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The 2018 Lombok, Indonesia, earthquake sequence demonstrated the important interplay between the deep root of an active volcano and rupture patterns on the Flores Thrust. This backarc fault runs along the north side of the Lesser Sunda Islands and is co‐located with several other large volcanoes that also likely influence its seismogenic zone. As a first step toward better understanding these fault‐volcano interactions, we construct a new 3D model of the Flores Thrust using relocated seismicity and geological constraints. We then develop simple thermal models of the volcanoes and map their interaction with the seismogenic zone of the Flores Thrust. The geometry of our preferred fault model varies along strike, with southward dips ranging from 25° to 32° in eastern Java to Bali, 7°–38° in eastern Bali to western Sumbawa, 31°–47° in eastern Sumbawa to Komodo, 41°–54° in western Flores, and 31°–38° in eastern Flores. Our models exhibit flat‐ramp geometry between eastern Bali and western Sumbawa, corresponding to the extent of the North Bali‐Lombok Basin where its basement acts as a thrust décollement; farther east, the fault is a simple ramp. The maximum seismogenic depth is approximated based on the estimated depth of the 450°C isotherm, which varies depending on the presence of volcanoes. The complex geological history in the region, involving rifting and tectonic inversion, controls the segmented and varying dips of the thrust. Moreover, the progressive growth of the Flores Thrust and the evolution of arc volcanism would also mutually influence each other, modifying both fault geometry and volcano distributions. Key Points: The Flores Thrust extends in the backarc from Bali to Flores with varying dips along the strikes, consisting of a flat and a ramp geometryVolcanoes in the Lesser Sunda Islands influence the seismogenic zone by modifying the depth of the brittle‐ductile transition zoneThe proximity between the Flores Thrust and volcanoes suggests the interplay during thrust development and arc evolution [ABSTRACT FROM AUTHOR]

Pre-seismic stress-induced electric charges might be generated in the fault zone for producing the electromagnetic (EM) precursors. The peroxy defect (PD) theory has been considered to interpret the generation of stress-induced charges. The critical-earthquake (CE) model has been applied to explain the underground electric currents. It is necessary to explore both the presence of PD in the seismogenic zone of earthquakes and the production of EM precursors based on the CE model. First, we study the relationship between the PD theory and the seismogenic zone. Secondly, we examine the CE model based on the estimates of the depth ranges for generating stress-induced charges/currents for yielding an average normal stress that is strong enough to generate the electric charges and depth-dependent electric properties of fault zones. Results show that the dip angle and faulting type are significant factors in influencing the depth ranges that are similar for thrust and strike-slip faults and wider for normal faults. For a same depth range, the ability of producing electric currents is almost the same for thrust and strike-slip faults, but lower for normal faults. The underground spatial distribution of resistivity may control the stress-induced currents. The permeability barrier or transition layer also influences the stress-induced charges. The low-resistivity lower crust would reduce the up-going p-holes and thus is not beneficial for generating ground currents as expected by the CE model. [ABSTRACT FROM AUTHOR]

The Main Recent Fault (MRF) is an important boundary fault that accommodates the oblique convergence motion between the Arabian and Eurasian plates. Several large earthquakes including the 1909 Mb 7.4, 1958-1963 three events with magnitude larger than Mw 6.0, and the 2006 Mw 6.1 earthquake had partly ruptured the central and southeast segments of MRF. However, there is no large earthquake occurred in the northwest segment in the past hundred years. In this study, the interseismic surface deformation was firstly mapped from the Synthetic Aperture Radar (SAR) images observed by the Sentinel-1 satellite. Then, the ununiformed fault slip rate, coupling ratio and locking depth are estimated based on the measured interseismic surface deformation. The results show that the interseismic fault slip rate slightly varies from ~3.7 mm/yr to ~6.1 mm/yr along the MRF. Furthermore, it is also found a gradual increasing extensional component with magnitude of 2-7 mm/yr from Morvarid to Sahneh segments of MRF. In addition, total 9 sub-segments could be divided based on the fault coupling distribution and historical earthquakes in the interest zone. A seismogenic zone is identified from the historical earthquakes, which has a good space consistency with the transition between the significant shallow locking and deep free slip zones. According to the potential seismic magnitude and estimated recurrence interval, it is found that several segments are approaching the end of their interseismic recurrence interval. Meanwhile, one segment (SF-8) may be in the early stage of a new recurrence period due to the recent several large earthquakes and a long recurrence interval. [ABSTRACT FROM AUTHOR]

Abstract The depths of most moderate‐sized oceanic earthquakes are poorly constrained because of the lack of local recording stations and noisy teleseismic recordings. This hampers our understanding of slip behaviors along oceanic faults and the mechanical properties of the oceanic lithosphere. In this study, we develop a new method to enhance the weak body‐wave signals, particularly the depth phases, associated with earthquakes on oceanic transform faults using large‐aperture arrays in the teleseismic range. We simulate the enhanced teleseismic signals to refine the centroid depths of moderate‐sized earthquakes. We validate the new approach using synthetic waveforms and show it outperforms conventional methods when dealing with noisy signals. We obtain the depth estimates for three moderate‐sized earthquakes on the Chain transform fault in the equatorial Atlantic Ocean and find two of them are consistent with a local catalog derived using oceanic bottom seismometers. Application of the new method to the past decades of teleseismic recordings of moderate‐sized earthquakes on the large and slow‐slipping transform faults will provide significantly improved constraints on the width of the seismogenic zone, thus advancing our understanding of the rheology of oceanic lithosphere and earthquake processes in oceanic settings and, by comparison, their more dangerous continental counterparts. The new method is not limited to oceanic transform earthquakes, but can be easily adapted to other seismological studies in which noisy but coherent signals could be revisited for better usage.

