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Slow earthquakes are a recently discovered phenomenon that mainly occur updip and downdip of the seismogenic zones of great earthquakes along the subducting plate interface. The spatiotemporal activity of various slow earthquakes occurring in the Nankai subduction zone is characterized by along-strike heterogeneity and along-dip systematic changes. Various slow earthquakes are horizontally distributed at their own depths and along-strike segments can be observed with respect to this distribution downdip of the locked zone; however, slow and great earthquakes occur in the same depth range near the Nankai Trough and Japan Trench axes. The frequently observed spatiotemporal interactions between different slow earthquakes can be attributed to their sensitivity and the stress transfer of the surrounding areas. This stress transfer is expected to extend to the adjacent sections in the seismogenic zone. Therefore, precise monitoring of slow earthquakes is important for future evaluations of great earthquakes, which requires the long-term maintenance and continuous improvement of the high-quality observation networks.

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

Megathrust geometric properties exhibit some of the strongest correlations with maximum earthquake magnitude in global surveys of large subduction zone earthquakes, but the mechanisms through which fault geometry influences subduction earthquake cycle dynamics remain unresolved. Here, we develop 39 models of sequences of earthquakes and aseismic slip (SEAS) on variably‐dipping planar and variably‐curved nonplanar megathrusts using the volumetric, high‐order accurate code tandem to account for fault curvature. We vary the dip, downdip curvature and width of the seismogenic zone to examine how slab geometry mechanically influences megathrust seismic cycles, including the size, variability, and interevent timing of earthquakes. Dip and curvature control characteristic slip styles primarily through their influence on seismogenic zone width: wider seismogenic zones allow shallowly‐dipping megathrusts to host larger earthquakes than steeply‐dipping ones. Under elevated pore pressure and less strongly velocity‐weakening friction, all modeled fault geometries host uniform periodic ruptures. In contrast, shallowly‐dipping and sharply‐curved megathrusts host multi‐period supercycles of slow‐to‐fast, small‐to‐large slip events under higher effective stresses and more strongly velocity‐weakening friction. We discuss how subduction zones' maximum earthquake magnitudes may be primarily controlled by the dip and dimensions of the seismogenic zone, while second‐order effects from structurally‐derived mechanical heterogeneity modulate the recurrence frequency and timing of these events. Our results suggest that enhanced co‐ and interseismic strength and stress variability along the megathrust, such as induced near areas of high or heterogeneous fault curvature, limits how frequently large ruptures occur and may explain curved faults' tendency to host more frequent, smaller earthquakes than flat faults. Plain Language Summary: Subduction zones, where one tectonic plate dives beneath another, generate the largest earthquakes worldwide. Our study investigates how the shape and tilt of these large offshore underground faults, termed "megathrusts," may determine the size of large earthquakes, how often they happen, and how similar or different subsequent events are. By creating computer simulations of earthquakes in subduction zones, we found that the angles and dimensions of the megathrust may set a limit on how big an earthquake can get. We also find that the presence of bends or curves along these faults can make earthquakes more unpredictable, sometimes leading to more variable series of smaller quakes before the biggest one hits. Our findings may help explain why some areas near subduction zones are prone to larger or more frequent earthquakes than others. Understanding these patterns can improve our ability to prepare for these natural disasters, potentially reducing their damaging effects on nearby communities and infrastructure. Key Points: Systematic earthquake cycle simulations reveal how subduction zone geometry controls megathrust earthquake size and timingDip and curvature affect characteristic slip style: periodic uniform ruptures versus supercycles of variably‐sized slow‐to‐fast eventsSeismogenic zone dip and dimensions limit maximum earthquake size; curvature‐linked stress and strength heterogeneity modulates recurrence [ABSTRACT FROM AUTHOR]

Earthquakes that rupture several faults occur frequently within the shallow lithosphere but are rarely observed for intermediate‐depth events (70–300 km). On 29 November 2007, the Mw7.4 Martinique earthquake struck the Lesser Antilles Island Arc near the deep end of the Wadati‐Benioff‐Zone. The sparse regional seismic network of 2007 previously hampered a detailed examination of this unusually complex event. Here, we combine seismic data from different studies with regional moment tensor inversion results and 3D full‐waveform modeling. We show that the earthquake is a doublet consisting of dip‐slip and strike‐slip motion along two oblique structures, both activated under extensional stress along the strike of the slab. Comparison with tectonic reconstructions suggests that the earthquake ruptured along a re‐activated ridge‐transform segment of the subducted Proto‐Caribbean spreading ridge. The unprecedented resolution of the source process highlights the influence of pre‐existing structures on localizing slab deformation also at intermediate‐depth. Plain Language Summary: Some earthquakes in continents and near the ocean floor are known to break multiple, differently oriented, faults. Such compound earthquakes are rarely observed in subducted plates in the intermediate‐depth region between 70 and 300 km. Intermediate‐depth earthquake mechanics and stress state are possibly different from shallower earthquakes, and maybe they hinder complex events. On 29 November 2007, the Mw7.4 Martinique earthquake occurred at a depth of ∼150 km, near the deep end of the regional seismic zone below the Lesser Antilles Arc. The regional seismic network in 2007 was relatively sparse; it revealed the earthquake had a complex mechanism but did not previously allow for an in‐depth study. In this study, we combine different types of data and methods, including full waveform information, based on a recently derived 3D regional velocity model. Our analyses shows that the Martinique earthquake consisted of at least two distinct sub‐events on perpendicular faults in the subducted plate—a source doublet. By comparing the orientations of the doublet faults with plate tectonic reconstructions, we infer that this intermediate‐depth earthquake broke along a fossil plate‐boundary. This indicates that such structures remain structural weaknesses even after subduction. Key Points: Moment tensor solutions, aftershock activity, back‐projection, and source‐time function suggest a complex rupture of the 29 November 2007, Mw7.4 Martinique earthquakeRegional Moment Tensor modeling and aftershock cross‐correlation identified the event as a source doubletEarthquake likely re‐activated a fossil ridge‐transform structure associated with the subducted Proto‐Caribbean spreading ridge [ABSTRACT FROM AUTHOR]

The 26 January 1700 CE Cascadia subduction zone earthquake ruptured much of the plate boundary and generated a tsunami that deposited sand in coastal marshes from northern California to Vancouver Island. Although the depositional record of tsunami inundation is extensive in some of these marshes, few sites have been investigated in enough detail to map the inland extent of sand deposition and depict variability in tsunami deposit thickness and grain size. We collected 129 cores in marshes of the Salmon River estuary in Oregon and reanalyzed 114 core logs from a 1987–88 study that mapped the inland extent of circa 1700 CE sandy tsunami deposits. The ca. 1700 CE tsunami deposit in the Salmon River estuary is easily recognized in cores ≤1 m deep in which a buried marsh peat is overlain by a well sorted sand bed with a sharp lower contact that thins and fines inland. We use tsunami deposit data and models of sandy tsunami sediment transport (using Delft3D‐FLOW) to test 15 rupture models that could represent a ca. 1700 CE earthquake. At least 12–16 m of slip offshore of the Salmon River, which results in 0.8–1.0 m of coastal coseismic subsidence, is required to match the ca. 1700 CE sand deposit's inland extent, which is consistent with models of heterogeneous megathrust slip in ca. 1700 CE. Our methods of detailed tsunami deposit mapping, combined with sediment transport modeling, can be used to test models of megathrust ruptures and their tsunamis to potentially improve earthquake and tsunami hazard assessments. Plain Language Summary: We rely on the geologic record of great earthquakes and tsunamis, often preserved in the stratigraphy of coastal estuaries, to determine the size, frequency, and location of prehistoric events. At the Cascadia subduction zone, the 1700 CE earthquake caused a meter or more of sudden coastal subsidence and a tsunami that not only inundated coastal areas in Cascadia but was also recorded in Japan. Computer modeling implies that the earthquake occurred around 9 p.m. on 26 January 1700. Although sandy tsunami deposits from the 1700 earthquake are widespread throughout Cascadia, few sites have been cored extensively enough to confidently identify the inland extent of sandy tsunami deposits. We used over 200 sediment cores from the Salmon River estuary in Oregon to map the extent of sandy tsunami deposits from 1700. We then ran numerical models of sediment transport driven by a variety of earthquake generation sources to determine that earthquakes causing at least 0.8 m of subsidence are needed to generate a modeled tsunami capable of recreating the 1700 tsunami deposits observed in cores. This study demonstrates that tsunami deposit mapping and modeling methods can be used to improve our understanding of the impacts of past great earthquakes and tsunamis. Key Points: Models of sediment transport test earthquake and tsunami sources by comparing the modeled and observed distribution of sandy depositsSandy deposits can define the minimum inundation of the last major tsunami, around 1700 CE, generated at the Cascadia subduction zoneTo match tsunami deposit data, models require an earthquake that caused ≥0.8 m of coastal coseismic subsidence at the Salmon River [ABSTRACT FROM AUTHOR]

