves (FZHWs) are an essential diagnostic signal that provides high-resolution imaging of fault interface properties at seismogenic depth. In this study, we validate the existence of a bi-material interface in the Xianshuihe–Anninghe Fault (XAF) zone around their intersection and determine the cross-fault velocity contrast. We employ a semi-automatic workflow to detect and pick FZHWs and direct P waves. In addition, to improve the identification ability of potential FZHWs in the automatic picking process, we adopt a 'forward-detecting and backward-picking' strategy combining the short-term average/long-term average (STA/LTA) algorithm with a kurtosis detector. The polarization and characteristic periods of the waveforms are then used to manually refine the picks and evaluate the quality. The results indicate that the average velocity contrast along the southern Xianshuihe Fault is 3–5 per cent, with the northeast side characterizing a faster P-wave velocity, in agreement with tomographic results. A systematic moveout between FZHWs and the direct P waves over a 100 km long fault segment reveals a single continuous interface in the seismogenic zone. The single bi-material fault structure might be conducive to the preparation of large earthquakes and further influences the corresponding dynamic rupture processes. [ABSTRACT FROM AUTHOR]

Historical and instrumental earthquake catalogs in low strain rate regions are not necessarily indicative of the long-term spatio-temporal distribution of seismicity. This implies that probabilistic seismic hazard analysis (PSHA) should also consider geologic and geodetic data through fault-based seismogenic sources. However, it is not always clear how on-fault magnitude-frequency distributions (MFDs) should be described and, if the seismogenic layer is especially thick, how fault sources should be extrapolated down-dip. We explore these issues in the context of a new PSHA for Malawi, where regional extensional rates are 0.5–2 mm yr−1, the seismogenic layer is 30–40-km thick, the instrumental catalog is ∼60 yr long and fault-based sources were recently collated in the Malawi Seismogenic Source Model. Furthermore, Malawi is one of several countries along the East African Rift where exposure to seismic hazard is growing, but PSHA does not typically consider fault sources. We use stochastic event catalogs to explore different fault source down-dip extents and MFDs. Our PSHA indicates that hazard levels are highest for a Gutenberg–Richter on-fault MFD, even at low probabilities of exceedance (2 per cent in 50 yr), whilst seismic hazard levels are also sensitive to how relatively short (<50 km) fault sources are extrapolated down-dip. For sites close to fault sources (<40 km), seismic hazard levels are doubled compared to previous instrumental-seismicity based PSHA in Malawi. Cumulatively, these results highlight the need for careful fault source modelling in PSHA of low strain rate regions and the need for new fault-based PSHA elsewhere in the East Africa Rift. [ABSTRACT FROM AUTHOR]

It is well known that large earthquakes often exhibit significant rupture complexity such as well separated subevents. With improved recording and data processing techniques, small earthquakes have been found to exhibit rupture complexity as well. Studying these small earthquakes offers the opportunity to better understand the possible causes of rupture complexities. Specifically, if they are random or are related to fault properties. We examine microearthquakes (M  < 3) in the Parkfield, California, area that are recorded by a high-resolution borehole network. We quantify earthquake complexity by the deviation of source time functions and source spectra from simple circular (omega-square) source models. We establish thresholds to declare complexity, and find that it can be detected in earthquakes larger than magnitude 2, with the best resolution above M2.5. Comparison between the two approaches reveals good agreement (>90 per cent), implying both methods are characterizing the same source complexity. For the two methods, 60–80 per cent (M 2.6–3) of the resolved events are complex depending on the method. The complex events we observe tend to cluster in areas of previously identified structural complexity; a larger fraction of the earthquakes exhibit complexity in the days following the M w 6 2004 Parkfield earthquake. Ignoring the complexity of these small events can introduce artefacts or add uncertainty to stress drop measurements. Focusing only on simple events however could lead to systematic bias, scaling artefacts and the lack of measurements of stress in structurally complex regions. [ABSTRACT FROM AUTHOR]

Earthquakes often occur in regions with complex material structure, such as sedimentary basins or mantle wedges. However, the majority of co-seismic modelling studies assume a simplified, often homogeneous elastic structure in order to expedite the process of model construction and speed up calculations. These co-seismic forward models are used to produce Green's functions for finite-fault inversions, so any assumptions made in the forward model may introduce bias into estimated slip models. In this study, we use a synthetic model of a sedimentary basin to investigate the impact of 3-D elastic structure on forward models of co-seismic surface deformation. We find that 3-D elastic structure can cause changes in the shape of surface deformation patterns. The magnitude of this effect appears to be primarily controlled by the magnitude of contrast in material properties, rather than the sharpness of contrast, the fault orientation, the location of the fault, or the slip orientation. As examples of real-world cases, we explore the impact of 3-D elastic structure with a model of the Taipei basin in Taiwan and a simulated earthquake on the Sanchaio fault, and with a 3-D geologic model of the San Francisco Bay Area and a slip model of the 1984 Morgan Hill earthquake on the Calaveras fault. Once again, we find that the presence of the basin leads to differences in the shape and amplitude of the surface deformation pattern, but we observe that the primary differences are in the magnitude of surface deformation and can be accounted for with a layered elastic structure. Our results imply that the use of homogeneous Green's functions may lead to bias in inferred slip models in regions with sedimentary basins, so, at a minimum, a layered velocity structure should be used. [ABSTRACT FROM AUTHOR]

The Brawley Seismic Zone (BSZ) is a ∼58-km-long section of the Pacific–North America Plate boundary that connects the southernmost San Andreas Fault (SAF) and the Imperial Fault in southern California. We analyse the BSZ as two segments: a north segment, dominated by SAF tectonics, and a south segment that accommodates a higher level of seismicity. The south segment includes a ∼6 km wide right lateral step-over that includes the Salton Sea geothermal field (SSGF) and Holocene subaerial lava domes at the south end of the Salton Sea, called Salton Buttes. In general, the 40 yr (1981–2021) of BSZ seismicity is characterized by transitory sequences that often are accommodated simultaneously or in quick succession on intricate ladder-like faults. These sequences rarely reactivate faults associated with previous seismicity alignments but rather activate adjacent faults, sometimes located within less than 1 km. They can include several events of similar size as the mainshock, followed by bursts of aftershocks sometimes located away from the mainshock rupture. The seismicity rate and the spatial geometrical complexity varies between the BSZ-north and BSZ-south segments. The 24-km-long BSZ-north accommodates a ∼12-km-long linear trend of seismicity that extends from the SAF terminus into the Salton Sea, where moderate-sized northeast striking sequences form the rungs in a ladder-structure in a weak crust. The seismicity in this area is most likely influenced by the stress state of the SAF. In contrast, the 34 km long BSZ-south segment, which also has a weak crust, has accommodated larger sequences that illuminate irregular ladder-type faulting, with aftershocks defining linear distributions striking either north or northeast. The focal mechanisms exhibit a mostly strike-slip style of faulting with minor dip-slip faulting in the south Salton Sea area as well as the Mesquite basin to the south. The state of stress, as determined from focal mechanisms, consists of almost horizontal σ 1, and σ 3 with vertical σ 2. The activation angle between the trend of σ 1 and the preferred nodal plane of the largest nearby event decreases systematically from north to south along the long-axis of the BSZ. In the step-over zone, the Holocene volcanism and the frequent seismicity sequences suggest crustal extension as well as associated reduced crustal strength as compared to other parts of the BSZ. The presence of weak, thin, hot crust and distributed ladder-like or conjugate predominantly strike-slip faulting suggests that the whole BSZ acts as a several kilometre wide fault zone where no prominent principal slip surfaces are required to accommodate moderate-sized (M 6+) earthquakes. [ABSTRACT FROM AUTHOR]