• Serpentinites entrained from the mantle wedge affects megathrust seismogenesis. • Different serpentine polymorphs exhibit different frictional behaviour. • Lizardite-serpentinites form a soft barrier to seismic slip at mantle wedge corner. • Antigorite patches farther downdip facilitate rupture propagation and initiation. • Case studies along the Chile margin support the soft-barrier hypothesis. How temperature and petrology control the seismogenesis of the deeper megathrust and associated hazard is poorly understood. It was previously proposed that the serpentinized mantle wedge would limit seismic rupture, but modern observations show many recent earthquakes to have ruptured far deeper than the mantle wedge corner (MWC). However, the notion of mantle wedge serpentinite impeding seismic rupture is still of great scientific value. Here, by comparing model predictions of megathrust temperature and petrology with instrumentally constrained slip distributions of megathrust earthquakes along the Chile margin, we propose a new hypothesis based on the contrasting frictional behaviours of different serpentine polymorphs widely reported in the literature. At low slip rates, lizardite (and chrysotile) exhibits rate-strengthening and hence stable-sliding, but antigorite, given high enough temperatures, exhibits rate-weakening and hence stick-slip. However, they both undergo dynamic weakening at high slip rates and thus can facilitate rupture propagation. Along most of the Chile margin, as in most other global subduction zones, the megathrust at and immediately downdip of the MWC is rich in lizardite. The lizardite-serpentinite impedes but cannot always stop seismic rupture, forming a soft barrier. Farther downdip, the megathrust is rich in antigorite. Antigorite patches and their dense collections can facilitate rupture propagation and even initiation, and some of the stronger patches can radiate abundant short-period seismic energy. This MWC "soft-barrier" hypothesis explains the different modes of downdip slip distribution observed along Chile: bimodal slip straddling the MWC, slip confined downdip or updip of the MWC, and slip predominantly updip of the MWC with a deep extension crossing the MWC. The hypothesis is qualitatively compatible with global observations of megathrust seismicity and embraces the exception of very warm subduction zones. [ABSTRACT FROM AUTHOR]

To investigate structural characteristics associated with variations in slip behavior on the plate boundary in the Nankai subduction zone, we conducted seismic reflection surveys in the central‐western Nankai Trough. Data were processed by pre‐stack depth migration. The resulting depth map of the top of the subducting Philippine Sea plate shows detailed topographic features of the plate boundary, including subducting seamounts and large‐scale undulations of the plate surface. We report the presence of a structurally anomalous region, potentially with low velocity, in the overriding plate at the boundary between the zone of large coseismic slip during the 1946 Nankai earthquake and the area producing slow earthquakes. This anomaly appears to be related to depth‐dependent variations in plate boundary slip style and plate coupling in the central‐western Nankai Trough. Plain Language Summary: The central‐western Nankai Trough off southwestern Japan is one of the best locations to study variations in slip style on the plate boundary fault in subduction zones because the fault slip distribution during the latest great earthquake, the distribution of slow earthquakes, and interplate coupling are known from previous studies. We conducted seismic surveys along densely distributed survey lines to image detailed subsurface structures in the central‐western Nankai Trough. A three‐dimensional map of the plate boundary fault geometry was created from seismic profiles used in this study. We found a structural anomaly, interpreted as a low‐velocity zone in the overriding plate, at the boundary between the slip zone of great earthquakes and the zone of slow earthquakes. This anomaly might be related to the variation of the plate boundary fault slip styles. Key Points: The topography of the subducting plate is mapped with densely spaced seismic profiles in the central‐western Nankai TroughAn arcuate structural anomaly in the overriding plate ∼100 km long with potentially low velocity coincides with the Tosabae TroughThe arcuate anomaly is located at the boundary between regions of strong and weak plate coupling [ABSTRACT FROM AUTHOR]

In contrast to common subduction, the young and thin part of the Antarctic Plate subducts first to the south of the Chile Triple Junction (CTJ), followed by the old and thick part, corresponding to wedge subduction. A finite element model was used to simulate the wedge subduction of the Antarctic Plate and to compare it with the slab subduction of the Nazca Plate. The results show that the CTJ is not only a wedge subduction boundary but also an important factor controlling the lithospheric thermal structure of the overriding plate. The computed heat flow curves are consistent with the data observed near the trench of the two selected profiles. The different slab dips to the north and south of the CTJ are considered to be caused by wedge subduction. When the slabs are young and at the same age, the deep dip of the Antarctic slab is 22° smaller than the Nazca slab. Southward from the CTJ, the slab age of the wedge subduction increases, which leads to a larger slab dip, a colder slab, and a wider seismogenic zone. The effect of the slab age of wedge subduction on the focal depth is smaller than that of the convergence rate. A 4.8-cm/year difference in convergence rate of the wedge subduction results in an 11-km difference in the width of the seismogenic zone and a 10-km difference in the depth of the downdip limit. Among these controlling factors, the convergence rate plays a major role in the different focal depths south and north of the CTJ.

Frictional instabilities along tectonic faults produce a full spectrum of slip behaviors including aseismic creep, slow earthquakes, and regular earthquakes. Thermally controlled rock friction provides new insights into the frictional properties of rock faults and associated seismic behaviors with increasing crustal depth. We first review the standard rate and state friction laws. Then we focus on discussing hydrothermal friction experiments on single, mixed, and natural gouges. Steady-state friction and velocity dependence vary significantly with increasing temperature. As temperature increases, steady-state friction coefficients of rock gouges may increase progressively, keep roughly constant, or fluctuate. At high temperatures some gouges may show a brittle-ductile transition. In addition, with increasing temperature, the rock friction switches between velocity-strengthening and velocity-weakening. This variation indicates thermal constraints on the fault stability transition and provides a possible explanation for the temperature and depth distribution of seismic activity. We examine four friction models for hydrothermal conditions: the classical rate-, state-, and temperature-dependent friction model, the friction to flow model, the power-law rate-, state- and temperature-dependent friction, and the extended microphysical (Chen-Niemeijer-Spiers) model. The hydrothermal friction experiments and physically-based models can enhance our understanding of fault friction. Further research is needed on the effects of pore fluids, multiple deformation and healing mechanisms, microstructural evolution at elevated temperatures, and extrapolation of experimental data and friction models to natural and induced earthquakes. [ABSTRACT FROM AUTHOR]