The top of the subducting plate is a thick, complex zone where the heterogeneous structure likely controls earthquake rupture processes. Imaging this heterogeneous channel typically involves active‐source methods with limited depth penetration, or low‐resolution teleseismic methods. To access short wavelengths at greater depth, we use high‐frequency P‐to‐S (PS, 1–15 Hz) mode‐converted arrivals from nearby earthquakes >50 km deep to image the plate interface at vertical scales <1 km. We use 37 broadband stations in southcentral Alaska between 2007 and 2008 at 10–15 km spacing, spanning the great 1964 earthquake rupture zone and adjacent deeper slow slip and tremor regions. The central 21 stations record high‐amplitude PS arrivals converting from the megathrust region, at depths corresponding to the top of a prominent low‐velocity zone (LVZ) in receiver function images. The PS/P amplitude ratio (APS_P) varies along strike and with depth of the conversion point but is independent of earthquake location and varies slowly between adjacent stations. APS_P changes with slab depth, indicating changes in lithology or fluid content of the plate interface, consistent with transitions in slip behavior from locked to slow slip. High APS_P cannot be explained by a velocity step or a single low‐velocity, high Vp/Vs layer, but requires several alternating high and low‐velocity layers. These observations indicate that the LVZ is a highly heterogeneous channel at multiple scales, resembling a subduction channel or sheared zone of metasediment and altered crust as observed in many exhumed subduction zones. Plain Language Summary: The largest earthquakes in the world occur in subduction zones. The contact between the overriding and subduction plates, the megathrust, hosts great earthquakes and less hazardous slow slip and tremor events. The location of these depends on the fault's physical properties, but there is a discrepancy between the structures different seismic imaging techniques observe. To address this issue, our study uses a previously underutilized P‐to‐S (PS) signal from local in‐slab earthquakes, which start as a P wave and converts to an S wave at the plate interface. The conversion occurs in the megathrust fault zone, and the arrival amplitude depends on the properties across it. The high amplitudes of PS compared to P across the array favor a model that represents the interface as a broad shear zone, where the velocity and number of layers create variability in the amplitudes. Mapping the amplitudes shows high variability in the fault properties at a finer scale than typical passive seismic imaging techniques. Key Points: Southcentral Alaskan stations record high amplitude arrivals converting from P to S at the megathrust for local earthquakes >50 km deepThe timing of P‐to‐S conversions varies between stations requiring a conversion from top of the subducting plateArrival amplitudes change coherently across the megathrust, indicating lithology or fluid changes at tens of km spatial scale [ABSTRACT FROM AUTHOR]

In this study, alternatively to the megathrust, we identify upper plate normal faults orthogonal to the trench as a possible tsunami source along the Lesser Antilles subduction zone. We study the Morne Piton Fault system, a trench-perpendicular upper crustal fault affecting the Lesser Antilles forearc at the latitude of Guadeloupe. By the means of seismic reflection, high resolution bathymetry, Remotely Operated Vehicle images and dating, we reassess the slip rate of the Morne Piton Fault at 0.2 mm.yr-1 since fault inception (i.e. 7 Ma), dividing by five previous estimations and thus increasing the earthquake time recurrence and lowering the associated hazard. We evidence a metric scarp with striae at the toe of the Morne Piton Fault system suggesting a recent fault rupture. We estimate a fault rupture area of ~ 450-675 km2 and then a magnitude range for the seismic event around Mw 6.5 ± 0.5. We present results from a multi-segment tsunami model representative for the worst-case scenario which gives an overview of what could happen in terms of tsunami generation if the whole identified Morne Piton Fault segments ruptured together. Our model illustrates the potential impact of local tsunamis on the surrounding coastal area as well as local bathymetric controls on tsunami propagation as (i) shallow water plateaus act as secondary sources and are responsible for a wrapping of the tsunami waves around the island of Marie-Galante, (ii) canyons are focusing and enhancing the wave height in front of the most touristic and populated town of the island, (iii) a resonance phenomenon is observed within Les Saintes archipelago showing that the waves' frequency content is able to perturbate the sea-level during many hours after the seismic rupture. [ABSTRACT FROM AUTHOR]

We employed seismic tomography to examine the velocity structure of the upper mantle in the Southernmost Mariana subduction zone. Our study focuses on data collected during a six-month experiment from 15 December 2016 to 12 June 2017, using 11 ocean bottom seismometers. By examining over 3700 local arrival times, we are able to determine the three-dimensional Vp and Vs structure. The subducting slab in this region displays a P- and S-wave velocity 2~6% higher than normal mantle and a lower Vp/Vs, with an average dip of 45° at depths ranging from 50 to 100 km. Additionally, our velocity images also shed new lights to the velocity anomalies of the mantle wedge region on top of the subducting slab, from the trench to the remnant arc. We observed slower velocity anomalies in the mantle wedge beneath the Southwest Mariana Rift, the West Mariana Ridge, and the forearc. In the outer forearc, a low-velocity anomaly is observed at depths shallower than 50 km, indicating mantle serpentinization and the presence of water. Additionally, a melt production region is observed beneath the central part of the forearc block at a depth of 40–60 km suggesting the possibility of melting processes in this region. [ABSTRACT FROM AUTHOR]

We investigate spatiotemporal changes of intermediate‐depth earthquakes in the double seismic zone beneath Central and Northeastern Japan before and after the 2011 magnitude 9 Tohoku earthquake. We build a template‐matching catalog 1 year before and 1 year after the Tohoku earthquake using Hi‐net recordings. The new catalog has a six‐fold increase in earthquakes compared to the Japan Meteorological Agency catalog. Our results show no significant change in the intermediate‐depth earthquake rate prior to the Tohoku earthquake, but a clear increase in both planes following the Tohoku earthquake. The regions with increased intermediate‐depth earthquake activity and the post‐seismic slips following the Tohoku earthquake are spatially separate and complementary with each other. Aftershock productivity of intermediate‐depth earthquakes increased in both planes following the Tohoku earthquake. Overall, aftershock productivity of the upper plane is higher than the lower plane, likely indicating that stress environments and physical mechanisms of intermediate‐depth earthquakes in the two planes are distinct. Plain Language Summary: Intermediate‐depth earthquakes occur at 70–350 km depth below the Earth's surface. Because of the high pressure and temperature at such depths, the physical mechanism of intermediate‐depth earthquakes is still poorly understood. In this study, we try to obtain insights into this problem by investigating the behaviors of intermediate‐depth earthquakes before and after the 2011 magnitude 9 Tohoku earthquake. We build an intermediate‐depth earthquake catalog in Central and Northeastern Japan around the Tohoku earthquake. This new catalog contains six times more earthquakes and is more complete than the standard catalog used in previous studies. After analyzing the earthquakes from the new catalog, we find no significant precursory acceleration of intermediate‐depth earthquake activities prior to the Tohoku earthquake. However, we observe a clear increase in intermediate‐depth earthquake rate following the Tohoku earthquake. We also observe that the aftershock productivity of intermediate‐depth earthquakes increased following the Tohoku earthquake. The subduction slab beneath our studied area is characterized by a double seismic zone, and we find that aftershock productivity in the upper plane of seismicity is higher than that in the lower plane. This phenomenon may be due to differences in the environments or physical mechanisms in the two planes of intermediate‐depth earthquakes. Key Points: We build a new catalog of intermediate‐depth earthquakes in Japan 1 year before and 1 year after the 2011 M9 Tohoku earthquakeThere is no significant increase in intermediate‐depth earthquake activities prior to the Tohoku earthquake but a clear increase after itAftershock productivities of double seismic zones increase after the Tohoku earthquake, and it in upper plane is higher than in lower plane [ABSTRACT FROM AUTHOR]