This study uses an ensemble Kalman method for near-surface seismic site characterization of 154 network earthquake monitoring stations in California to improve the resolution of S -wave velocity (VS) and P -wave velocity (VP) profiles—up to the resolution depth—coupled with better quantification of uncertainties compared to previous site characterization studies at this network. These stations were part of the Yong et al. site characterization project, with selected stations based on future recordings of ground motions that are expected to exceed 10 per cent peak ground acceleration in 50 yr. To estimate VS and VP from experimental dispersion data, Yong et al. investigated these stations using linearized (local search and iteration) routines, and Yong et al. later studied a subset of these stations using nonlinear (global search and optimization) routines. In both studies, the selection of model parameters—that is, discretization of the VS and VP profiles with only five fixed thickness layers—was mainly based on trial and error. In contrast, this paper uses an approximate Bayesian method to assimilate experimental dispersion data and sequentially update an ensemble of particle estimates that span the VS and VP parameter spaces. Doing so, we systematically determine the most probable profiles conditioned on the experimental dispersion data, the introduced noise levels, and a priori knowledge in the form of physical constraints. We consider two configurations to discretize the soil depth from the surface to half of the maximum discernible wavelength obtained from the experimental dispersion data, namely refined and coarse models, and two initial models for each configuration to study solution multiplicity. Our results suggest that using the refined model for the top surface layers improves the resolution of near-surface site characteristics and the model's success rate in capturing dispersion data at high frequencies. All models result in similar VS but distinct VP profiles, with increasing uncertainty at deeper layers, suggesting that the fundamental mode of Rayleigh wave dispersion data is not adequate to constrain the P -wave velocity profile and the S -wave velocity close to the resolution depth. [ABSTRACT FROM AUTHOR]

We investigate active tectonics in southwestern Turkey along the trend between Fethiye, near the eastern end of the Hellenic subduction zone, and Burdur, on the Anatolian plateau. Previously, regional GNSS velocities have been used to propose either (1) a NE-trending zone of strike-slip faulting coined the Fethiye–Burdur Fault Zone, or (2) a mix of uniaxial and radial extension accommodated by normal faults with diverse orientations. We test these models against the available earthquake data, updated in light of recent earthquakes at Arıcılar (24 November 2017, M w 5.3), Acıpayam (20 March 2019, M w 5.6) and Bozkurt (8 August 2019, M w 5.9), the largest in this region in the last two decades. Using Sentinel-1 InSAR and seismic waveforms and arrival times, we show that the Arıcılar, Acıpayam and Bozkurt earthquakes were partially or fully buried ruptures on pure normal faults with subtle or indistinct topographic expressions. By exploiting ray paths shared with these well-recorded modern events, we relocate earlier instrumental seismicity throughout southwestern Turkey and incorporate these improved hypocentres in an updated focal mechanism compilation. The southwestern Fethiye–Burdur trend is dominated by ESE–WNW trending normal faulting, even though most faults evident in the topography strike NE–SW. This hints at a recent change in regional strain, perhaps related to eastward propagation of the Gökova graben into the area or to rapid subsidence of the Rhodes basin. The northeastern Fethiye–Burdur trend is characterized by orthogonal normal faulting, consistent with radial extension and likely responsible for the distinct physiography of Turkey's Lake District. We find that the 1971 M w 6.0 Burdur earthquake likely ruptured a NW-dipping normal fault in an area of indistinct geomorphology near Salda Lake, contradicting earlier studies that place it on well-expressed faults bounding the Burdur basin, and further highlighting how damaging earthquakes are possible on faults that would prove difficult to identify beforehand. Overall, our results support GNSS-derived kinematic models that depict a mix of uniaxial and radial extension throughout southwestern Turkey, with no evidence from focal mechanisms for major, active strike-slip faults anywhere along the Fethiye–Burdur trend. Normal faulting orientations are consistent with a stress field driven primarily by contrasts in gravitational potential energy between the elevated Anatolian plateau and the low-lying Rhodes and Antalya basins. [ABSTRACT FROM AUTHOR]

The Nordland region, Northern Norway, situated in an intraplate continental setting, has the highest seismicity rate in mainland Norway. However, the exact cause of seismicity in this region is still debated. Better understanding of factors that influence the seismicity in Nordland can help increase knowledge of intraplate seismicity in general. Here, we address this problem with the aid of a new high-resolution 3-D VP and VP / VS ratio images of the crust in Nordland using seismic traveltime tomography. These images show the existence of a localized, 10–15 km Moho step that runs parallel to the coast. The north–south extent of this step coincides with the region that exhibits the highest rates of seismicity. Focal mechanisms of selected earthquakes computed in this study are dominated by normal and oblique-normal, indicating a coast-perpendicular extension. The coast-perpendicular extensional stress regime deviates from the regional compression imposed by the ridge push from the North Atlantic. This deviation is thought to stem from the additional interference with local flexural stress caused by sediment redistribution and glacial isostatic adjustment, and possibly exacerbated by gravitational potential energy stress associated with the Moho step. The deformation due to the extensional regime is localized on pre-existing faults and fractures along the coastline. The tomography result shows that two distinct seismic swarms occurred in the coastal area with low VP and variable VP / VS ratio anomalies, pointing towards fractured crust and possibly the presence of fluids. The existence of fluids here can change the differential stress and promote seismic rupture. [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]

As early as the Palaeozoic era, the Tianshan Mountains constituted a vast tectonic convergent zone within the Eurasian Continent; by this time, they had already experienced multiple intense crustal movements. The Indian Plate is currently advancing northwards by more than 50 mm yr–1; it affects the Tianshan Mountains even though they are located more than 1000 km away. However, many of the geological issues of the Tianshan Mountains, especially in eastern Tianshan, which remains an active tectonic zone today, are not yet fully understood. There have been few studies into the seismogenic characteristics of this intense seismic zone. Here, the full gravity gradient tensor (GGT) within the eastern Tianshan Mountains was calculated using a stable spatial frequency domain algorithm, based on high-spatial resolution (7.2 arcsec or approximately 200 m) Global Gravity Model plus (GGMplus) surface gravity data. In addition, the features of the geological structures in the eastern Tianshan Mountains were interpreted using different combinations of GGT components. Moreover, the possibilities of using different combinations of GGT components to identify the distributions and strikes of faults was discussed regarding the potential earthquake rupture risks in this study area. The results show that (1) the distributions and strikes of the main fault zones in the study area are highly consistent with the linear geological features within the mountains, as extracted from a combination of different GGT components, including terrain effects. (2) Removing the terrain effects revealed that the different components of GGT (derived from the complete Bouguer gravity anomaly) vary gently, showing few linear geological features that could be extracted. This implies that satellite gravity data are not sufficient to analyse the characteristics of small underground geological structures (spatial scales less than ∼10 km). (3) Comparing the spatial distributions of fault traces depicted at different ages with the linear geological features of the study area revealed that discovering more unknown faults increased the coincidence ratio of linear geological features to fault traces in the area (from 27 to 40 per cent). In conclusion, the findings of this study represent a valuable reference for further revealing the seismogenic characteristics of this area, where there is a lack of detailed surface fault structure measurements due to its inaccessibility. The proposed method could also recognize faults in other similar areas with harsh conditions that are not feasible for ground-based surveys, such as forests and glaciers. [ABSTRACT FROM AUTHOR]

Although the Dead Sea Transform (DST) fault system has been extensively studied in the past, little has been known about the present-day kinematics of its southernmost portion that is offshore in the Gulf of Aqaba. Here, we present a new GPS velocity field based on three surveys conducted between 2015 and 2019 at 30 campaign sites, complemented by 11 permanent stations operating near the gulf coast. Interseismic models of strain accumulation indicate a slip rate of |$4.9^{+0.9}_{-0.6}~\mathrm{ mm}\,\mathrm{ yr}^{-1}$| and a locking depth of |$6.8^{+3.5}_{-3.1}~\mathrm{ km}$| in the gulf's northern region. Our results further indicate an apparent reduction of the locking depth from the inland portion of the DST towards its southern junction with the Red Sea rift. Our modelling results reveal a small systematic left-lateral residual motion that we postulate is caused by, at least in part, late post-seismic transient motion from the 1995 M W 7.2 Nuweiba earthquake. Estimates of the moment accumulation rate on the main faults in the gulf, other than the one that ruptured in 1995, suggest that they might be near the end of their current interseismic period, implying elevated seismic hazard in the gulf area. [ABSTRACT FROM AUTHOR]