The aftershock distribution of the 2014 Mw 8.1 Iquique earthquake offshore northern Chile, identified from a long‐term deployment of ocean bottom seismometers installed eight months after the mainshock, in conjunction with seismic reflection imaging, provides insights into the processes regulating the updip limit of coseismic rupture propagation. Aftershocks updip of the mainshock hypocenter frequently occur in the upper plate and are associated with normal faults identified from seismic reflection data. We propose that aftershock seismicity near the plate boundary documents subduction erosion that removes mass from the base of the wedge and results in normal faulting in the upper plate. The combination of very little or no sediment accretion and subduction erosion over millions of years has resulted in a very weak and aseismic frontal wedge. Our observations thus link the shallow subduction zone seismicity to subduction erosion processes that control the evolution of the overriding plate. Plain Language Summary: To better understand the controls on shallow seismicity and subduction erosion following large subduction earthquakes, we use marine recordings of the Mw 8.1 2014 Iquique earthquake aftershocks and long‐offset multi‐channel seismic data. By comparing the aftershock locations and seismic imaging, we observe that most aftershocks occurred in the upper continental plate and abruptly stopped in the frontal forearc. The amplitude characteristics of upper‐crust reflections indicate a fractured and fluid‐filled outer forearc, which is associated with the absence of aftershocks. Large‐scale faulting, as evidenced by disrupted reflections in the seismic image, can be correlated to upper plate seismicity. We propose that the aftershocks updip of the main earthquake area reflect active subduction erosion processes. Key Points: We investigate structure and seismicity at the updip end of the 2014 Iquique earthquake rupture using amphibious seismic dataSeismicity updip of the 2014 Iquique earthquake occurs over a broad range likely interpreted to be related to the basal erosion processesCoseismic stress changes and aftershocks activate extensional faulting of the upper plate and subduction erosion [ABSTRACT FROM AUTHOR]

The geometries (i.e., dip angles) of active faults from the surface to the seismogenic zone are the most important factors used to evaluate earthquake ground motion, which is crucial for seismic hazard assessments in urban areas. In Osaka, a metropolitan city in Japan, there are several active faults (e.g., the Uemachi and Ikoma faults), which are inferred from the topography, the attitude of active faults in surface trenches, the seismic reflection profile at shallow depths (less than 2 km), and the three-dimensional distribution of the Quaternary sedimentary layers. The Uemachi and Ikoma faults are N–S-striking fault systems with total lengths of 42 km and 38 km, respectively, with the former being located ~ 12 km west of the latter; however, the geometries of each of the active faults within the seismogenic zone are not clear. In this study, to examine the geometries of the Uemachi and Ikoma faults from the surface to the seismogenic zone, we analyze the development of the geological structures of sedimentary layers based on numerical simulations of a two-dimensional visco-elasto-plastic body under a horizontal compressive stress field, including preexisting high-strained weak zones (i.e., faults) and surface sedimentation processes, and evaluate the relationship between the observed geological structures of the Quaternary sediments (i.e., the Osaka Group) in the Osaka Plain and the model results. As a result, we propose geometries of the Uemachi and Ikoma faults from the surface to the seismogenic zone. When the friction coefficient of the faults is ~ 0.5, the dip angles of the Uemachi and Ikoma faults near the surface are ~ 30°–40° and the Uemachi fault has a downward convex curve at the bottom of the seismogenic zone, but does not converge to the Ikoma fault. Based on the analysis in this study, the dip angle of the Uemachi fault zone is estimated to be approximately 30°–40°, which is lower than that estimated in the previous studies. If the active fault has a low angle, the width of the fault plane is long, and thus the estimated seismic moment will be large. [ABSTRACT FROM AUTHOR]

We investigated a mud volcano (MV) in a fault zone located at the southern edge of the Kumano Basin, the largest forearc basin along the Nankai Trough. Existing seismic sections show a truncated bottom-simulating reflection by a conduit below a topographic high, indicating the presence of an MV. New shipboard acoustic observations show that the fluid may be seeping through the seafloor, which in turn indicates that there are sufficient fluids for larger scale fluid migration in this area. Autonomous underwater vehicle-based high resolution acoustic observations and pH measurements indicate that soft sediment covers most of the MV and surrounding seafloor and that mud and small amounts of “high-backscattered materials” are sprinkled within the crater and around the MV. The MV type is different from those in the landward part of the Kumano Basin: the southern MV is smaller in size, has a steeper slope angle than the those in the northern Kumano Basin, and is located in a fault zone. The characteristics of this 14th Kumano Basin MV suggest that it is an expression of the larger scale fluid and sediment migration along the southern edge of the Kumano Basin, which may provide information regarding fluid and sediment migration along fault systems in the Nankai Trough accretionary prism. [ABSTRACT FROM AUTHOR]

The organizations in different countries involved in seismic zoning modify the maps in response to various seismic events. The predictive ground motion is used in the analyses which may not be estimated using a single attenuation relation, because it is established on theoretical basis not with the actual ones and hence always errors are included in the analysis. However, the impact of faults is not considered in many of such seismic zoning efforts in several countries. An earthquake is generally triggered along the fault line; magnitude of generated earthquake depends on many factors, mainly the fault characteristics. Hence, the fault is the basic source of seismic activity and its presence and importance directly influences the seismic status of a region. In the present study a new term "fault importance index (FII)" is proposed which is a numeric value estimated from fault characteristics. The FII value for each of linear feature is calculated and relation is derived between the FII with earthquake intensity. The generated polynomial equations relate the past intensity spread with FII. The seismic risk zone map is then generated on the basis of FII. The developed seismic risk map has been compared to few earlier seismic maps and it can be used for design of buildings especially near the earthquake source. [ABSTRACT FROM AUTHOR]