An Mw 6.2 earthquake occurred in Suzu, northeastern Noto Peninsula, Japan, on 5 May 2023, followed by many aftershocks. Before this mainshock‐aftershock sequence, an intense earthquake swarm lasted in the vicinity for 2.5 years. Here, we estimated the rupture process of the Mw 6.2 mainshock and relocated >20,000 surrounding small earthquakes. The results show that systematic upward migration occurred via a complex network of faults in the preceding swarm period and that the mainshock rupture was initiated near the shallow end of the swarm earthquakes. The mainshock rupture propagated farther updip, followed by many aftershocks in the shallow extension. Upward fluid movement likely caused systematic upward earthquake migration from a depth of 18–5 km. The present results indicate the importance of monitoring swarm events since large (M > 6) and dangerous earthquakes can occur during such swarms. Plain Language Summary: Growing evidence suggests that fluid movements at depth can cause earthquake swarms in the overriding plates of subduction zones. However, there is less evidence of fluid involvement in the occurrence of large (M > 6) earthquakes, and it has been reported that the generation environments of large earthquakes and swarm earthquakes tend to be different. During intense swarm activity in the northeastern Noto Peninsula, Japan, starting at the end of 2020, an M6.2 earthquake occurred on 5 May 2023, followed by many aftershocks. To clarify the occurrence mechanism of the M6.2 mainshock, we relocated >20,000 small earthquakes and estimated the rupture process. The results showed a systematic upward migration of earthquakes in the preceding swarm period on the deep side of the mainshock fault (depth from 20 to 12 km). The mainshock rupture initiated near the shallow end of the swarm activity and propagated updip (depth from 12 to 9 km). Many aftershocks followed in shallower part of the same fault (depth from 10 to 5 km). Upward fluid migration likely caused the observed systematic upward earthquake migration throughout the sequence. The results indicate that earthquakes as large as M > 6 can be triggered by fluid migration during enhanced seismicity in swarms. Key Points: An Mw 6.2 event initiated near the shallow end of a fault where the preceding earthquake swarm showed systematic upward migrationThe Mw 6.2 rupture propagated farther updip, followed by many shallow aftershocks on the faultUpward fluid migration can trigger earthquakes as large as Mw > 6 [ABSTRACT FROM AUTHOR]

The Cascadia subduction zone has well‐documented geological records of megathrust earthquakes with the latest ∼Mw 9 rupture occurring in CE 1700. The paleoseismic observations suggest that southern Cascadia is mature for future earthquakes since the last event. Consequently, it is crucial to investigate the potential rupture scenarios. Although existing interseismic locking models share similar moment deficits, whether future earthquakes would be margin‐wide or segmented, as suggested by paleoseismic records, remains unknown. Accordingly, we aim to investigate: (a) possible rupture segmentation patterns, (b) whether south Cascadia can host margin‐wide ruptures, and (c) whether the existing locking models suggest similar future rupture scenarios. Assuming constant effective normal stress, we estimate the stress distribution constrained by the locking models (i.e., Gamma model from Schmalzle et al. (2014, https://doi.org/10.1002/2013GC005172), the preferred model from S. Li et al. (2018, https://doi.org/10.1029/2018JB015620), and the best‐fit model from Lindsey et al. (2021, https://doi.org/10.1038/s41561-021-00736-x)) from static calculation and discover they lead to different stress distributions, indicating distinct seismic potentials. Our dynamic rupture scenarios show that the south can generate both segmented ruptures (>Mw 7.3–8.4) and margin‐wide ruptures (>Mw 8.6) depending on the hypocenter location. The along‐strike rupture length appears to coincide with rupture lengths hypothesized from paleoseismology, and the Schmalzle model produces reasonable coastal subsidence amplitudes from the CE 1700 event. Therefore, we propose that three high‐slip trench‐breaching patches are sufficient for reproducing historical subsidence records. Our results can be further applied in seismic and tsunami hazard assessment, and serve as a comparison for non‐trench‐breaching scenarios. Plain Language Summary: The Cascadia subduction zone is a region off the coast of North America where one tectonic plate (Juan de Fuca) slowly slides beneath another (North America). Although there have been no significant earthquakes in Cascadia recently, geological records indicate that the last earthquake, estimated as a magnitude 9, occurred in CE 1700. In between earthquake occurrences, stress is accumulated and manifested in strain build‐up measurable by instruments. Existing models infer stress accumulation from these measurements but are variable along the length of the earthquake fault. Based on stress distributions suggested by the models, we design potential future earthquake scenarios using three‐dimensional computer simulations. Our results show how the final earthquake size and expected ground‐shaking change depending on where it is allowed to initiate in the simulation. Key Points: We conduct 3D dynamic rupture simulations for the future possible scenarios in Cascadia with constraints from interseismic locking modelsDifferent hypocenters in southern Cascadia reveal rupture segmentation and the southernmost unilateral rupture gives greater ground‐shakingOur dynamic rupture scenarios have reasonably consistent segmentation extents and coastal subsidence patterns with paleoseismic observations [ABSTRACT FROM AUTHOR]

Volcanic glass and its mixture with smectite are commonly observed in shallow parts of subduction zones. As volcanic glass layers often act as glide planes in submarine landslides, and because its alteration product, smectite, is one of the frictionally weakest geological materials, the frictional characteristics of volcanic glass‐smectite mixtures are important for fault slip behavior in shallow parts of subduction zones. We performed a series of friction experiments on volcanic glass‐smectite mixtures with different smectite contents from 0% to 100% at various velocity conditions from 10 μm/s to 1 m/s under an effective normal stress of 5 MPa and pore pressure of 10 MPa. In general, apparent friction coefficients negatively depend on the smectite content at any velocity tested. We found that samples with smectite contents of 15%–30% showed a drastic slip‐weakening behavior at intermediate velocities of 1–3 mm/s. Finite element method modeling shows that thermal pressurization does not contribute to the observed weakening behavior. The critical nucleation length estimated from the slip‐weakening behavior is approximately 1–10 km, which is large enough to prevent the slip to accelerate to seismic slip velocity. Therefore, gouges with minor amount of clay, such as subducting volcanic ash layers, may contribute to the occurrence of the slow earthquakes at shallow depths in subduction zones. Plain Language Summary: Materials erupted from volcanoes deposit on the seafloor and subduct at the trench. One such material, volcanic glass, easily alters into mechanically weak clay minerals such as smectite that can cause an enormous slip during an earthquake in subduction zones. In this study, we experimentally examined the frictional properties of mixtures of volcanic glass and smectite to elucidate fault slip behavior at shallow depths of subduction zones. Experiments with varying smectite content showed that a drastic reduction in fault frictional strength was induced when a small amount of smectite was present at moderately high velocity conditions. This behavior may induce slow earthquakes in shallow subduction zones. Key Points: Small amount of clay minerals in volcanic glass induces a drastic slip‐weakening at ∼1 mm/sThermal processes do not contribute to the weakening behaviorSlip weakening and large critical nucleation length at intermediate slip velocities might cause slow earthquakes [ABSTRACT FROM AUTHOR]

Off Cape Muroto area, along the Nankai Trough in southwest Japan, is a typical area with adjacent occurrences of slow and megathrust earthquakes. High‐resolution monitoring of slow earthquakes is necessary to understand tectonic conditions. In the off Muroto area, distributed acoustic sensing (DAS) measurement, which provides high‐density strain data, has been conducted using offshore fiber‐optic cable. We observed shallow tremors, a type of slow earthquakes, using DAS measurement for the first time. The characteristics of the tremor signals recorded by DAS were longer durations than those recorded in seismographs and composed of several phases with apparent velocities of several hundreds of m/s to several km/s that are coherent only in tens of meters. By combining DAS and seismograph data, we located these tremors around a subducted seamount peak. Therefore, spatial relationship between slow earthquakes and structural characteristics is suggested. This is a pioneering study of slow earthquake observation using DAS. Plain Language Summary: Distributed acoustic sensing (DAS) measurement is a recent technology that uses a fiber‐optic cable as a strain sensor array. As DAS provides high‐density observation data, it has often been used for the observation of regular earthquakes in recent years. However, few studies have used DAS measurement to observe slow earthquakes, which have longer characteristic durations compared to regular earthquakes. We observed shallow tremors, a type of slow earthquake, using DAS for the first time off Cape Muroto, where slow and huge regular earthquakes occur in the neighboring areas. The characteristics of tremor signals in DAS data are long durations and composition of several phases with variable apparent velocities that are coherent only in tens of meters. We located tremors by picking the arrival of signals by visually checking the waveforms of DAS and an offshore seismograph network and locating them at the point where the difference between synthetic and observed arrival times was the smallest. Most tremors occurred around 134.7°E, 32.8°N, which corresponds to a subducted seamount peak; therefore, a spatial relationship between slow earthquakes and a subducted seamount is suggested in this region. Key Points: We first observed shallow tremors by distributed acoustic sensing (DAS) using offshore fiber‐optic cable in the Nankai TroughTremor signals of DAS are composed of several phases of variable apparent velocities that are coherent only in tens of metersSpatial relationship between shallow tremors and subducted seamount is suggested along the dip direction [ABSTRACT FROM AUTHOR]