The 26 May 2019 M W 8.0 Peru intraslab earthquake ruptured the subducting Nazca Plate where the dip angle of the slab increases sharply and the strike angle rotates clockwise from the epicentre to north. To obtain a detailed seismic source model of the 2019 Peru earthquake, including not only the rupture evolution but also the spatiotemporal distribution of focal mechanisms, we performed comprehensive seismic waveform analyses using both a newly developed flexible finite-fault teleseismic waveform inversion method and a back-projection method. The source model revealed a complex rupture process involving a back-propagating rupture. The initial rupture propagated downdip from the hypocentre, then unilaterally northward along the strike of the slab. Following a large slip occurring ∼50–100 km north of the hypocentre, the rupture propagated bilaterally both further northward and back southward. The spatial distribution of focal mechanisms shows that the direction of T -axis azimuth gradually rotated clockwise from the epicentre northward, corresponding to the clockwise rotation of the strike of the subducting Nazca Plate, and the large-slip area corresponds to the high-curvature area of the slab iso-depth lines. Our results show that the complex rupture process, including the focal-mechanism transition, of the Peru earthquake was related to the slab geometry of the subducting Nazca Plate. [ABSTRACT FROM AUTHOR]

The 2016 M w 7.8 Kaikoura, New Zealand, earthquake occurred along the eastern margin of the transition region between active subduction in the North Island and oblique collision in the South Island. To infer crustal properties, we imaged Q (1/seismic attenuation) by combining selected M > 3.5 aftershocks with data from previous Q models. For 158 distributed aftershocks, we fit spectral decay on temporary stations and all Geonet stations, providing 6194 t * p and 19 497 t * s. Considering the varied rheology and faults, we also used 2.5-D numerical models to study ductile strain development. The complex earthquake ruptured an ∼180-km-long zone of multiple faults, which involved jumping around the complicated eastern end of the Hope fault, without significant slip on the Hope fault. The Qs and Qp results show features in the upper and lower crust which correlate to the distribution and types of fault rupture. This earthquake involved numerous faults over a region of greywacke crust, where the underlying high Q Cretaceous slab is about 30-km depth. It initiated with ∼5-m slip on the Humps fault in a region of background seismicity and low Q lower crust, adjacent to the Hope fault. The central region near Kaikoura shows a high Q crustal block, which appears to have inhibited rupture; as the rupture progressed over several small faults to jump offshore of the apparently strong block. Underlying the Kaikoura greywacke crust, below 20-km depth, there is a 40-km-long region of increased Vp, Qs and gravity, which likely represents an intraplate plutonic complex emplaced into the Hikurangi Plateau, forming an elevated section which influences deformation. In the northern section, in a region with relatively uniform moderate to low Q , the earthquake evolved into the relatively continuous ∼80-km-long major rupture along the Jordan, Kekerengu and Needles faults, with ∼6–20 m dextral slip at depth and surface displacements of ∼10 m dextral and ∼2 m vertical. The northern progression of the rupture stopped when it approached an abrupt change to high Q crust across Cook Strait. At 20–30-km depth northwest of the rupture, deeper zones with low Q are consistent with regions of distributed ductile shear and creep where the observed afterslip may have occurred, where the underlying slab is 25–40 km deep. The numerical model shows that ductile deformation localizes in this area of lower crust above the relatively strong slab, connecting outer faults (Kekerengu) to inland faults (Clarence, Awatere, Wairau), and demonstrates that no subduction thrust is required under the Marlborough region. [ABSTRACT FROM AUTHOR]

In order to generate plate tectonics, the near surface layer should not be too strong, but the causes for not-so-strong near surface layer remains unclear. We conduct mantle convection modelling in the spherical geometry to investigate the influence of the strength of the near surface layer. We explore a range of friction coefficients including the static high friction coefficient (∼0.6) as well as the reduced friction coefficients by fast fault motion in earthquakes. When the friction coefficient is low enough (<0.03), the surface layer is yielded by the convective stress, and the style of mantle convection appears the mobile-lid mode (plate tectonics style of convection). This style is relevant for the Earth where fault motion is unstable because of the low surface temperature. In contrast, for a high friction coefficient, the surface layer is too strong, generating the stagnant-lid mode. This case corresponds to Venus where fault motion is stable because of high surface temperature. Our calculations show that, in plate tectonic style of convection, the mantle convection is likely to be more vigorous, inducing the high convective stress that helps the operation of plate tectonics. In contrast, when stagnant-lid mode of convection appears, the convective vigor is likely to be low, inducing the low convective stress. Therefore, in each case, the interplay between the surface strength and convective stress tends to maintain the same mode of convection in a self-consistent way. We also investigate the relationship between mantle temperature and heat flux for two different modes of convection upon a change in friction coefficient. We found that the heat flow associated with mobile lid convection caused by low friction is less sensitive to the mantle temperature compared to a conventional mantle convection model, where the heat flow is highly sensitive to mantle temperature. This provides a possible mechanism to solve the thermal runaway paradox. [ABSTRACT FROM AUTHOR]

The Groningen gas field located in the northern Netherlands is the largest gas field in Europe. Many induced earthquakes have occurred since the gas extraction commenced in the 1960s, especially in the last two decades, which have caused concerning social and economic problems. To better quantify the seismic hazards caused by the induced earthquakes, the near surface attenuation in the Groningen area needs to be characterized from actual earthquakes. The spectral ratio method based on multiwindow analysis has proven to be a powerful tool to estimate the S -wave attenuation parameters of shallow sediments. In this study, waveforms from earthquake events recorded by the dense network of 70 boreholes, each of which is equipped with four geophones are used to estimate the S -wave attenuation parameters of shallow sediments. There are strong lateral variations in the near-surface attenuation, which is structurally similar to the S -wave model from a previous study in the same area. Besides, the calculated |$Q_S^{ - 1}$| values are inversely proportional to the frequency and depth. The averaged |${\rm{\ }}Q_S^{ - 1}$| values for depths between 0–150 m and 0–200 m are close to those of the soft soil Sendai basin in Japan. The |$Q_S^{ - 1}$| values from this study have been used to model a surface peak ground acceleration (PGA) map. As expected, both near surface |$Q_S^{ - 1}$| values and source mechanism have a profound effect on the modelled PGA. [ABSTRACT FROM AUTHOR]

Metropolitan France is a region of slow tectonic deformation with sparse seismicity. On 11 November 2019, the M L 5.4 Le Teil earthquake became the largest seismic event recorded in the last 16 yr. This event was recorded by the national seismic networks and also by a wide variety of other geophysical techniques including infrasound and InSAR measurements. These complementary technologies offer the opportunity to investigate in detail the earthquake source characteristics and the associated ground motion attenuation. Both seismic waveform inversions and InSAR interferogram reveal a shallow rupture on a reverse fault with an associated moment magnitude of 4.8–4.9. Infrasound signals also provide fast evidences pointing towards the area of ground surface displacements, which coincides with La Rouvière fault, in the Cévennes fault system, known as a formerly active normal fault during the Oligocene. The very significant amount of seismic records also helps towards validating the GMPE laws available for the region. This multitechnology characterization documents the kinematics of this rare example of shallow intraplate fault reactivation. [ABSTRACT FROM AUTHOR]

As the largest and most active intracontinental orogenic belt on Earth, the Tien Shan (TS) is a natural laboratory for understanding the Cenozoic orogenic processes driven by the India–Asia collision. On 19 January 2020, a M w 6.1 event stuck the Kalpin region, where the southern frontal TS interacts with the Tarim basin. To probe the local ongoing orogenic processes and potential seismic hazard in the Kalpin region, both interseismic and instantaneous deformation derived from geodetic observations are used in this study. With the constraint of interseismic global navigation satellite system (GNSS) velocities, we estimate the décollement plane parameters of the western Kalpin nappe based on a 2-D dislocation model, and the results suggest that the décollement plane is nearly subhorizontal with a dip of ∼3° at a depth of 24 km. Then, we collect both Sentinel-1 and ALOS-2 satellite images to capture the coseismic displacements caused by the 2020 Kalpin event, and the interferometric synthetic aperture radar (InSAR) images show a maximum displacement of 7 cm in the line of sight near the epicentral region. With these coseismic displacement measurements, we invert the source parameters of this event using a finite-fault model. We determine the optimal source mechanism in which the fault geometry is dominated by thrust faulting with an E–W strike of 275° and a northward dip of 11.2°, and the main rupture slip is concentrated within an area 28.0 km in length and 10.3 km in width, with a maximum slip of 0.3 m at a depth of 6–8 km. The total released moment of our preferred distributed slip model yields a geodetic moment of 1.59 × 1018 N |$ \cdot $| m, equivalent to M w 6.1. The contrast of the décollement plane depth from interseismic GNSS and the rupture depth from coseismic InSAR suggests that a compression still exists in the Kalpin nappe forefront, which is prone to frequent moderate events and may be at risk of a much more dangerous earthquake. [ABSTRACT FROM AUTHOR]