The northwest of Iberia experiences important tectonic activity that has led to several episodes of seismic swarms. Despite substantial knowledge of the surface structures that could be responsible for this activity, their distribution at depth remains unclear. Geophysical research and rheological data display important discrepancies concerning the deep structure of the area. In order to shed light on the processes that control deep seismicity and their relationship with surface structures, we combine the analysis of seismic activity in northwest Iberia with the interpretation of sandbox experiments simulating tectonic structures similar to those observed. The structural interpretation of the modeling results was conducted by analyzing the surface particle image velocimetry field. Additionally, yield strength and brittle‐ductile transition zone (BDTZ) maps were constructed from model sections. The modeling results show a direct relationship between the main active structures developed along pre‐existing weak zones and the observed variations in crustal thickness. Moreover, these thickened zones correspond to the sectors where the BDTZ in the crust deepens. Therefore, the anomalous thickness and lateral variations of the seismogenic zone cannot be solely explained by rheological or structural changes but result from a combination of factors such as fluid pressure and the location of fractures at depth, which controls the fluid migration responsible for the failure of crustal faults. These factors can trigger seismic activity at depth and may control the observed thickness variations in the BDTZ. Key Points: New insights into the lateral‐strength variations and depth to the brittle‐ductile transition zoneTectonic implications of the seismic activity in NW IberiaMechanisms of brittle failure and depth variations to the brittle‐ductile transition zone in NW Iberia [ABSTRACT FROM AUTHOR]

Abstract Ocean drilling in the Nankai Trough forearc suggests a new scenario for the evolution of the Nankai subduction zone. Continuous subduction since the Late Cretaceous has been a common tectonic scenario, although the plate subduction was transferred from the Pacific Plate to the Philippine Sea Plate during the Miocene. Seismic reflection studies coupled with drilling have demonstrated that two episodes have controlled the recent evolution of the Nankai forearc: a resurgence of subduction at ~ 6 Ma after cessation since ~ 12 Ma and rapid growth of the accretionary prism since ~ 2 Ma because of the influx of large amounts of terrigenous sediments from the Japan Alps in central Japan. Both episodes were synchronous with large-scale plate reorganizations. The westward subduction of the Philippine Sea Plate initiated both in the Ryukyu and the Philippine trenches at ~ 6 Ma. Rifting in the Okinawa and Mariana troughs started at ~ 6 Ma. Compressive tectonics in northeast Japan started at ~ 3–2 Ma, and resultant mountain building with active surface erosion commenced in central Japan at ~ 2 Ma. This recent compressive tectonic phase might be due to the initiation of convergence of the Amurian Plate with the Okhotsk or North American Plate along the eastern margin of the Japan Sea. In addition to this event, the strong collision and indentation of the Izu-Bonin Arc since ~ 2.5 Ma was also enhanced in central Japan.

What controls subduction megathrust seismogenesis downdip of the mantle wedge corner (MWC)? We propose that, in the region of the 2010 Mw = 8.8 Maule, Chile, earthquake, serpentine minerals derived from the base of the hydrated mantle wedge exert a dominant control. Based on modeling, we predict that the megathrust fault zone near the MWC contains abundant lizardite/chrysotile‐rich serpentinite that transforms to antigorite‐rich serpentinite at greater depths. From the MWC at 32–40 km depth to at least 55 km, the predominantly velocity‐strengthening megathrust accommodated dynamic propagation of the 2010 rupture but with small slip and negative stress drop. The downdip distribution of interplate aftershocks exhibits a gap around the MWC that can be explained by the velocity‐strengthening behavior of lizardite/chrysotile. Interspersed velocity‐weakening and dynamic weakening antigorite‐rich patches farther downdip may be responsible for increased abundance of aftershocks and possibly for some of the high‐frequency energy radiation during the 2010 rupture. Plain Language Summary: A subduction megathrust rupture may extend from the trench to beneath populated coastal area. To understand what controls the megathrust seismogenic behavior of its deeper part, we conduct a case study of the 2010 magnitude 8.8 Maule, Chile, earthquake and its aftershocks. With numerical thermal modeling, we find that the deep seisomogenic behavior may be controlled by serpentine materials derived from the base of the overlying hydrated mantle wedge. The deep part of the megathrust ruptured "passively" against increasing resistance (negative stress drop). Lower‐temperature serpentines lizardite and chrysotile that are known to facilitate aseismic slip may be responsible for an observed gap in aftershock distribution in the downdip direction. Higher‐temperature serpentine antigorite that is known to facilitate seismic slip may be responsible for increased aftershocks farther downdip and possibly radiation of high‐frequency energy during the 2010 Maule earthquake. Key Points: Seismogenic behavior of subduction megathrust may be affected by materials derived from the base of the serpentinized forearc mantle wedgeThermopetrologic modeling suggests abundance of lizardite and chrysotile around the mantle wedge corner but antigorite farther downdipSerpentine frictional behavior may explain distributions of stress drop and seismic energy radiation in 2010 rupture and aftershocks [ABSTRACT FROM AUTHOR]

To study the coupling mechanism between lithosphere, atmosphere and ionosphere, the ionosonde based earthquake (EQ) anomalies provide clear indications for the future events. The Lithospheric-Atmosphere-Ionospheric coupling (LAIC) provides information about pre and post seismic precursors over the seismogenic zone. This study presents the variations in ionospheric critical frequency (f o F2) of the F2 layer and Total Electron Content (TEC) from Global Ionospheric Maps (GIMs) associated with the two M w ≥ 6.0 EQs at Pakistan-Iran border (here-in-after, Saravan EQ) and Afghanistan. The ionosonde stations data located at Islamabad and Sonmiani provided significant f o F2 variation for the April 16, 2013 (M w 7.7) and June 22, 2022 (M w 6.0) EQs in Saravan and Afghanistan, respectively. Furthermore, the f o F2 values showed significant variations before the main shocks of both the events. Moreover, the TEC maps over the epicenter supported the variations of ionosonde data with maximum variations beyond the bound of the statistical method as anomalous positive variations before the two events. Moreover, prominent variations occur before both the EQs in ionosonde and TEC data but the Afghanistan event showed more pronounced variations than Saravan EQ. Additionally, it has been observed that more prominent variation occurs in both the stations within 5–10 days before the main shocks to aid another development in LAIC hypothesis. • Seismoionospheric anomalies have been studied for M w ≥ 6.0 EQ. • Ionosonde and GIM-TEC data showed variations near epicenter. • Anomalies became more significant near the epicenters. • Storms time ionospheric are also prominent in the data. [ABSTRACT FROM AUTHOR]