In this study, we investigated the relationship between the strain accumulation before a slow slip event (SSE) and the strain release during the SSE for three recent SSEs along the Suruga Trough, Sagami Trough, and Nankai Trough, which are subduction zones in central to southwest Japan. The three analysed SSEs were the 2013–2016 Tokai long-term SSE (L-SSE), the 2018 Boso-Oki short-term SSE (S-SSE), and the 2019–2021 Central Shikoku L-SSE. We applied exponential and logarithmic functions to remove the postseismic deformations caused by the Mw 9.0 2011 Tohoku-Oki earthquake. We discovered a strong negative correlation between strain accumulation and strain release in the dilatation of all three SSEs and in the maximum shear strain of the 2018 Boso-Oki S-SSE. A comparison of the amount of strain accumulation with that of strain release revealed that approximately 30% of the strain was released in the 2013–2016 Tokai L-SSE, that 40% of the strain was released in the 2019–2021 Central Shikoku L-SSE, and that approximately 60% of the strain was released in the 2018 Boso-Oki S-SSE. This finding suggests that all of the accumulated strains are not necessarily released by the SSEs. [ABSTRACT FROM AUTHOR]

Subducting seamounts are recognized as one of the key features influencing megathrust earthquakes. However, whether they trigger or arrest ruptures remains debated. Here, we use analog models to study the influence of a single seamount on megathrust earthquakes, separating the effect of topography from that of friction. Four different model configurations have been developed (i.e., flat interface, high and low friction seamount, low friction patch). In our models, the seamount reduces recurrence time, interseismic coupling, and fault strength, suggesting that it acts as a barrier: 80% of the ruptures concentrate in flat regions that surround the seamount and only smaller magnitude earthquakes nucleate above it. The low‐friction zone, which mimics the fluid accumulation or the establishment of fracture systems in natural cases, seems to be the most efficient in arresting rupture propagation in our experimental setting. Plain Language Summary: Seamounts ‐ extinct volcanoes ‐ are ubiquitous features of the seafloor of subducting plates. During their descent toward the mantle, seamounts are squeezed between the overriding‐ and subducting plates, creating a geometrical and mechanical discontinuity that is thought to control the largest earthquakes that occur on Earth. It is not known, however, whether they trigger or arrest earthquakes, as a variety of observations have been used to support one or the other hypothesis. Here, we tackle this subject using scaled laboratory models that allow reproduction of large earthquakes. Our models support a scenario where subducting seamounts primarily arrest rupture propagation, limiting therefore expected maximum magnitudes. Our models also suggest that subducting seamounts might influence the recurrence time of large earthquakes by decreasing their frequency. Key Points: We investigated the effect of seamount subduction on megathrust earthquakes by isolating the role of geometry from that of frictionSeamounts act primarily as barriers to earthquake propagation, promoting small ruptures and decreasing seismic couplingReduced friction displays maximum barrier efficiency, a configuration likely associated with fluids or fracture systems in nature [ABSTRACT FROM AUTHOR]

Large variations in the maximum earthquake magnitude ( M max ) have been observed among the world's subduction zones. There is still no universal relationship between M max and a given subduction-zone parameter, such as plate age, plate dip angle, or plate velocity, which suggests that multiple parameters control M max . Here, we conduct exhaustive variable selections that are based on three evaluation criteria; leave-one-out cross-validation errors (LOOCVE), Akaike information criterion (AIC), and Bayesian information criterion (BIC) to determine the combination of subduction-zone parameters that best explains M max . Multiple linear regression analyses are applied using 18 subduction-zone parameters as potential candidates for the explanatory variables of M max . The minimum BIC is obtained when five variables (trench sediment thickness, existence of an accretionary prism, upper-plate crustal thickness, bending radius of the subducting oceanic plate, and trench depth) are selected as explanatory variables; each variable contributes positively to M max . Minimum LOOCVE and AIC values are obtained when eight variables (the five parameters for BIC, plus the along-strike plate convergence rate, age of the subducting plate, and maximum depth of the subducting plate) are selected. Our selection of the trench sediment thickness and plate bending radius contributing to M max is consistent with previous studies. The results show that increasing upper-plate crustal thickness results in a large M max . In addition to smoothing the subducting-plate interface via subducted sediments, along-dip extension of the crustal area along the convergent plate boundary would be important for generating a large earthquake. [ABSTRACT FROM AUTHOR]

Various types of slow earthquakes occur in the Nankai subduction zone, among which the slow slip events (SSEs) seem to connect the shallow and deep part of the subduction plate and further trigger seismic or aseismic transients in surrounding areas. This study uses a version of space‐time epidemic‐type aftershock sequence (ETAS) model with non‐stationary background rate to detect earthquake swarms that may be related to SSEs in the Nankai region. We detect 999 swarm sequences including 13,387 earthquakes, accounting for about 18 percent of all M ≥ 1.0 earthquakes in the selected region along the Nankai trough. The detected swarms are mostly distributed in the Hyuga‐nada, Bungo Channel, Kii Channel, and Tokai regions, forming a complementary pattern to SSEs. Nearly 70 percent of swarm events are located in western Nankai, with the southern part characterized by a higher occurrence rate, which is consistent with the smaller recurrence interval of SSEs in this region. By comparing occurrence times of swarm events to those of SSEs, we find that some SSEs are accompanied by enhanced swarm activities during their occurrence periods. In addition, the swarm rates increase to higher levels in the southern part of western Nankai during the 2003 and 2010 Bungo SSEs, indicating the high sensitivity of earthquake swarms to the migrations of slow earthquakes in this region. Plain Language Summary: Aseismic slips in subduction zones, such as slow slip events (SSEs), can cause changes in seismicity rate, sometimes reflected by the occurrences of earthquake swarms. Such phenomena have been detected worldwidely in subduction zones, including the Japan, Aleutian and Hikurangi trenches. However, the swarm activities related to slow slip events along the Nankai trough remains unclear. Since SSEs along the Nankai subduction zone recur frequently and may affect the generation of large earthquakes in the updip locked areas, it is crucial to detect earthquake swarms and reveal the spatiotemporal relationship between swarms and SSEs in the Nankai region. Using a statistical branching model, we can detect earthquake swarms in the Nankai subduction zone, which might correlate with SSEs in space and time. We find that the detected earthquake swarms are mostly located in western Nankai, and generally complement with slips of SSEs. The occurrence rate of swarms in southern Hyuga‐nada with low interplate coupling is relatively high. Furthermore, the migrations of slow earthquakes in the southern part of western Nankai are accompanied by enhanced swarm rates. These results suggest that detecting seismicity changes is important for understanding the stress release process in subduction zones. Key Points: Earthquake swarms are mainly intraplate events in Nankai and form a complementary pattern with slow slip events (SSEs) in spaceAftershock productivity of large earthquakes positively correlates with interplate coupling in western NankaiNo significant spatiotemporal correlations between SSEs and swarms are observed in general, except for some individual SSEs [ABSTRACT FROM AUTHOR]

Slow earthquakes are now commonly found to display a wide range of durations, moments, and slip and propagation speeds. But not all types of slow earthquakes have been examined in detail. Here we probe tremor bursts with durations between 1 and 30 min, which are likely driven by few minute‐long bursts of aseismic slip. We use a coherence‐based technique to detect thousands of tremor bursts beneath Vancouver Island in Cascadia. Then we examine 17 of the ruptures by tracking their evolving tremor locations over an 8‐km region. We find that tremor migrates at rates of 3–25 m/s: faster than longer tremor bursts. Though some observational biases persist, the short events' speeds appear to fill a gap in the spectrum of observed slow earthquakes. They may provide further evidence that whatever fault zone process creates slow earthquakes, it must allow for faster slip and propagation in smaller ruptures. Plain Language Summary: Slow earthquakes, like earthquakes, are events with transient slip. But in slow earthquakes, faults slip slowly. Slip rates are thousands or millions of times slower than in earthquakes. There are a range of types of slow earthquakes, with durations from seconds to years. But not all types of slow earthquakes have been examined in detail. Here we examine slow earthquakes with durations between 1 and 30 min. Specifically, we examine the small bursts of seismic energy created by the slow slip. We develop techniques to precisely identify and locate that seismic energy, also known as tremor, and we are able to track the growth of slow earthquakes over the course of their minute‐long durations. We find that during the few‐minute‐long events, tremor migrates at rates of 3–25 m/s. This few minute‐long migration fills an observational gap in our knowledge of slow earthquakes. The new observations may help us understand how various‐duration slow earthquakes are related to each other and what causes them. Key Points: Identify thousands of few‐minute tremor bursts and track migration during 17 burstsMigration speeds range from 3 to 25 m/sObserved ruptures fill an observational gap in the slow earthquake spectrum [ABSTRACT FROM AUTHOR]