The 2011 M w 9.0 Tohoku-oki earthquake ruptured the subduction megathrust fault in the central Japan Trench. We investigated the aftershock activity in the southern Japan Trench to the south of the main rupture area using ocean bottom seismographs deployed both landward and seaward of the trench. In the trench-outer rise region seaward of the trench axis, we identified several ∼100-km-long linear earthquake trends both parallel and oblique to the southern Japan Trench. The earthquake trend oblique to the southern Japan Trench is a southward extension of the trench-parallel linear earthquake trend in the central to northern Japan Trench. The trench-parallel normal-faults in the trench-outer rise region could extend linearly, despite the change of the trench strike from N–S to NNE–SSW to the south of the main rupture area. Normal-faults oblique to the trench should be considered as substantial parts of large intraplate normal-faulting earthquakes. In addition, intraplate seismicity coinciding with the lower velocity oceanic mantle suggest that the structure heterogeneity would be indicative of normal-faults extending into the mantle. In the trench landward area, earthquake activity showed along-trench variations. Earthquakes along the shallow megathrust interface near the trench were observed south of 37°N. These shallow near-trench regular earthquakes, which are located close to the episodic tremors and temporally correlated with the tremor activities, suggest that the afterslip on the plate interface likely extended to the shallow plate interface close to the trench axis. Smaller spatial scale structure heterogeneity, such as the thickness variation in the channel-like low-velocity sedimentary unit, likely relate to the proximity of the regular earthquakes and slow slip which results in the formation of diverse slip behaviours in the shallow subduction zone of the southern Japan Trench. [ABSTRACT FROM AUTHOR]

The ∼ M 6 1799 Bouin earthquake is considered as one of the largest earthquakes to have struck Western France. However, the seismogenic source potentially responsible for this event remain marginally documented. We present results from a focused offshore-onshore multidisciplinary survey in its meizoseismal area in order to identify the fault segments that potentially ruptured during this earthquake. Based on macroseismic data and the geology, we focused our study on the so-called Machecoul Fault as a potential source of the 1799 Bouin event. Our survey includes extensive high-resolution seismic reflection, high resolution bathymetry and a 1-yr seismological survey. These data were combined with existing topography, onshore gravity data and drill data to document the geometry of the Marais Breton/Baie de Bourgneuf basin, the past tectonic activity and the current local microearthquakes at depth along its bounding faults. Offshore and onshore observations suggest a recent activity of the segmented Machecoul Fault bounding the basin to the North. Offshore, the planar contact between the Plio-Quaternary sediments and the basement along the fault trace as well as the thickening of these sedimentary units near this contact suggests tectonic control rather than erosion. Onshore, the recent incision of the footwall of the fault suggests a recent tectonic activity. The temporary local seismological experiment deployed between 2016 and 2017 recorded a diffuse microseismicity down to the depth of 22 ± 5 km along the southward dipping Machecoul Fault, associated with predominantly normal fault mechanisms. Altogether, these results suggest that the Machecoul Fault is a serious candidate for being the source of the historical Bouin 1799 earthquake. [ABSTRACT FROM AUTHOR]

Two key parameters control the localization of deformation and seismicity along and surrounding crustal faults: the strength and roughness of the pre-existing fault surface. Using 3-D discrete element method simulations, we investigate how the anisotropy and amplitude of roughness control the mechanical behaviour of healed faults within granite blocks during quasi-static triaxial compression. We focus on models in which the uniaxial compressive strength of the healed faults is about 25 per cent of that strength of the surrounding host rock. These models provide insights into the evolution of fracture network localization, fault roughness, gouge production, fault slip and stress concentrations along initially healed faults of varying roughness. In contrast to expectations, the uniaxial compressive strengths of models that host faults with root-mean-squared roughness amplitudes of 0.2–1.4 mm do not vary more than the change produced by variations in particle packing. To assess if this lack of influence arises from the evolving roughness of the faults, we track the roughness amplitudes parallel and perpendicular to the downdip direction throughout fault failure and slip. The de facto roughness does not provide an explanation for the lack of influence of roughness on compressive strength because the roughness of the faults does not evolve to similar values with slip. Rather, smoother faults remain smoother than rougher faults throughout the simulation. However, the rougher faults produce larger volumes of gouge than the smoother faults. The gouge lubricates the fault and thereby reduces the influence of roughness on compressive strength. These observations suggest that fault topography and the asperities that build this topography do not exert a significant impact on deformation. To quantify the influence of asperities on slip, we calculate correlation coefficients between the fault surface topography and components of the slip vectors. The observed negative correlation coefficients between the fault topography and fault-plane parallel slip quantify the degree to which asperities slow slip in the downdip direction. The observed positive correlation coefficients between the topography and fault-plane perpendicular movement quantify the degree to which asperities promote opening. Thus, this analysis shows how asperities control slip by acting as speed bumps that hinder fault-plane parallel slip and promote fault-plane normal opening as the healed faults slide. The asperities do not significantly control fault movement during the unlocking and failure of the healed faults, but only following the peak axial stress as the faults slide and damage zones develop. These models thus provide unparalleled access to the dynamics of reactivated healed faults. [ABSTRACT FROM AUTHOR]

The M w 6.4 26 November 2019, earthquake has been the strongest in the last decades in Albania, causing damages of intensity VIII to IX EMS in the epicentral region around Durres. The region north of Durres has experienced a maximum uplift of ca. 11 cm, based on SAR interferometry, which represents the main environmental effect induced by the earthquake. Other coseismic environmental effects were liquefaction mostly in the coastal area north and south of Durres, lateral spread in the Erzen river banks and possibly minor rock falls. As a whole, the observed effects are indicative of an intensity VIII to IX in the ESI scale. The rupture parameters that best fits the earthquake data (seismic moment, hypocentre depth, GPS data, deformation field from SAR interferometry), based on Coulomb modelling, show a reverse slip of 0.6 m on a NW–SE trending plane dipping 25° northeast, 20 km long and ca. 12 km wide, from 19.5 to ca. 15 km deep. The surface projection of the upper tip of the rupture is on the coast north of Durres. The inferred Coulomb stress change does not impose any significant load on the surrounding major faults, that is Kruja thrust, Lezha transfer fault, and the offshore thrust fault responsible for the 1979 M w 7.1 Montenegro earthquake. The historical earthquakes and the regional tectonic setting, dominated by plate collision and important transfer fault zones suggest that the last earthquake might not be representative of the actual maximum seismic and surface faulting hazards in northwestern Albania, a region of fast industrial and touristic growth. This calls for detailed active tectonics studies with a palaeoseismological perspective in the region surrounding the epicentral area, where the two main towns in Albania lie. [ABSTRACT FROM AUTHOR]

Earthquake source parameters such as seismic stress drop and corner frequency are observed to vary widely, leading to persistent discussion on potential scaling of stress drop and event size. Physical mechanisms that govern stress drop variations are difficult to evaluate in nature and are more readily studied in controlled laboratory experiments. We perform two stick-slip experiments on fractured (rough) and cut (smooth) Westerly granite samples to explore fault roughness effects on acoustic emission (AE) source parameters. We separate large stick-slip events that generally saturate the seismic recording system from populations of smaller AE events which are sensitive to fault stresses prior to slip. AE event populations show many similarities to natural seismicity and may be interpreted as laboratory equivalent of natural microseismic events. We then compare the temporal evolution of mechanical data such as measured stress release during slip to temporal changes in stress drops derived from AEs using the spectral ratio technique. We report on two primary observations: (1) In contrast to most case studies for natural earthquakes, we observe a strong increase in seismic stress drop with AE size. (2) The scaling of stress drop with magnitude is governed by fault roughness, whereby the rough fault shows a more rapid increase of the stress drop–magnitude relation with progressing large stick-slip events than the smooth fault. The overall range of AE sizes on the rough surface is influenced by both the average grain size and the width of the fault core. The magnitudes of the smallest AE events on smooth faults may also be governed by grain size. However, AEs significantly grow beyond peak roughness and the width of the fault core. Our laboratory tests highlight that source parameters vary substantially in the presence of fault zone heterogeneity (i.e. roughness and narrow grain size distribution), which may affect seismic energy partitioning and static stress drops of small and large AE events. [ABSTRACT FROM AUTHOR]