Abstract The alteration and dehydration of predominantly basaltic subducting oceanic crustal material are thought to be important controls on the mechanical and hydrological properties of the seismogenic plate interface below accretionary prisms. This study focuses on pillow basalts exposed in an ancient accretionary complex within the Shimanto Belt of southwest Japan and provides new quantitative data that provide insight into clay mineral reactions and the associated dehydration of underthrust basalts. Whole-rock and clay-fraction X-ray diffraction analyses indicate that the progressive conversion of saponite to chlorite proceeds under an almost constant bulk-rock mineral assemblage. These clay mineral reactions may persist to deep crustal levels (~320 °C), possibly contributing to the bulk dehydration of the basalt and supplying fluid to plate-boundary fault systems. This dehydration can also cause fluid pressurization at certain horizons within hydrous basalt sequences, eventually leading to fracturing and subsequent underplating of upper basement rock into the overriding accretionary prism. This dehydration-induced breakage of the basalt can explain variations in the thickness of accreted basalt fragments within accretionary prisms as well as the reported geochemical compositions of mineralized veins associated with exposed basalts in onland locations. This fracturing of intact basalt can also nucleate seismic rupturing that would subsequently propagate along seismogenic plate interfaces. Graphical abstract Schematic model showing the setting for décollement step-down into the ocean-plate basalts at the subdution plate interface. The diagram also shows textures of intact basalt and its expected frictional properties and behaviors associated with seismogenesis.

The earthquake and tsunami that occurred in 1185 in the North Azov region is considered here using the multidisciplinary approach for the first time in professional literature, based on indications from the Old Russian written source, The Tale of Igor's Campaign. Problems such as the relationship between earthquake and tsunami, shaking intensity, geographic position, and other location conditions are addressed. The epicenter area is established to be the northeastern shore of the Sea of Azov. Information is given about earthquakes in the region for 200 years and their confinement to the sublatitudinally extending northeastern shore of the sea is found. The revealed seismogenic zone is correlated with the North-Azov fault, a large tectonic structure of the same orientation. Finally, based on archeoseismic data, which have not been previously employed in a seismic hazard assessment in the framework of making General Seismic Zoning maps, the question is raised of assessing the seismic potential of the zone. [ABSTRACT FROM AUTHOR]

We investigate 3‐D seismic structures (Vp, Vs, and Poisson's ratio) and Vp azimuthal anisotropy in the source area of the 2018 Eastern Iburi earthquake (M 6.7) in Hokkaido, Japan. Its mainshock occurred at the edge of a high‐Vp (2–4%) seismogenic zone. Significant low‐Vs (−1% to −3%) and high Poisson's ratio (2–7%) anomalies are imaged in and below the source zone and extend to the upper surface of the subducting Pacific slab, most likely reflecting ascending fluids released by the slab dehydration. A high consistency between the fault plane and the low‐Vs and high Poisson's ratio anomalies indicates that the fluids may have entered the fault and affected the rupture nucleation. A high‐V (1–3%) anomaly is revealed in the fore‐arc mantle wedge and connects with the high‐V seismogenic zone, probably reflecting a lithospheric fragment and contributing to cool down the mantle wedge. Complex seismic anisotropy is revealed in the crust in and around the source area, which may reflect complicated stress regime and strong structural heterogeneities there. Key Points: The 2018 Eastern Iburi earthquake occurred at the edge of a high‐Vp seismogenic zoneAscending fluids from slab dehydration affected rupture nucleation of this earthquakeA high‐V anomaly exists below the source zone, which may cool down the mantle wedge [ABSTRACT FROM AUTHOR]

Accurate prediction of earthquakes has long been an objective in the earthquake monitoring community. This study considered abnormal fluctuations of the aerosol optical depth (AOD) that occurred both before and after the 2008 Wenchuan and 2013 Lushan earthquakes, and it was found that these fluctuations had good correlation with both seismic earthquake events, excluding meteorological factors (wind speed, temperature, relative humidity). A buffer zone around the fault was determined optimal as the seismogenic zone to explore the AOD abnormal fluctuations, which suggested that the fracture zone was the preferred channel via which aerosols were transported into the atmosphere. By exploring the relationship between the 2008 Wenchuan and 2013 Lushan earthquakes, it was established that AOD fluctuation in the Lushan buffer zone had good correlation with the Wenchuan earthquake. It proved that the Wenchuan earthquake had a promoting effect on the Lushan earthquake. [ABSTRACT FROM AUTHOR]

The central Vanuatu forearc is characterized by a reduced convergence rate at the trench, significant uplift of the overriding plate, and the presence of large forearc islands. Volcanic activity and intermediate‐depth seismicity behind the forearc are among the highest on Earth. These features are presumed to be associated with the subduction of a large seamount chain and an immersed ridge. We used a catalog of P and S arrivals from a local seismological network to construct the first 3‐D velocity model of the region and to relocate earthquakes beneath the forearc. The 3‐D model reveals a highly heterogeneous velocity distribution in the first 40 km beneath the surface. Trench‐parallel low P and S velocity zones in the upper tens of kilometers beneath the western edges of the two largest forearc islands correlate to the major features entering into subduction and suggest highly fractured and probably water‐infiltrated features. Trench‐parallel high‐velocity zones at 5–15‐km depth, further to the east, may be part of a continuous consolidated rock structure that acts as a backstop. Thick overriding plate crust (29 ± 3 km) in the forearc is consistent with the presence of continental remnants. The earthquake distribution is generally heterogeneous, suggesting a complex fault structure and variable stress. Earthquakes are, however, well aligned at the plate interface in between the subducting features, where they constrain the angle of subduction to be 15° on average, down to 10–15‐km depth. Key Points: Trench‐parallel low P and S velocity anomalies beneath the forearc correlate to major reliefs entering subductionA trench‐parallel high‐velocity anomaly may be part of a continuous consolidated rock structure (backstop)The earthquake distribution is heterogeneous, suggesting a complex stress regime and fault structure [ABSTRACT FROM AUTHOR]