Slow earthquakes are episodic slow fault slips. They form a fundamental component of interplate deformation processes, along with fast, regular earthquakes. Recent seismological and geodetic observations have revealed detailed slow earthquake activity along the Japan Trench—the subduction zone where the March 11, 2011, moment magnitude (Mw) 9.0 Tohoku-Oki earthquake occurred. In this paper, we review observational, experimental, and simulation studies on slow earthquakes along the Japan Trench and their research history. By compiling the observations of slow earthquakes (e.g., tectonic tremors, very-low-frequency earthquakes, and slow slip events) and related fault slip phenomena (e.g., small repeating earthquakes, earthquake swarms, and foreshocks of large interplate earthquakes), we present an integrated slow earthquake distribution along the Japan Trench. Slow and megathrust earthquakes are spatially complementary in distribution, and slow earthquakes sometimes trigger fast earthquakes in their vicinities. An approximately 200-km-long along-strike gap of seismic slow earthquakes (i.e., tectonic tremors and very-low-frequency earthquakes) corresponds with the huge interplate locked zone of the central Japan Trench. The Mw 9.0 Tohoku-Oki earthquake ruptured this locked zone, but the rupture terminated without propagating deep into the slow-earthquake-genic regions in the northern and southern Japan Trench. Slow earthquakes are involved in both the rupture initiation and termination processes of megathrust earthquakes in the Japan Trench. We then compared the integrated slow earthquake distribution with the crustal structure of the Japan Trench (e.g., interplate sedimentary units, subducting seamounts, petit-spot volcanoes, horst and graben structures, residual gravity, seismic velocity structure, and plate boundary reflection intensity) and described the geological environment of the slow-earthquake-genic regions (e.g., water sources, pressure–temperature conditions, and metamorphism). The integrated slow earthquake distribution enabled us to comprehensively discuss the role of slow earthquakes in the occurrence process of the Tohoku-Oki earthquake. The correspondences of the slow earthquake distribution with the crustal structure and geological environment provide insights into the slow-earthquake-genesis in the Japan Trench and imply that highly overpressured fluids are key to understanding the complex slow earthquake distribution. Furthermore, we propose that detailed monitoring of slow earthquake activity can improve the forecasts of interplate seismicity along the Japan Trench. [ABSTRACT FROM AUTHOR]

Nucleation of earthquake slip at the plate boundary fault (décollement) in subduction zones has been widely linked to the frictional properties of subducting sedimentary facies. However, recent seismological and geological observations suggest that the décollement develops in the subducting oceanic crust in the depth range of the seismogenic zone, at least in some cases. To understand the frictional properties of oceanic crustal material and their influence on seismogenesis, we performed hydrothermal friction experiments on simulated fault gouges of altered basalt, at temperatures of 100–550°C. The friction coefficient (μ) lies around 0.6 at most temperature conditions but a low μ down to 0.3 was observed at the highest temperature and lowest velocity condition. The velocity dependence of μ, (a−b), changes with increasing temperature from positive to negative at ∼100°C and from negative to positive at ∼450°C. Compared to gouges derived from sedimentary facies, the altered basalt gouge showed potentially unstable velocity weakening over a wider temperature range. Microstructural observations and microphysical interpretation infer that competition between dilatant granular flow and viscous compaction through pressure‐solution creep of albite contributed to the observed transition in (a−b). Alteration of oceanic crust during subduction produces fine grains of albite and chlorite through interactions with interstitial water, leading to reduction in its frictional strength and an increase in its seismogenic potential. Therefore, shear deformation possibly localizes within the altered oceanic crust leading to a larger potential for the nucleation of a megathrust earthquake in the depth range of the seismogenic zone. Plain Language Summary: Megathrust earthquakes in subduction zones have long been studied to mitigate damage by ground shakings and tsunamis. Frictional properties of plate boundary fault are fundamental information to understand the slip activities causing megathrust earthquakes. In this study, we focused on altered basalt, which is a rock of subducting oceanic crust and has not been focused on as much as sedimentary rocks. Laboratory friction experiments were performed at a wide temperature range with water pressurized conditions. We found that the altered basalt shows frictionally unstable behavior of the type that is prerequisite for generating an earthquake in the temperature conditions of seismogenic zone. This unstable behavior is likely to be result of alteration of oceanic crust during subduction. Because the altered basalt can be mechanically as weak as sedimentary facies, subducting oceanic crust forms an additional or perhaps even preferred candidate for sourcing megathrust earthquakes in subduction zones. Key Points: Altered basalt shows unstable frictional behavior under a wide range of hydrothermal conditionsProduction of fine‐grained albite during subduction promotes the frictional instability via pressure solution creepSubducting oceanic crust may serve as a source of nucleating megathrust earthquakes in subduction zones [ABSTRACT FROM AUTHOR]

The distribution of earthquakes at intermediate depths corresponding to pressures <2 GPa in several hot subduction zones (such as Cascadia and southwestern Japan) coincides with the breakdown of antigorite to forsterite and talc; thus, this reaction may have triggered these earthquakes. However, previous studies have overlooked the potential significance of this reaction. Here, we performed a series of time-dependent dehydration experiments on antigorite at a pressure of 200 MPa and a temperature range of 500-650 °C. The results show that dehydration is controlled by a heterogeneous nucleation and growth mechanism and has an activation energy of 354 ± 24 kJ/mol. The formation of fine-grained forsterite and large talc crystals is consistent with kinetic results indicating Avrami exponents n = -- 1.4-1.1 and ~2.7, respectively. Fluid production rates at 600 and 650 °C are 2.54 x 10-6 and 4.69 x 10-5 mfluidm-3cks-1, respectively, which are much faster than those of mantle deformation, causing high fluid pressure in hot subducting mantle but not necessarily embrittlement. We emphasize the role of kinetic mechanisms in controlling the grain sizes of reaction products, which likely determine the mechanical behavior of serpentinized fault zones. Superplasticity or velocity weakening of fine-grained forsterite and velocity weakening of antigorite by water and/or talc may be responsible for earthquake nucleation and propagation in a heterogeneous system, which can be either dehydration products within a serpentinized fault zone or the mixture of antigorite fault and surrounding peridotite in hot subduction zones (<2 GPa). [ABSTRACT FROM AUTHOR]

Data on the stress–strain state of the Earth's crust obtained by precision observations were studied using strain-gage analysis based on the geometric theory of deformation. Ground-based optical distance observations were made on one of the geodynamic polygons in Petropavlovsk-Kamchatsky to calculate the major axes of compression and tension. Out of ten baselines, four variants of the strain-gauge rosette were taken to calculate the invariant characteristics of deformation from measured displacements of the Earth's surface. The strain-gage analysis of the observation results made it possible to determine the main deformations and calculate the angle between the dominant loading axis and the shortening axis. Two sources of impact were proposed as factors that specify the dominant loading axis: magmatism (nearby active volcanoes) and the subduction zone (the impact of the subducting Pacific Plate). Since the region is seismically active, the analysis involved strong (M > 5.5) earthquakes to identify a possible relationship between earthquakes and deformation characteristics. [ABSTRACT FROM AUTHOR]