On 2018 September 28, 18:02:44 local time, the magnitude 7.5 earthquake accompanied by a tsunami and massive liquefaction devastated Palu region in Central Sulawesi, Indonesia. Comprehensive post-disaster surveys have been conducted, including field survey of surface ruptures, LiDAR, multibeam-bathymetry mapping and seismic-reflection survey. We used these data to map fault ruptures and measure offsets accurately. In contrast to previous remote-sensing studies, suggesting that the earthquake broke an immature, hidden-unknown fault inland, our research shows that it occurred on the mappable, mature geological fault line offshore. The quake ruptured 177-km long multifault segments, bypassing two large releasing bends (first offshore and second inland). The rupture onset occurred at a large fault discontinuity underwater in a transition zone from regional extensional to compressional tectonic regimes. Then, it propagated southward along the ∼110-km submarine fault line before reaching the west side of Palu City. Hence, its long submarine ruptures might trigger massive underwater landslides and significantly contribute to tsunami generation in Palu Bay. The rupture continued inland for another 67 km, showing predominantly left-lateral strike-slip up to 6 m, accompanied by a 5–10 per cent dip-slip on average. The 7 km sizeable releasing bend results in a pull-apart Palu basin. Numerous normal faults occur along the eastern margin. They cut the Quaternary sediments, and some of them ruptured during the 2018 event. Our fault-rupture map on mature straight geological fault lines allows the possible occurrence of early and persistent 'supershear', but significant asperities and barriers on segment boundaries may prohibit it. [ABSTRACT FROM AUTHOR]

We investigate the coseismic and post-seismic deformation due to the 6 February 2018 M w 6.4 Hualien earthquake to gain improved insights into the fault geometries and complex regional tectonics in this structural transition zone. We generate coseismic deformation fields using ascending and descending Sentinel-1A/B InSAR data and GPS data. Analysis of the aftershocks and InSAR measurements reveal complex multifault rupture during this event. We compare two fault model joint inversions of SAR, GPS and teleseismic body waves data to illuminate the involved seismogenic faults, coseismic slip distributions and rupture processes. Our preferred fault model suggests that both well-known active faults, the dominantly left-lateral Milun and Lingding faults, and previously unrecognized oblique-reverse west-dipping and north-dipping detachment faults, ruptured during this event. The maximum slip of ∼1.6 m occurred on the Milun fault at a depth of ∼2–5 km. We compute post-seismic displacement time series using the persistent scatterer method. The post-seismic range-change fields reveal large surface displacements mainly in the near-field of the Milun fault. Kinematic inversions constrained by cumulative InSAR displacements along two tracks indicate that the afterslip occurred on the Milun and Lingding faults and the west-dipping fault just to the east. The maximum cumulative afterslip of 0.4–0.6 m occurred along the Milun fault within ∼7 months of the main shock. The main shock-induced static Coulomb stress changes may have played an important role in driving the afterslip adjacent to coseismic high-slip zones on the Milun, Lingding and west-dipping faults. [ABSTRACT FROM AUTHOR]

The unbroken section of the North Anatolian Fault beneath the Sea of Marmara is a major source of seismic hazard for the city of İstanbul. The northern and currently the most active branch, the Main Marmara Fault (MMF), is segmented within a shear zone and exhibits both partially creeping and locked behaviour along its 150 km length. In 2019 September, a seismic activity initiated near MMF, off-coast the town of Silivri, generating 14 earthquakes ≥  M w 3.5 in a week. The M w 5.8 Silivri earthquake, is the largest in the Marmara Sea since the 1963 M w 6.3 Çınarcık earthquake. Our analyses reveal that the activity started in a narrow zone (∼100 m) and spread to ∼7 km following an M w 4.7 foreshock within ∼2 d. The distribution of relocated aftershocks and the focal mechanisms computed from regional waveforms reveal that the M w 5.8 earthquake did not occur on the MMF, but it ruptured ∼60° north-dipping oblique strike-slip fault with significant thrust component located on the north of the MMF. Finite-fault slip model of the main shock shows 8 km long rupture with directivity toward east, where the ruptured fault merges to the MMF. The narrow depth range of the slip distribution (10–13 km) and the aftershock zone imply that the causative fault is below the deep sedimentary cover of the Marmara Basin. The distribution of aftershocks of the M w 5.8 event is consistent with Coulomb stress increase. The stress changes along MMF include zones of both stress decrease due to clamping and right-lateral slip, and stress increase due to loading. [ABSTRACT FROM AUTHOR]

We present 3-D Vp and Vp / Vs tomographic images for depths down to 22 km along with precise earthquake locations beneath the Sea of Marmara using the seismological data sets collected during the Ocean Bottom Seismometer (OBS) experiments conducted in 2001 and 2006. The unique data set collected by 52 OBSs and augmented by 14 land stations include 3852 P and 2643 S arrival times from 434 earthquakes and 4744 P arrival times from 557 air-gun shot records. From the tomographic depth sections and relocated seismicity, we identify two shear zones which are marked by narrow-width (∼10 km), low Vp (4.3–5.3 km s–1) and high Vp / Vs ratio (∼2.0) anomalies coinciding with the surface trace of the Main Marmara Fault (MMF) and extending near vertically from 8 km depth below the basins into the deeper part of the crust. We consider these shear zones to be the strike-slip deformation signature of the MMF at depth. The western shear zone imaged lies between the Western High and the eastern end of the Central Basin including the 60-km-long segment of the MMF and contains higher seismicity and is visible in the 8–22 km depth range. The eastern shear zone, coinciding with the NW–SE trending surface trace of the MMF, is imaged beneath the Çınarcık Basin with lower seismicity and tomographic resolution. Furthermore, these shear zones may contain fractures filled with deep crustal fluids allowing partial aseismic creep. Within these shear zones, we also identify small-scale, high Vp (6 km s–1) anomalies with low seismicity indicative of strong patches of the fault at depth that can be considered as asperities. Between these shear zones, our tomographic images clearly show a central zone that has homogenously high Vp (∼6.0–6.5 km s–1) and low Vp / Vs ratio (∼1.7) over a large area including the 50-km-long central segment of the MMF lying between the eastern end of the Central Basin and western end of the Çınarcık Basin. Thus, we infer that the central MMF is in a locked state based on its high shear strength, low seismicity and a lack of detectable geodetic slip. Furthermore, a large earthquake might not nucleate on the central MMF since it may be strongly locked at the current level of stress loading, and thus it may behave as a seismic barrier or anti-asperity. On the other hand, the central MMF is a good candidate for a supershear rupture because it has nearly uniform elastic properties and low background seismicity indicating the homogeneity of friction and pre-stress across the fault, which are presented as the main characteristics of supershear fault ruptures. From the seismic hazard perspective, we speculate that a large earthquake may be expected to nucleate within either of the imaged shear zones with the possibility of the central MMF acting as a seismic barrier to stop or to join the rupture in a supershear mode. The imaged features have important implications for various aspects of seismic hazard and crustal dynamics in the Sea of Marmara region. [ABSTRACT FROM AUTHOR]

Here, we present the results of a kinematic slip model of the 2020 M w 6.7 Doğanyol-Sivrice, Turkey Earthquake, the most important event in the last 50 yr on the East Anatolian Fault Zone. Our slip model is constrained by two Sentinel-1 interferograms and by 5 three-component high-rate GNSS (Global Navigation Satellite System) recordings close to the earthquake source. We find that most of the slip occurs predominantly in three regions, two of them at between 2 and 10 km depth and a deeper slip region extending down to 20 km depth. We also relocate the first two weeks of aftershocks and find a distribution of events that agrees with these slip features. The HR-GNSS recordings suggest a predominantly unilateral rupture with the effects of a directivity pulse clearly seen in the waveforms and in the measure peak ground velocities. The slip model supports rupture propagation from northeast to southwest at a relatively slow speed of 2.2 km s−1 and a total source duration of ∼20 s. In the absence of near-source seismic stations, space geodetic data provide the best constraint on the spatial distribution of slip and on its time evolution. [ABSTRACT FROM AUTHOR]