To reveal the detailed deformation structures related to plate subduction in the Nankai Trough, we applied up‐to‐date technologies to improve our 3‐D seismic images. This region is dominated by a megasplay fault system that consists of a coseismic out‐of‐sequence thrust branching from the plate interface and separating the inner and outer accretionary prism. The 3‐D seismic volume was acquired off Kumano in 2006 as a preliminary site survey as part of the Nankai Trough Seismogenic Zone Experiment. The preprocessed data quality was improved by recovering the broadband responses and by better attenuating multiple reflections and noise. New reflection images were then produced using prestack time migration and prestack depth migration with updated velocity models. The new velocity model suggests the possible existence of a high‐velocity zone just above the megasplay fault, which might indicate petrophysical alteration in the seismogenic zone. The fault geometry with spatial dip angle variation and the overburden velocities are important factors for further estimating the force distribution along the coseismic fault. Deformation structures newly imaged beneath the Kumano Basin and dipping layers above the megasplay fault imply complex thrusting and possible fluid flow paths within the inner prism. Fine‐scale deformation features are clarified in the shallow areas from the outer prism to the transition zone, which are useful for reconstructing the accretionary prism development. A low‐reflectivity zone, including an isolated layered block, may be originally underthrusted sediments that have been remobilized during later strike‐slip faults along the edge of Kumano Basin. Plain Language Summary: Deformed sediments that have been stripped off the subducting plate and added to a growing mass of accreted sediments in the Nankai Trough, off the coast of southwest Japan, are cut by a major active thrust fault that is considered to be the potential primary location of periodic great earthquakes that generate destructive tsunamis. To reveal the complex geological structures around the fault, we improved seismic reflection profiles by reprocessing legacy 3‐D seismic data using the latest signal processing technologies. The improved profiles show a thick high‐velocity zone that may potentially indicate hardening of rocks due to mineral precipitation and subsequent filling of fractures above the fault. We also better imaged deformation structures that were generated during the past multiple faulting events in the plate subduction process. Key Points: Seismic reflection imaging in the Nankai Trough off Kumano has been improved by applying recent data processing technologiesA high‐velocity zone identified above the megasplay fault may indicate petrophysical alteration at the depth of the seismogenic zoneComplex thrust structures and possible fluid flow paths in the accretionary prism are inferred from the new depth images [ABSTRACT FROM AUTHOR]

Moderate to large earthquakes repeatedly occur beneath the southernmost Ryukyu arc and forearc, but the seismogenic structures were poorly studied. To better understand the southernmost Ryukyu seismogenic structures, we have deployed ocean bottom seismometers to record the aftershocks of a large earthquake in 2015. As a result, several groups of aftershocks are associated with the seismogenic structures. A deep group coincides with the north‐dipping seismogenic zone that terminates near the tip of the mantle wedge beneath the Ryukyu Arc. A shallow group dips southward from the southern slope of the Ryukyu Arc to the southern end of the aftershocks; this group shows south‐dipping normal faulting character and accompanies the lower crust exhumation of Ryukyu Arc above the seismogenic zone. A plate interface step‐down beneath the seismogenic zone is inferred. A crustal underplating and exhumation around the seismogenic zone have taken place synchronously during the earthquake event. Plain Language Summary: Large earthquakes frequently occur at the plate boundary where two plates converge. Those earthquakes usually appear at a segment of the downgoing plate surface, called a seismogenic zone. Seismogenic zones are extensively studied in order to understand the possible earthquake cycles, crustal stress variation, mechanisms of earthquake nucleation, rupture propagation, and so on. Historically, the southernmost Ryukyu seismogenic zone has generated numerous earthquakes but its seismogenic structures are still unclear. By studying aftershocks of a large earthquake in 2015, we show the general characters of the southernmost Ryukyu seismogenic zone and indicate a synchronous occurrence of the uplift of the south Ryukyu Arc and accumulation of some subducted material beneath the Ryukyu Arc. Key Points: A better resolution of the tectonic events and plate interface geometry beneath the southernmost Ryukyu arc and forearcTectonic exhumation occurs in the frontal portion of the southernmost Ryukyu seismogenic zoneTectonic underplating occurs beneath the southernmost Ryukyu seismogenic zone [ABSTRACT FROM AUTHOR]

Along subduction zone megathrust faults, the transition from smectite to illite with depth occurs at temperatures of ∼150 °C and has long been hypothesized to be one of the dominant controls on the updip limit of the seismogenic zone. Despite this, there have been few experimental studies performed at in-situ conditions to test the frictional stability behavior of subduction zone faults across the smectite-illite transition. In this study, we simulated in-situ temperature-pressure-mineral conditions along the shallow plate boundary in the Nankai Trough, and conducted friction experiments to further constrain the role of smectite-illite transition in seismogenesis in subduction zones. We found that the friction coefficient of the simulated sediment increased with the progress of illitization and followed the Reuss average of friction coefficients of the mineral phases. The obtained friction coefficients and the Coulomb wedge model inferred that the pore pressure conditions around the plate boundary may be over-pressured but not as high as lithostatic pressure. Higher friction coefficients of sediment due to diagenetic processes would be required to sustain pore pressure close to lithostatic along the plate boundary. The velocity dependence of friction coefficient (a − b) was always positive (i.e., velocity strengthening, not seismogenic) except for slip rates less than ∼10 μm/s at 171 °C where the smectite-illite transition has almost completed. Therefore, shallow slow earthquakes that occur beneath the outer prism in the Nankai Trough cannot be explained only by the frictional instability of unconsolidated sediment. Since the smectite-illite transition mostly completes at ∼30 km landward from the trough axis, the transition from velocity strengthening to velocity weakening observed at 171 °C is caused by temperature-dependent frictional behavior of illite-rich material. Hence, the updip limit of the seismogenic zone in subduction zones is mainly controlled by the frictional properties of illite-rich material, not by the smectite-illite transition. • Frictional properties of décollement in the shallow Nankai Trough were examined. • Smectite-illite transition has little influence on frictional instability. • Shallow slow earthquakes occur within frictionally stable sediments. • Seismogenic zone is controlled by temperature-dependent illite friction. [ABSTRACT FROM AUTHOR]