A strong earthquake with a magnitude Mw = 7.0 occurred on February 13, 2020, at a depth of 142 km in the vicinity of the Southern Kuril Islands. Over recent decades, there has been an anomaly near the focal zone in the earthquake distribution over the layers of the double seismic focal zone in accordance with the seismic dislocation kinematic type compared to the other areas of the intermediate depths of the southern part of the Kuril arc. Inversion of the earthquake focal mechanisms of intermediate depth was performed for the southern part of the Kuril Islands along the upper and lower layers of the seismic focal zone. In the upper layer, compression prevails along the slab; the axis of maximum compression σ3 is mainly parallel to the slab and is oriented at an angle of 140–160° (320–340°) to the direction of its dip, i.e., subparallel to the Pacific Plate motion vector in the mantle in this area. The stress state of extension along the slab prevails in the lower layer; the axis of minimum compression σ1 is parallel to the slab and is mainly rotated by 20° (200°) clockwise from the direction of its dip. This orientation of the principal stress axes could indicate that, in addition to the slab unbending, their formation is affected by the mantle resistance to its movement. In the upper and lower layers, we marked the areas where the shear stress prevails that correspond to the kinematic type of transform faults, which are presumably seismically active on the descending plate. The most complex pattern of the stress state of the medium was obtained near the focus of the earthquake that occurred on February 13, 2020. Here, in the upper layer, an area with the stress state of extension and orientation of the principal axes corresponds more to the lower layer. Seismic quiescence is observed in the lower layer, near the focus of this earthquake within the area 60 km wide along the island arc and at the entire interval of intermediate depths. [ABSTRACT FROM AUTHOR]

The Vp/Vs kinematic parameter for the Kamchatka seismic subduction zone has been long monitored on a regular basis. This has allowed estimating the behavior of the parameter and revealing its features during the precursory period and actual rupturing of the March 25, 2020 earthquake which occurred in the Pacific plate at the contact between the Pacific plate and the continental slope of Kamchatka. The March 25, 2020 earthquake with its high magnitude (ML = 7.6, MW = 7.5) is a rare enough event to occur in the Pacific plate. Knowledge of earth structure in the source region of the earthquake was updated based on the results of deep seismic sounding and continuous seismic profiling which was carried out in the area in the past. It was shown that the earthquake source area is at an inflection in the descending Pacific plate. The source area lies in the layer at the boundary between the asthenosphere and the crust and mantle. The layer is at depths of 30‒60 km and has lower Vp and Vs velocities. The inflection region along Kamchatka shows lower Vp/Vs values, which is a sign of extension along the plate. The source area is in the contact between the Pacific plate, which is subducting under the southern end of Kamchatka, and the North Kuril Islands, as well as the Sea-of-Okhotsk plate. The region shows a distinct change in the anomalous values of the parameter from lower to higher, with the latter being typical for compression. Our study has allowed estimating possible regions of tension and compression in the geological medium. [ABSTRACT FROM AUTHOR]

Geodetic and seismological observations have shown discrepancies between azimuths of maximum contraction (strain rate) and maximum compression (stress). These discrepancies can be the results of the superposition of localized or transient mechanical processes such as fault coupling during seismic cycles. Rich sets of recent geophysical data allow us to conduct spatiotemporal imaging of the discrepancies. Here, we estimate the spatiotemporal evolution in the strain-rate fields of the Japanese Islands with optimized smoothing distances from 1997 to 2021 using Global Navigation Satellite System (GNSS) data, and investigate how the maximum contraction axes of horizontal strain rates differ from those of horizontal stress based on earthquake focal mechanisms. Several characteristic results are observed for each region within the Japanese Islands. Both azimuths of the strain rates and stress differ by more than 60° over hundreds of kilometers from the Kanto region to along the Nankai Trough, related to seismotectonics due to the dual subduction of the Philippine Sea plate and the Pacific plate beneath the Japanese Islands. The differences in the azimuths imply the effect of the very long-term stable subduction of the Pacific plate. We find that the azimuthal differences tend to be small along tectonic zones with active inland earthquakes and high strain rates on the back-arc sides. We also find that the 2011 off the Pacific coast of Tohoku earthquake caused notable azimuthal differences in the strain rates and the stress in the Tohoku region. The strength of fault may cause lower response sensitivity of seismological stress to major earthquakes than geodetic strain rate. Our dataset has wide spatiotemporal coverage and can serve as a basis for further research, for example, to estimate the current fault conditions during seismic cycles. [ABSTRACT FROM AUTHOR]

Shallow slow earthquakes in subduction zones are related to spatiotemporal variations of stress on the plate boundary adjacent to a megathrust earthquake. A dense network (DONET) of cabled ocean‐bottom seismometers (OBSs) deployed in the Nankai Trough seismogenic zone, southwest Japan, has enabled real‐time monitoring of tremors. Because the amplitudes of tremor signals are very low, even when recorded by OBSs, we developed a hybrid method that simultaneously uses traveltimes based on cross‐correlations and amplitudes of waveforms to determine tremor hypocenters. We applied our method to waveforms observed during five years of continuous DONET recording from April 2016 to September 2021 to detect more than 6,500 tremors, including three major episodes in 2016, 2018, and 2020–2021. The tremors of each episode were distributed over distances of >100 km along the trench axis in two regions, southeast of the Kii Peninsula and off Cape Muroto. We also estimated seismic energy rates and found that for the three major tremor episodes, energy rates increased gradually with time. The 2018 and 2020–2021 episodes began near a subducted seamount and a subducted ridge, respectively, and were likely related to fluid movements controlled by the topography of the downgoing plate. We showed that the tremors were spatiotemporally synchronized with slow slip events, and their spatial variations of energy rate corresponded to stress heterogeneity on the shallow plate boundary. Future real‐time monitoring of shallow tremors will contribute to our understanding of changes of stress state due to slow earthquakes at the shallow plate boundary. Plain Language Summary: In recent years, slow earthquakes with lower frequency content than ordinary earthquakes have been observed near tectonic plate boundaries. These slow earthquakes are thought to be related to the accumulation and release of stress at the shallow part of the plate boundary, which is where large earthquakes could occur. However, it is extremely difficult to detect slow earthquakes because their signals are very weak. The dense seismic network (DONET) deployed recently on the seafloor of the Nankai Trough has made it possible to capture signals from these earthquakes because the recording instruments are much closer to the earthquake sources than previously used onshore seismic recording systems. We analyzed the waveforms recorded by DONET from 2016 to 2021 to determine the epicenters of tremors that occurred during that period. We detected more than 6,500 tremors and determined their spatiotemporal distribution around the shallow plate boundary. We showed that the tremors tend to occur in areas of the shallow plate boundary where stress is relatively low. Future analyses of DONET data will allow more detailed monitoring and provide a better understanding of changes in the stress state in the immediate vicinity of areas prone to large earthquakes. Key Points: We developed a new hybrid system to monitor tectonic tremors by using their envelope cross‐correlations and amplitudesWe detected more than 6,500 tremors along the Nankai Trough including three major and several minor episodes of tremor activityReal‐time tremor monitoring will add to knowledge of their relation to other earthquakes and of stress changes in shallow subduction zones [ABSTRACT FROM AUTHOR]

Detailed imaging of accretionary wedges reveals splay fault networks that could pose a significant tsunami hazard. However, the dynamics of multiple splay fault activation during megathrust earthquakes and the consequent effects on tsunami generation are not well understood. We use a 2‐D dynamic rupture model with complex topo‐bathymetry and six curved splay fault geometries constrained from realistic tectonic loading modeled by a geodynamic seismic cycle model with consistent initial stress and strength conditions. We find that all splay faults rupture coseismically. While the largest splay fault slips due to a complex rupture branching process from the megathrust, all other splay faults are activated either top down or bottom up by dynamic stress transfer induced by trapped seismic waves. We ascribe these differences to local non‐optimal fault orientations and variable along‐dip strength excess. Generally, rupture on splay faults is facilitated by their favorable stress orientations and low strength excess as a result of high pore‐fluid pressures. The ensuing tsunami modeled with non‐linear 1‐D shallow water equations consists of one high‐amplitude crest related to rupture on the longest splay fault and a second broader wave packet resulting from slip on the other faults. This results in two episodes of flooding and a larger run‐up distance than the single long‐wavelength (300 km) tsunami sourced by the megathrust‐only rupture. Since splay fault activation is determined by both variable stress and strength conditions and dynamic activation, considering both tectonic and earthquake processes is relevant for understanding tsunamigenesis. Plain Language Summary: In subduction zones, where one tectonic plate moves beneath another, earthquakes can occur on many different faults. Splay faults are relatively steep faults that branch off the largest fault (the megathrust) in a subduction zone. As they are steeper than the megathrust, the same amount of movement on them could result in more vertical displacement of the seafloor. Therefore, splay faults are thought to play an important role in the generation of tsunamis. Here, we use computer simulations to study if an earthquake can break multiple splay faults at once and how this affects the resulting tsunami. We find that multiple splay faults can indeed fail during a single earthquake due to the stress changes from trapped seismic waves, which promote rupture on splay faults. Rupture on splay faults results in larger seafloor displacements with smaller wavelengths, so the ensuing tsunami is bigger and results in two main flooding episodes at the coast. Our results show that it is important to consider rupture on splay faults when assessing tsunami hazard. Key Points: Multiple splay faults can be activated during a single earthquake by megathrust slip and dynamic stress transfer due to trapped wavesSplay fault activation is facilitated by their favorable orientation with respect to the local stress field and their closeness to failureLong‐term geodynamic stresses and fault geometries affect dynamic splay fault rupture and the subsequent tsunami [ABSTRACT FROM AUTHOR]