The May 2018 activity at Kīlauea Volcano, Hawaii involved magma transport and dyke intrusion along the East Rift Zone (ERZ) and nucleation of the 4 May 2018 M 6.9 earthquake along the basal décollement of Kīlauea's mobile south flank. Combined Global Positioning Systems (GPS) and Interferometric Synthetic Aperture Radar (InSAR) measurements captured the deformation sequence associated with the dyke intrusion, main shock and eruption episode along the ERZ. The earthquake was encouraged by static stress changes from the preceding magma reservoir inflation, ERZ expansion and fault creep on the décollement downdip of the rupture. Slip models derived from the inversion of GPS displacements indicate peak coseismic slip of 2–3 m. Our model analyses, including of the pre-May 2018 deformation, suggest that prior to this event there was no slip on the section of the décollement that ruptured in the earthquake. The observed magma inflation, rapid fault creep on the décollement and coseismic rupture reflect complex cyclic interactions between the magmatic and faulting processes. [ABSTRACT FROM AUTHOR]

The effects of fault waviness on the fault slip modes are unclear. Laboratory study on the effects of the centimetre-scale fault contact distribution, which is mainly controlled by the fault waviness, on granodiorite stick-slip instabilities may help to unveil some aspects of the problem. The fast and slow stick-slip motions were separately generated in two granodiorite samples of the same roughness but different fault contact distributions in the centimetre scale in the laboratory. The experimental results show the following: (1) the fault with the small contact area and heterogeneous contact distribution generates fast stick-slip instabilities, while the fault with the large contact area and homogeneous contact distribution produces slow stick-slip events; (2) the nucleation processes of the fast stick-slip events are characterized by abrupt changes once the nucleation zones expand to the critical nucleation length that is observed to be shorter than the fault length, while the slow stick-slip events appear as a gradual evolution of the nucleation zones leading to total fault sliding. These indicate that, unlike the micron-scale fault contact distribution controlled by roughness, which depends mainly on the grain size of the abrasives used for lapping the fault surface, the centimetre-scale fault contact distribution, which depends mainly on the waviness of the fault surface profile, also plays an important role in the fault slip modes. In addition, the effects of the fault waviness on the fault friction properties are preliminarily analysed based on the rate- and state-dependent friction law. [ABSTRACT FROM AUTHOR]

Mesas, which consist of an elevated area of land with a flat top and steep cliffs at the sides, are one of the common geological formations present in the Los Alamos region. Previous research has shown that geological formations such as sedimentary canyons can amplify soil response during earthquakes. There have also been parametric studies to understand the response of an idealized and isolated mountain/canyon under inclined plane waves. In this study, a 2-D linear anelastic soil domain, with topography modelled after the Los Alamos region with many mesas and canyons, is considered to understand site-specific topographic effects in the presence of non-isolated topographical features. Various earthquake sources configurations ranging from point sources to finite faults with varying rupture length, dip angles and spatial slip distributions are considered. To isolate the effect of topography, three other soil domains—a homogeneous half-space, homogeneous half-space with mesas and canyons, and a computational domain with just the soil properties from the Los Alamos region on a flat soil domain is also considered. 2-D site-response analyses of these soil domains under earthquake excitation show that the free-field response of the soil can be amplified or de-amplified depending on the topography of the region and the location of the station relative to the fault. These studies also show that even relatively small mesas with height less than 100 m can significantly amplify the response (by a factor of 2 or more), which differ from the much smaller amplification factors (≤1.2) specified by standard building codes such as Eurocode-8 (2000) for topographical features with a similar slope. De-amplifications are also fairly common, especially close to the canyons. The results from this study agree qualitatively with those from the ambient vibration study conducted by Stolte et al. on a mesa from the Los Alamos region. Thus, such site-specific studies provide important insights into the variability of the topographic amplification factors within a region of interest. Such knowledge is important in the design of safety-related critical infrastructure located within that region. [ABSTRACT FROM AUTHOR]

In this paper, we model the crack growth in an elastic medium constituted by two welded half-spaces with different rigidities. We implement a 2-D boundary element method (BEM) computing shear and normal tractions acting on the crack and the slip accommodating stress drop from an arbitrary initial configuration to a final frictional configuration. The direction of crack growth follows the criterion of maximum energy release (strain and gravitational energy) provided that it overcomes the surface fracture energy and the work dissipated by friction. The energetic criterion leads to estimates of the dip angle of seismic faults depending on the amplitude of the initial stress and it includes the classical Anderson's results as a particular case. Moreover, in presence of a sharp rigidity contrast, the direction of crack growth is strongly deflected. The model simulates non-planar, complex, fault geometries, as in the case of detachment and listric faults and it explains the increase of dip angles for both normal and reverse faults, when they enter soft sedimentary layers. [ABSTRACT FROM AUTHOR]

The distribution of historic earthquakes in the Sumatra subduction zone reveals, in the forearc region, the intense seismic activity and frequent occurrences of M w > 8 earthquakes throughout the whole area. In contrast, the neighbouring region has less dense seismicity and no large earthquake greater than M w8 has been observed in the Java subduction zone. Such different seismic behaviours may be due to distinct degrees of the stress accumulation and release. In this study, the strength of plate coupling inferred from mantle lithosphere buoyancy (H m) estimation is used to explain the seismogenic behaviour in the Sunda–Andaman subduction zone. Strong and weak plate coupling status are obtained in the Sumatra and Java subduction zones, respectively. These results can explain the significant differences in seismogenic behaviours in the Sunda–Andaman subduction zone. In assessing the global implications of this finding, we observe that uplifted serpentinized forearc mantle peridotite is the critical phenomenon in weak plate coupling cases and leads to a limit on the width of the coupling zone. Strong plate coupling can cause a relatively low gravity anomaly as well as a negative trench-parallel gravity anomaly (TPGA) in the forearc regions and correlates well with the occurrence of large earthquakes, whereas weak plate coupling can cause a positive TPGA and constrain the potential occurrence of large earthquakes. [ABSTRACT FROM AUTHOR]

We present a numerical method for simulating both single-event dynamic ruptures and earthquake sequences with full inertial effects in antiplane shear with rate-and-state fault friction. We use the second-order form of the wave equation, expressed in terms of displacements, discretized with high-order-accurate finite difference operators in space. Advantages of this method over other methods include reduced computational memory usage and reduced spurious high frequency oscillations. Our method handles complex geometries, such as non-planar fault interfaces and free surface topography. Boundary conditions are imposed weakly using penalties. We prove time stability by constructing discrete energy estimates. We present numerical experiments demonstrating the stability and convergence of the method, and showcasing applications of the method, including the transition in rupture style from crack-like ruptures to slip pulses for strongly rate-weakening friction and the simulation of earthquake sequences in a viscoelastic solid with a fully dynamic coseismic phase. [ABSTRACT FROM AUTHOR]