The vigorous collision between the Eurasian plate and Philippine Sea plate in Taiwan causes a series of imbricate fold and thrust belts to develop at the deformation front. The Chukou Fault (CKF), characterized by a thrust type fault, located in Chiayi County, southwest (SW) Taiwan, is a prominent boundary between the fold-thrust belts and the Western Coastal Plain. Most of the seismicity in SW Taiwan is associated with this fault and its neighboring fault systems. The seismotectonic structures in the CKF zone, especially in the east, are complex due to the interactions among fault systems with distinct slip motions. To gain better insights into the seismogenic characteristics in the CKF zone, we used 1661 microearthquakes recorded by a temporary dense broadband seismic network and the Central Weather Bureau Seismic Network (CWBSN) between 2003 and 2004 to investigate the physical properties of the crust in the CKF zone. A waveform cross-correlation technique was applied to 143086 pairs of waveform data to determine the relative differential travel time between the P- and S-waves. By combining both the absolute and relative differential travel time data, we were able to obtain a new 3-D crustal P-wave velocity structure and Vp/Vs ratios. This study suggests that by using both absolute and relative differential travel time data in tomographic inversion can obtain precise 3-D velocity images and also gain better correlation between seismic events and fault structures, which is crucial for understanding the seismogenic process in our study area.

Structural-paragenetic and kinematic methods of tectonophysics are applied to study earthquake focal mechanisms of the Zagros system. Nodal planes of focal mechanisms are identified as L-, L′- and R-, R′-shears by the first method, whereby coordinates of principal stress axes P, T and N (i.e. in tectonophysics, σ1, σ3 and σ2, if σ1 ≥ σ2 ≥ σ3) are defined. ‘Working’ nodal planes corresponding to activated ruptures are revealed. Axes of the main normal stresses are combined into local groups on the basis of the kinematic identity of planes of seismogenic ruptures (Figure 2). The second method is applied to construct stereograms of the main axes P, T and N, to construct and interpret stereograms of vectors of seismogenic shifts (Figure 3), and to more clearly define coordinates of principal axes σ1, σ3 и σ2. As evidenced by their comparison, coordinates of the principal axes obtained by the two tectonophysical methods are well coincident (see Figure 2). Five groups of seismogenesis are distinguished; they differ in combination of deformation regimes and kinematic conditions. Locations are determined of the areas wherein earthquake foci of similar parameters are located. This means that seismogenic zones are distinguished; structural and kinematic characteristics of such zones are determined by parameters of stereographic models of corresponding types of seismogenesis (Figures 4 and 5). It is established that the region is dominated by shear and upthrust deformation regimes due to regional submeridional compression and SW-NE compression (see Figures 4 and 5). Submeridional subhorizontal compression is explained by the northward movement of the Arabian plate, and SW-NE compression is explained by divergent processes within the limits of the Red Sea rift. The time pattern of the seismogenic processes from 1979 to 2001 shows that submeridional compression and SW-NE compression are associated with different deep mechanisms. Processes of SE-NW compression, which are observed in the northern part of the Arabian plate, are caused by its interaction with the Eastern Black Sea microplate.

An unusual foreland-dipping seismicity band has been identified at a depth between 7 and 25 km beneath the Coastal Plain, a foreland basin, in southwestern Taiwan. This seismogenic zone is closely associated with abrupt changes in Vp/Vs based on our new high-resolution seismic tomography. Rocks with high Vp/Vs ratios of up to 1.84 were found directly above the inclined seismicity band. Below it, the Vp/Vs ratio dramatically drops to about 1.72–1.78. We found that this change may result from illite-bearing slates transforming to muscovite-bearing phyllite/mica-schist. Previous studies suggested that the frictional instability regime for illite–quartz gouge and muscovite–quartz gouge falls in the range of 250–500 °C. We calculated the isotherms based on regional heat flow data and found that the foreland-dipping seismicity band can well be bounded between the 250 °C and 500 °C isotherms. In addition, in the same temperature range wet illite/hydromuscovite would transform to muscovite and release water. This dehydration may elevate pore pressure and facilitate the formation of an incipient decollement at the base of the seismicity band in front of the mountain belt. The hypothesis that such a decollement exists is supported by the existence of a corresponding zone of low resistivity and also by the geometry of the earthquake of October 22, 1999. The latter clearly shows a fault branched off the basal decollement with an angle about 45°. This finding implies that the effective friction coefficient of the basal decollement is at least 23% less than that of the overlying rocks. [ABSTRACT FROM AUTHOR]

Coseismic frictional melting and the production of quenched glass called pseudotachylyte is a recurring process during earthquakes. To investigate how glassy materials affect the postseismic strength and stability of faults, obsidian gouges were sheared under dry and wet conditions from 200°C to 300°C at ~150 MPa effective normal stress. Dry glass exhibited a brittle rheology at all conditions tested, exhibiting friction values and microstructures consistent with siliciclastic materials. Likewise, wet glass at 200°C exhibited a brittle rheology. In contrast, wet gouges at 300°C transitioned from brittle sliding to linear-viscous (Newtonian) flow at strain rates <3 × 10−4 s−1, indicating melt-like behavior. The viscosity ranged from 2 × 1011 to 7.8 × 1011 Pa-s. Microstructures show that viscous gouges were fully welded with rod-shaped microlites rotated into the flow direction. Fourier transform infrared spectroscopy along with electron backscatter imaging demonstrate that hydration of the glass by diffusion of pore water was the dominant process reducing the viscosity and promoting viscous flow. As much as 5 wt % water diffused into the glass. These results may provide insight into postseismic-slip behaviors and challenge some interpretations of fault kinematics based on studies assuming that pseudotachylyte formation and flow is solely coseismic. [ABSTRACT FROM AUTHOR]