Natural faults host various types of migrating slow earthquake phenomena, with migration speeds much lower than seismic wave speeds and different moment‐duration scaling from regular earthquakes. To advance the obtained quantitative understanding of the migration process and long duration of slow earthquakes, I study a chain reaction model in a population of brittle asperities based on a rate‐ and state‐dependent friction on a 3‐D subduction plate boundary. Simulation results show that the migration speed is quantitatively related to frictional properties by an analytical relation derived here. By assuming that local pore water in front of the migration drives rapid tremor reversal and is so local as to hold a constant stress drop, the application of the analytical solution to observational results suggests that (a) the temporal changes of observed migration speeds for the rapid tremor reversal could be explained by about 70% reduction of the effective normal stress; (b) effective normal stress for the deeper extension of seismogenic segment in the western part of Shikoku is about 1.5 times greater than that in the central part. Applying rupture time delay between slow earthquake asperities for the duration longer than regular earthquake, I also conclude that (c) the characteristic slip distance of rate‐and‐state friction for low‐frequency earthquakes is roughly between 30 µm and 30 mm; (d) the stress and strength drops of very low‐frequency earthquakes is much smaller than 1 MPa. Plain Language Summary: Previous computer simulations and a few observational studies suggest that large subduction earthquakes may be preceded by slow earthquakes (not felt by humans) whose migration speed increases as the occurrence of the large earthquake approaches. So far, this precursory process has only been discussed qualitatively. In this study, I consider a chain reaction model on a heterogeneous fault made of small brittle asperities embedded in a viscous matrix: when a small asperity breaks rapidly, it generates a wave of slow slip around it, which in turn triggers the rupture of neighboring asperities, and so on. I develop a theoretical relation between slow earthquake migration speed and frictional properties. The model helps explain why slow earthquakes are slow, and provides a basis for precursory slow earthquake migration phenomena. The theoretical relation also provides an estimate of the characteristic slip distance of slow earthquakes, a rock property that is difficult to extrapolate it to actual fault from laboratory experiments to natural scales. Key Points: I propose a quantitative relation between slow earthquake migration speed and friction properties with observed stress dropI attribute the long duration of slow earthquakes to rupture time delays due to aseismic slip between seismic slip patchesCharacteristic slip distance for low‐frequency events is estimated as 30 μm∼30 mm, which is derived from propagation speed [ABSTRACT FROM AUTHOR]

With five earthquakes (1938, 1946, 1957, 1964, and 1965), the Aleutian‐Alaska subduction plate boundary ruptured a length of 3,548 km. We revisit these five earthquakes—first studied in detail by Sykes (1971, https://doi.org/10.1029/JB076i032p08021)—through probabilistic relocation of carefully selected mainshocks and aftershocks. Our final catalog of 324 events is established from a set of 12 mainshocks that includes all Mw ≥ 7.7 megathrust earthquakes. Using the relocated catalog, we create revised aftershock regions delimited both parallel and normal to the trench. These aftershock regions exhibit significant differences from previous studies, with the following basic findings: the 10 November 1938 Mw 8.3 earthquake extended further west, to the Shumagin Islands, and further east, into the Kodiak region, relative to the prevailing aftershock region established by McCann et al. (1979, https://doi.org/10.1007/978-3-0348-6430-5%5f2). The 01 April 1946 Mw 8.6 sequence was anomalously concentrated near the trench, which implies near‐trench coseismic slip that contributed to the exceptionally large tsunami. The 09 March 1957 Mw 8.6 aftershocks spanned a 1,230 km length with numerous aftershocks within the outer‐rise region of the incoming Pacific plate. The 28 March 1964 Mw 9.2 aftershocks extended east into the Pamplona thrust system (south of Icy Bay, Alaska), suggesting coseismic rupture into this region; this is consistent with coseismic static displacements, as well as current estimates of interseismic locking. The post‐1965 events we examine are all smaller than the earlier events, but since they occurred in an era of improved data collection (seismic, geodetic, and tsunami), they provide a better opportunity for assessing the link between the distribution of aftershocks and the occurrence of coseismic, postseismic, and interseismic slip. Plain Language Summary: The Aleutian‐Alaska subduction zone is one of the world's most active settings, having produced five large earthquakes (1938 M8.3, 1946 M8.6, 1957 M8.7, 1964 M9.2, and 1965 M8.6) along a length of 3,500 km. Fundamental information about these earthquakes was previously determined from studying the spatial coverage of aftershocks following these five mainshocks. The aftershock regions established the approximate region of rupture during the mainshocks. Here, we revisit a study undertaken in 1971 by relocating 324 earthquakes using globally recorded arrival times. We use the earthquake relocations to determine revised aftershock regions. We apply this analysis to the five largest earthquakes, as well as to all other plate boundary earthquakes since 1965 with magnitude greater than 7.7. The resulting set of aftershock regions enables a better understanding of how slip occurs along the Aleutian‐Alaska subduction zone, both during earthquakes and between earthquakes. Key Points: A relocated catalog of 324 aftershocks from 12 earthquakes (1938–2021) along the Aleutian‐Alaska megathrust is presentedThe catalog establishes revised aftershock regions, with smaller events (Mw = 7.7–8.2) postdating five larger, earlier events (1938, 1946, 1957, 1964, and 1965)The aftershock regions provide better resolution of the trench‐normal extent of aftershocks, enabling a nuanced view of coseismic slip [ABSTRACT FROM AUTHOR]

Strike-slip earthquakes near major subduction zones have received less attention than thrust or reverse earthquakes in subduction zone areas. The occurrence of the 2018 Palu Mw 7.5 earthquake in eastern Indonesia provides an unprecedented opportunity to investigate the characteristics of one of these events. The Palu earthquake occurred on the left-lateral, north–south-striking Palu–Koro fault, which is the main plate boundary structure accommodating the convergence between blocks in a triple junction area. It excited a significant tsunami, which unusually is associated with strike-slip earthquakes, and also ruptured at a supershear speed, which is mostly observed on strike-slip faults in continents. Based on our fieldwork, we speculate that the normal slip component of the offshore rupture section in Palu bay on the middle segment probably favours tsunami genesis. Our field investigation has revealed evidence of a simple geometry as well as slip partitioning of dip-slip and strike-slip motion on two subparallel strands on the main segment, both of which may have contributed to the supershear of the rupture propagation. Instead of only a transtensive behaviour of the middle segment, our results also illustrate the transpressional property of the northern and southern rupture segments, which shows more complex behaviour than that of a common continental strike-slip fault. [ABSTRACT FROM AUTHOR]

Coastal areas concentrate a large portion of the country's population around urban areas, which in subduction zones commonly are affected by drastic tectonic processes, such as the damage earthquakes have registered in recent decades. The seismic cycle of large earthquakes primarily controls changes in the coastal surface level in these zones. Therefore, quantifying temporal and spatial variations in land level after recent earthquakes is essential to understand shoreline variations better, and to assess their impacts on coastal urban areas. Here, we measure the coastal subsidence in central Chile using a multi-temporal differential interferometric synthetic aperture radar (MT-InSAR). This geographic zone corresponds to the northern limit of the 2010 Maule earthquake (Mw 8.8) rupture, an area affected by an aftershock of magnitude Mw 6.8 in 2019. The study is based on the exploitation of big data from SAR images of Sentinel-1 for comparison with data from continuous GNSS stations. We analyzed a coastline of ~300 km by SAR interferometry that provided high-resolution ground motion rates from between 2018 and 2021. Our results showed a wide range of subsidence rates at different scales, of analyses on a regional scale, and identified the area of subsidence on an urban scale. We identified an anomalous zone of subsidence of ~50 km, with a displacement <−20 mm/year. We discuss these results in the context of the impact of recent earthquakes and analyze the consequences of coastal subsidence. Our results allow us to identify stability in urban areas and quantify the vertical movement of the coast along the entire seismic cycle, in addition to the vertical movement of coast lands. Our results have implications for the planning of coastal infrastructure along subduction coasts in Chile. [ABSTRACT FROM AUTHOR]