Slow slip events (SSEs) and other forms of slow and transient fault slip are becoming increasingly recognized as important, due to their influence on seismicity and potential to provide information on fault zone properties. The majority of our current knowledge of slow fault slip has been obtained from geodetic and seismologic field measurements, but laboratory data of slow slip are comparatively scarce. Here, I present the results of laboratory friction experiments conducted on 11 natural samples from major fault zones around the world, all obtained by scientific drilling. The experiments are conducted water saturated, at room temperature and 10 MPa effective normal stress, representative of in-situ conditions on the shallow portions of fault zones from which the samples were recovered and where slow slip is known to occur. A key component of these experiments is shearing at realistically slow driving rates of cm yr–1, accurately simulating tectonic driving rates. In most samples, these cm yr–1 driving rates produce laboratory SSEs, which are instances of accelerating slip accompanied by a stress drop. The peak slip velocities and stress drops measured in these laboratory SSEs are comparable with those of natural SSEs measured or estimated from geodetic data. A strong correlation is observed between reduced pre-SSE velocity and higher peak slip velocity for the entire laboratory SSE data set. In contrast to the velocity data, significant scatter is observed in the percentage stress drop measurements. The source of this scatter can be attributed to samples with a significant expandable clay component, which tend to exhibit larger stress drops. Results of velocity-stepping tests at cm yr–1 rates show a tendency for velocity-weakening friction not observed at higher sliding velocities, and that the materials with lower values of the rate-dependent friction parameter a–b tend to produce faster SSEs. Critical stiffness analyses within the framework of rate-and-state friction laws show that most of the SSEs observed in this study do not satisfy the condition for slip instability. The SSEs are more consistent with accelerating stable slip, although the stiffness condition allowing such behaviour is not always satisfied. Considering the laboratory SSEs to be accelerating stable slip, I present a conceptual model for their nucleation. Key elements of the model are a healing-dominated departure from steady-state causing partial locking and velocity decrease, followed by a transition to a velocity-dominated phase representing the actual slip event. The model is consistent with observations from geodetic measurements and the experimental observations in this study. In general, characteristics of SSE-producing fault portions such as the ability to strengthen and store elastic strain energy released as stress drops may be expected to enhance coseismic slip from remotely nucleating earthquakes, an effect which may be quite limited but should be investigated further. [ABSTRACT FROM AUTHOR]

Oceanic plates play an important role in transporting water from Earth's surface into the mantle. The trench–outer rise region is one of the places where hydration of the oceanic plate occurs. We investigated oceanic plate hydration in the trench–outer rise region of the Japan Trench on the basis of the seismic velocity structure as estimated by a tomographic analysis of data from ocean bottom seismograph arrays. This analysis demonstrates a reduction in P- wave and S- wave velocity (Vp and Vs) within the uppermost oceanic mantle of the incoming Pacific plate in the direction towards the trench axis. This seismic velocity reduction, which was observed in at least the uppermost 15 km of the mantle, is consistent with hydration of the uppermost mantle due to infiltration of water through the oceanic crust along bending-related normal faults. Under the assumption that the velocity reduction is caused solely by serpentinization of the mantle, the Vp changes suggest that water content increases by about 1.1 to 2.9 wt per cent at depths as great as 15 km below the oceanic Moho. An increase in water-filled fractures can also account for this velocity reduction with a smaller amount of water. Aside from the general trend of velocity reduction towards the trench axis, the Vp structure of the incoming plate shows spatial heterogeneity related to mantle seismicity, in that earthquakes within the uppermost mantle generally coincide with areas of low Vp. This spatial seismic heterogeneity is consistent with localized hydration or formation of water-filled fractures along bending-related normal faults. Hence, a low- Vp anomaly within the uppermost oceanic mantle could be an indicator of possible sites of large outer-rise normal-faulting earthquakes in which the rupture extends into the mantle. [ABSTRACT FROM AUTHOR]

On 2012 August 11, an earthquake doublet (M w6.5 and M w6.3), separated in time by 11 min, occur in the northwest of Iran. The hypocentres of these earthquakes are close (∼6 km) and located near the cities of Ahar and Varzaghan. The rupture process of both main shocks is retrieved by inverting the near-field strong motions data and using the elliptical subfault approximation method. Our calculations show that the two earthquakes are occurring on two distinct fault planes: the first main shock (M1) has nucleated at a depth of ∼8.5 km, and is located ∼4 km east of the eastern termination of the E-W trending surface rupture. The slip reaches the ground surface west of the hypocentre on an E-W striking fault (N88°E) that dips almost vertically (80°S). This earthquake exhibits a right-lateral strike-slip mechanism. The entire slip is imaged on a single patch that ruptures with an average speed of 2.4 km s−1. The rupture duration is ∼5.6 s and the earthquake releases a seismic moment of ∼8.41E + 18 N·m. The slip reaches the surface with a right-lateral dislocation value of ∼1 m, which is consistent with the observed surface rupture. About 11 min later, the second main shock (M2) nucleates ∼5 km to the west and 4 km to the north with respect to the hypocentre of the M1, and at a depth of ∼16.5 km. The M2 rupture evolves toward shallower depths and to the west on an ENE-WSW oriented fault plane (strike ∼256°) with a dip of ∼60° northward. The slip is essentially distributed on two distinct patches with strike-slip and reverse mechanisms, respectively. The first patch has a pure right-lateral strike-slip mechanism, and ruptures at a relatively fast speed of over 2.8 km s−1, and last for about 2.6 s until it reaches the second patch. The latter has a reverse mechanism (rake∼112°) and extends the rupture toward shallow depths, and to the west at a speed of ∼2.5 km s−1, and its rupture lasts for ∼2.5 s. The top of the slip distribution of M2 stops at a depth of ∼8 km. We observe that aftershocks surround the M1 and most of the M2 slip models. They are not distributed in the region of high slip (∼3.1 m) of M1. We show that the rupture of M2 is controlled by the static Coulomb stress changes caused by M1, with the maximum slip of M2 located in the positive Coulomb stress caused by M1. The M2 rupture stops where it reaches the area of high negative Coulomb stress change (over −10 bars). The cumulative Coulomb stress fields of both main shocks show a transfer of positive static Coulomb stress change of >0.1 bars on the eastern segment of the North Tabriz Fault. This segment did not rupture since the 1721 M∼7.6–7.7 event that has destroyed the city of Tabriz, and that currently hosts 2 million people. The occurrence of this earthquake doublet with different mechanisms reveals the slip partitioning of the oblique convergence regime of NW Iran on the Ahar–Varzaghan complex fault system. [ABSTRACT FROM AUTHOR]

The formation of fragments due to avalanche-like growth of damage under impulsive forces is a process central to numerous studies ranging from shaped charge jet break up and rock blasting to bolide impacts, and, more recently, earthquake rupture. In the latter case, pulverized rocks found millimetres to tens of metres from the principal slip zones of large faults have been associated with fast, even supershear, rupture propagation. It has been postulated that earthquake source characteristics directly affect the degree of fragmentation, and the study of fragment size distribution may shed light on the energy budget of individual earthquakes as well as long-term effects on fault zone properties. The actual fragmentation process, and the partitioning of dissipated energy at fast loading rates, however, is still enigmatic. We use modified Split Hopkinson Pressure Bar experiments, in which we can control stressing rate, amplitude and duration, as a laboratory analogue for the complex natural prototype source processes. In our experiments, we characterize the velocity distribution of ejected fragments from Westerly Granite specimens resulting in a range of fragmentation states, from weakly fragmented to pulverized. Analysis of the velocity distributions (and the related kinetic energy) reveals spatial domains that are free of ejected fragments; these so-called 'zero kinetic energy modes' are related to the fragmentation state: increasing fragmentation corresponds to a reduction of zero mode domains. The evolution of these zero modes with strain rate reveals that the transition from low strain rate fracturing to high strain rate pulverization is a smooth, continuous transition, rather than a sharp boundary. Furthermore, our results yield important insights into the process of fragmentation in earthquake process zones, including how dissipated energy is partitioned during fragmentation, and indicate that delocalization of energy is systematically coupled with source parameters. [ABSTRACT FROM AUTHOR]

Interpretation of surface fault scarps and palaeoseismic trenches is a key component of estimating fault slip rates, earthquake recurrence rates and maximum magnitudes for hazard assessments. Often these analyses rely on the assumption that successive earthquakes all breached the surface and that the ruptures are recorded topographically, or by the deposits exposed in a trench. The M w7.2 1992 Suusamyr earthquake, Kyrgyzstan, is an apparently problematic case for such analyses because its ruptures show significant displacement but are only mapped as having broken the surface along small, disparate portions of the fault. Here we present the results of surveys conducted along the Suusamyr Fault to establish whether that is the case. Two sets of ruptures were identified following the earthquake. They are unusually short for their displacement and are separated by a 25 km gap. Using satellite imagery, high-resolution digital elevation models and palaeoseismic trenching we first reassess the distribution of the 1992 ruptures and then reconstruct the Holocene earthquake record to establish the extent to which the 1992 earthquake is representative of the rupture behaviour of this fault. We find evidence for at least two prehistoric surface rupturing earthquakes in the Holocene: one ∼3 ka and one >8 ka that, along with the modern event, gives recurrence intervals of ∼3 and ∼5 kyr. Within spatial gaps between segments of the 1992 ruptures there are clear prehistoric surface ruptures and the ruptures in each prehistoric earthquake were discontinuous. We conclude that there is significant variability in the surface rupture pattern of successive earthquakes on the Suusamyr Fault, with implications for the completeness of palaeoseismic records obtained from thrust scarps. [ABSTRACT FROM AUTHOR]