We studied the seismic activity in the Maravatío-Acambay and Actopan regions in Central Mexico. These regions are of great importance due to the occurrence of shallow crustal normal-faulting earthquakes that caused widespread destruction near their epicenter and as far away as Mexico City. That was the case of the 19 November 1912 Acambay ( M w 6.9). We determined statistical seismicity characteristics such as the Båth's law (the size of largest aftershock with respect to that of the mainshock), the b -value and p -value. For the Maravatío aftershock sequence, we obtained a b -value of 0.88 and p -value of 0.68. Based on reported seismicity, we obtained a b -value of 1.12 for the Actopan region. We estimated the size of the largest aftershock of the Acambay event in the range of 4.7 < M w < 5.9. By using the fragment-asperity model, we also performed the analysis of the magnitude distribution. The analysis showed similar values for the non-extensivity parameter ( q -value): q = 1.6465 and q = 1.6555 for the Maravatío aftershock sequence and for the seismicity in the Actopan region, respectively. The similitude in the q -values could be associated with the similarities in the tectonic environment. In this model, the constant of proportionality ( a ) between released energy and the size of fragments showed that more energy is released for the Maravatío aftershock sequence than for the regular seismicity rate in the Acambay region. Finally, we analyzed the relation of the seismicity and the tectonic environment by quantifying the seismic coupling and the thickness of the seismogenic layer. The estimated seismogenic layer for the Maravatío-Acambay and Actopan regions are 14.6 km and 20.8 km, respectively. The seismic coupling coefficient at the Venta the Bravo fault in the Maravatío-Acambay region and Actopan region are 0.21 and 0.46, respectively. Our estimation of the seismic coupling coefficients shows that the regions can be classified as low-to-intermediate-coupling zones. [ABSTRACT FROM AUTHOR]

This paper presents the first whole Taiwan-scale spatial variation of the seismogenic zone using a high-quality crustal seismicity catalog. The seismicity onset and cutoff depths (i.e., seismogenic depths) are determined by the earthquake depth–moment distribution and used to define the upper and lower boundaries of the seismogenic zone, respectively. Together with the published fault geometries and fault area–moment magnitude relations, the depth difference in the onset and cutoff depths (i.e., seismogenic thickness) is used as the fault width to determine the moment magnitudes of potential earthquakes for the major seismogenic faults. Results show that the largest ( Mw 7.9–8.0) potential earthquake may occur along the Changhua fault in western Taiwan, where the seismic risk is relatively high and seismic hazard mitigation should be a matter of urgent concern. In addition, the first-motion focal mechanism catalog is used to examine the relation between the seismogenic depths and earthquake source parameters. For crustal earthquakes (≤ 50 km), the shallowest onset and cutoff depths are observed for normal and strike-slip events, respectively. This observation is different from the prediction of the conventional continental-rheology model, which states that thrust events have the shallowest cutoff depth. Thus, a more sophisticated rheology model is necessary to explain our observed dependence of the seismogenic depths on faulting types. Meanwhile, for intermediate to large crustal ( Mw ≥ 4; depth ≤ 50 km) earthquakes, thrust events tend to occur at the bottom region of the seismogenic zone, but normal and strike-slip events distribute at a large depth range. [ABSTRACT FROM AUTHOR]

In order to determine fluid-mineral reactions and the fluid budget in subduction zones, we have examined rocks from two accretionary complexes, the Kodiak (Alaska) and Shimanto (Japan), that were buried at the temperature conditions of the seismogenic zone. Maximum burial temperatures based on Raman spectroscopy of carbonaceous material in both examples range from 230 to 350 °C, whereas pressures reached up to ⁓3 and ⁓4kbars (based on multi-equilibrium modelling) in the higher-grade units of the Japanese and Alaskan complexes, respectively. At 230–260 °C, chlorite is interpreted as a coproduct of the lower-grade smectite-to-illite transformation. Chlorite content increases as temperature increases to 330–350 °C, as a result of illite-to-chlorite transformation. Other reactions include pyrite oxidation, dissolution of titanite and precipitation of anatase. Whole-rock chemistry points to the absence of systematic chemical variations in the temperature range between 250 and 350 °C and only local transport of quartz and albite from the matrix, to veins nearby, is observed. Qualitative analyses of the composition of fluid inclusions using decrepitation method show that the composition of the fluid is significantly different from the original seawater trapped in pores. In addition, the salinity of fluid inclusions in the quartz veins at 230–260 °C, analyzed using the Raman spectrum of water, is lower than seawater and interpreted as dilution of the original seawater by the fresh water released by the smectite-to-illite transformation. In contrast, veins formed at 330–350 °C contain fluid inclusions with a salinity on the order of or higher than that of seawater. The increase in salinity for temperatures of 250–350 °C reflects hydration reactions, which are mostly controlled by chlorite crystallization. This study demonstrates an uptake of pore fluid by mineral reactions at temperature corresponding to the base of the seismogenic zone, which is in contrast to the general trend of dehydration reactions occurring along rock burial in subduction zones. • Chlorite crystallization occur in the temperature range 260–350 °C. • Fluid inclusions salinity increase results from chlorite crystallization. • No bulk rock change; mass diffusive transfer is only local from the matrix to veins. • Mineral phase equilibrium prevails only at temperatures >300 °C. • Pore fluid chemistry is significantly affected by prograde reactions. [ABSTRACT FROM AUTHOR]