In microseismic monitoring of hydrofracturing, the stress state around the wellbore determines the occurrence and extension of the hydraulic fractures when repetitive fracturing is taking place. A number of methods have been proposed to invert stress state. However, on account of the micromesh region scale and complex distribution form of the hydraulic fractures, it is difficult to obtain the ideal stress models that can satisfy a set of focal mechanisms in a research region. For this reason, we sufficiently discussed the mechanism characteristics and tried to make use of them to improve stress inversion results. After evaluating the partition and choosing the optimal number of clusters, a fuzzy clustering algorithm is first applied to classify the microseismic events into several clusters by taking advantage of the directions of P axes, and then the inversion of stress state from focal mechanisms is implemented for each cluster, respectively. As a result, stress models with smaller-fit angles are obtained, proving the validity of this inversion scheme. In addition, the stress regime categories and propagation of hydraulic fractures are analysed using the inverted stress models, and it is more accurate in estimating the stress state of a research region. [ABSTRACT FROM AUTHOR]

We propose a new wavelet-based compressive sensing method to image coseismic radiation sources with teleseismic array data. Compared to the sliding-window-based technique, our method avoids choosing the window length by taking advantages of the adaptive resolution feature of the wavelet transform. Thus artefacts caused by inappropriate window length and waveform truncation can be removed. As waveforms usually become less aligned at late arrivals due to the increasing distance of actual radiation sources with respect to the hypocentre, we introduce a time-correction scheme to realign waveforms with the reference point obtained from beamforming at every time step. Synthetic tests indicate that the new method can better resolve the seismic radiation sources with more reliable locations and source times. To demonstrate its feasibility, we apply the method to the M w9.0 Tohoku earthquake. We find similar spatial rupture and seismic radiation patterns, such as repeating rupture in the region close to the hypocentre and along-dip frequency-dependent radiation, as indicated by previous models from finite-fault inversion and back projection. In addition, our results show structure-controlled rupture characteristics, with the seismic radiation in the first 80 s mainly confined in the off-Miyagi high-wave-speed region. The rupture speed shows clear along-dip variations, with updip rupture slower than that of downdip, which corresponds well with the depth-varying characteristics along the megathrust. [ABSTRACT FROM AUTHOR]

Pre-existing weakness zones in the lithosphere such as transform faults/fracture zones and extinct mid-oceanic ridges have been suggested to facilitate subduction initiation in an intra-oceanic environment. Here, we propose that the additional forcing coming from the mantle suction flow is required to trigger the conversion of a fracture zone/transform fault into a converging plate boundary. This suction flow can be induced either from the slab remnants of former converging plate boundaries or/and from slabs of neighbouring active subduction zones. Using 2-D coupled thermo-mechanical models, we show that a sufficiently strong mantle flow is able to convert a fracture zone/transform fault into a subduction zone. However, this process is feasible only if the fracture zone/transform fault is very close to the mid-oceanic ridge. Our numerical model results indicate that time of subduction initiation depends on the velocity, domain size and location of mantle suction flow and age of the oceanic plate. [ABSTRACT FROM AUTHOR]

Fractures often play an important role in controlling the fluid flow in hydrocarbon reservoirs. When the seismic wave propagates through media containing fracture corridors, significant scattering dispersion and attenuation can occur. In this work, we study the P -wave dispersion and attenuation due to the scattering caused by 2-D fluid-saturated aligned fractures with finite thickness, which are embedded in an isotropic elastic background medium. Using the Foldy approximation and the representation theorem, the P -wave dispersion and attenuation are related to the displacement discontinuities across the fractures. These fracture displacement discontinuities are obtained from the boundary conditions and the P -wave dispersion and attenuation can thus be calculated. A numerical example shows that the fracture thickness has significant influence on the dispersion and attenuation, especially in the low-frequency regime when the fracture size is smaller than the seismic wavelength. The effects of the fluid bulk modulus are also significant, which are opposite to those of the fracture thickness. However, the effect of the fluid viscosity is found to be negligible for the studied configurations. To validate the proposed model, the theoretical predictions are compared with ultrasonic measurements on fractured samples. The comparison shows overall good agreement between theory and experiment. This work reveals the important influence of fracture thickness and saturating fluid properties on the P -wave scattering dispersion and attenuation. Hence, it shows a potential to extract these parameters from seismic data. [ABSTRACT FROM AUTHOR]

For permafrost, information on unfrozen water content is critical for thermal modelling, understanding permafrost evolution and for predicting thaw susceptibility for infrastructure. We utilize electrical resistivity surveys for the detection and monitoring of unfrozen water content in cold continuous permafrost. We develop a data processing scheme based on a common subset of reciprocal measurements to filter data from irregular acquisitions in variable environmental conditions to a standard noise level, such that models recovered from disparate data sets can be compared quantitatively. Using temperature-corrected post-inversion model differencing, changes in recovered electrical resistivity models are used to calculate the relative change in unfrozen water content over common regions of high model resolution. The technique is applied to electrical resistivity data collected seasonally using a permanent electrode installation in cold continuous permafrost alongside airport infrastructure in the Canadian Arctic. The number of healthy data decreases over the experiment duration, with a correlation between data quality and ground surface temperature. Using the resistivity models, relative changes in unfrozen water content are estimated that are consistent with temperature records. Results are compared to an empirical temperature-dependent water content model obtained via calibrated thermal modelling at the study site. When compared to the empirical model, the changes in resistivity overpredict the changes in unfrozen water content due to temperature-induced phase changes alone after one year of acquisition. Prevalent changes in unfrozen water content occur in a zone below the active layer at temperatures of approximately 0 to −4.5 °C, and may be indicative of permafrost evolution involving ice formation or water movement over many months in moderately-saline permafrost beneath airport infrastructure. [ABSTRACT FROM AUTHOR]

While single point source is an oversimplified representation of medium to large earthquakes, finite fault models in many cases over parameterize the inversion due to the lack of sufficient near field data. Multiple point source solutions can fill in the gap in-between these two representations. Here, we propose a Markov-Chain-Monte-Carlo multiple point source inversion scheme, in combination with the advantage of the cut-and-paste technique, which cuts the seismogram into Pnl and surface portions and allows different time-shifts for each segment to align data with the synthetics. We apply the approach to the M w 6.2 foreshock in the 2016 Kumamoto earthquake sequence by using the strong-motion observations within 100 km. We are able to perform the inversion at relatively high-frequency ranges (0.02–0.28 Hz for Pnl and 0.02–0.20 Hz for surface waves) with confidence on the velocity model built on the M w 5.4 path calibration event. Our results show that the rupture was mainly composed of three subevents, with total duration of about 12 s and total M w of 6.2. The strikes of three subevents agree well with surface fault mapping where the Futagawa Fault intersects the Hinagu Fault with ∼30° difference in strike. Our solution shows that the first subevent dips to the southeast, while the second and the third subevents, located ∼3 km the north and ∼4 km to the southwest of the first subevent, dip to the northwest. The focal mechanism of the first subevent shows remarkable agreement with the first-motion solution. The fault geometry also shows well consistence with the relocated aftershocks, which delineate a SE-dipping fault around the first subevent and two NW-dipping faults to the north and south, respectively, corresponding to the second and the third subevent. The sum of moment tensor of subevents that have not only different geometry but also rake angles shows strong Compensated Linear Vector Dipole (CLVD) component (48–50 per cent). With a local 1D crustal model, a full-moment-tensor inversion using regional long-period waveform data also detects strong CLVD component of this earthquake. In contrast, using the PREM model results in an almost pure double-couple solution. In short, we have precisely resolved the rupture process, the intricate fault geometry, and the strong CLVD component with strong-motion data. This highlights the importance of extracting the relatively high-frequency information from the waveform data and with accurate velocity model in seismic source analyses of large earthquakes. [ABSTRACT FROM AUTHOR]