A detailed 3‐D model of P wave isotropic velocity and azimuthal anisotropy beneath SE Asia is obtained by jointly inverting local earthquake arrival times and teleseismic relative travel‐time residuals. Our results show that the high‐velocity (high‐V) subducting Australian slab has penetrated through the mantle transition zone (MTZ) and reached a depth of ∼1,200 km beneath Sumatra, whereas the high‐V slab has subducted toward the north and trapped within the MTZ beneath Java. The Hainan mantle plume is revealed clearly as a significant low‐velocity anomaly beneath the MTZ, which extends down to the bottom of our model (∼1,600‐km depth). The upwelling of the Hainan plume is resisted by the stagnant slab in the MTZ, resulting in divergent fast‐velocity directions of azimuthal anisotropy beneath the stagnant slab. The detailed results of seismic tomography and azimuthal anisotropy provide important new information on the complex mantle structure and dynamics of the SE Asian region, in particular, the subducting slabs and the Hainan plume, as well as their interactions. Plain Language Summary: The SE Asia curved subduction system is an ideal region to study plate tectonics and subduction dynamics due to the coexistence and interactions of the Hainan mantle plume and subducting slabs. However, detailed geometries and structures of the plume and slabs are still unclear because of the uneven distribution of seismic stations and earthquakes in SE Asia. In this work, we study the 3‐D seismic velocity structure beneath SE Asia by conducting P wave seismic tomography. We find that the Hainan plume upwelling is resisted by a stagnant slab in the mantle transition zone (MTZ, at depths of 410–660 km), generating divergent azimuthal anisotropies that are centered in significant low‐velocity zones beneath the MTZ. The coexistence of the mantle plume and the stagnant slab may have caused the complex tectonic history and geological features in SE Asia, such as many arc and back‐arc volcanoes, active intraplate volcanism, and widely distributed Cenozoic basalts. Key Points: A high‐resolution 3‐D model of P wave velocity and azimuthal anisotropy beneath SE Asia is determinedDifferent slab geometries beneath Sumatra and Java are related to the lithosphere age of the subducting slabThe Hainan plume upwelling is resisted by a stagnant slab in the MTZ, causing divergent shape azimuthal anisotropies [ABSTRACT FROM AUTHOR]

Fault slip activity at subduction zones is governed by sediment frictional properties, which in turn are affected by diagenetic processes. To study the spatial patterns in frictional properties across the Nankai Trough, SW Japan, and their relations to fault slip activity, we used sediment samples (10%–59% clay mineral content) along the Kumano transect covering a large spatial range from the inputs via the outer prism to the inner prism, including the deepest sample ever recovered to date. We performed laboratory friction experiments under in situ effective normal stresses and seawater‐saturated conditions. Our results generally demonstrate that the friction coefficient inversely correlates with the clay mineral content. However, the outer prism sediments show higher friction coefficients than sediments from the other locations for a comparable clay mineral content. All samples show velocity‐weakening behavior at low velocities, but the outer prism and the deep inner prism sediments show velocity strengthening at higher velocities. Based on the experimental results combined with a Coulomb wedge model, we propose that the lowest friction coefficient on the décollement occurs beneath the trenchward portion of the outer prism, whereas the minimum friction coefficient of the prism sediment occurs in the landward portion of the outer prism. In addition, the calculated critical nucleation length for slip instability suggests that the décollement beneath the outer prism area is more frictionally unstable than it is beneath the inner prism. This inference is consistent with the spatial distribution of very‐low‐frequency earthquakes and slow slip events along the shallow Nankai Trough. Plain Language Summary: Subduction zones host the largest earthquakes on earth, and such earthquakes cause severe damage to regions nearby. Therefore, how and where an earthquake is triggered are the most important questions for the future mitigation of seismic hazards. An important factor controlling seismicity is the frictional behavior of the subduction zone fault. In this study, we study the Nankai subduction zone, SW Japan, which hosts both fast and slow earthquakes. Conducting laboratory friction experiments, we examine the spatial patterns in frictional properties of sub‐seafloor sediments sampled by ocean drilling. Experimental results indicate that the friction coefficients of sediments vary with the position in the subduction system, which may reflect diagenetic processes or tectonic history. Combined with the topographical information, we estimate the friction coefficient of the plate boundary fault (décollement) and propose that the friction coefficient increases landward. Our results can provide useful information on the location of future fault slip activities at the Nankai Trough. Key Points: Friction measurements and Coulomb wedge modeling indicate that the décollement and prism strengths vary spatiallyNucleation length for slip instability suggests the décollement is more unstable beneath the outer prism than the inner prismOur results explain the spatial distribution of slip events at the shallow Nankai Trough [ABSTRACT FROM AUTHOR]

Megathrust earthquakes are strongly influenced by the elastic properties of rocks surrounding the fault. In contrast to friction, these properties can be derived in situ from geophysical measurements along the seismogenic zone. However, they are often overestimated in numerical simulations, particularly in the shallow megathrust. Here we explore the influence that realistic depth‐varying upper‐plate elastic properties along the megathrust have on earthquake rupture dynamics and tsunamigenesis using 3D dynamic rupture and tsunami simulations. We compare results from three subduction zone scenarios with homogeneous and heterogeneous elastic media, and bimaterial fault. Elastic properties in the heterogeneous model follow a realistic depth‐distribution derived from controlled‐source tomography models of subduction zones. We assume the same friction properties for all scenarios. Simulations in the heterogeneous and homogeneous models show that rigidity depth variations explain the depth‐varying behavior of slip, slip rate, frequency content, and rupture time. The depth‐varying behavior of slip, frequency content, and rupture duration quantitatively agree with previous predictions based on worldwide data compilations, explaining the main depth‐dependent traits of tsunami earthquakes and large shallow megathrust earthquakes. Large slip, slow rupture and slip rate amplification in bimaterial simulations are largely controlled by elastic rock properties of the most compliant side of the fault, which in subduction zones is the upper plate. Large shallow slip and trenchward increasing upper‐plate compliance of the heterogeneous model lead to the largest co‐seismic seafloor deformation and tsunami amplitude. This highlights the importance of considering realistic upper‐plate rigidity variations to properly assess the tsunamigenic potential of megathrust earthquakes. Plain Language Summary: The largest earthquakes on Earth occur along the megathrust fault in subduction zones. When these events reach the shallow portion of the fault (<5 km depth), they are slower, generate larger slip, and release lower frequency content than in the deep region of the fault (i.e., 25 km depth). The presence of large slip at shallow depths is responsible for the large seafloor displacement that triggers the generation of devastating tsunamis. Thus, it is important to understand which factors determine the depth‐dependent behavior of these rupture properties to properly assess the tsunamigenic potential of megathrust earthquakes. In this work, we use 3D numerical dynamic rupture and tsunami simulations to demonstrate that this particular depth‐varying behavior is highly influenced by realistic variations of elastic rock properties overlying the fault in the upper plate. The overestimation of these properties in the shallow portion of the megathrust lead to underestimation of slip and seafloor displacement, and thus tsunami hazard. Key Points: We use 3D simulations to evaluate the influence of realistic upper‐plate rigidity on megathrust earthquake properties and tsunamigenesisSimulations show that realistic upper‐plate rigidity variations explain the depth‐dependent behavior of earthquake dynamic propertiesOverestimation of upper‐plate rigidity leads to underestimation of co‐seismic seafloor deformation and tsunami amplitude in our simulations [ABSTRACT FROM AUTHOR]

Accurate earthquake locations are important for understanding earthquake processes. Recent advances in earthquake relocation methods have resulted in relative centroid locations with smaller uncertainties using cross‐correlation‐based methods. However, relative hypocenter locations, which are traditionally determined using the onset times of event waveforms, generally possess larger uncertainties than that of the centroids. Here, we develop a new cross‐correlation‐based relative hypocenter relocation method based on the network correlation coefficient method, whereby we incorporate an additional step to search for a suitable window to cross‐correlate the event waveform onsets. We then perform a joint inversion on the obtained relative hypocenter and centroid locations to directly compare their relative locations under a unified coordinate. We apply this method to three regions where repeating earthquakes have been detected, and reveal both the repetitive rupture of similar centroids by these repeating events and newly illuminated hypocenter hotspots that serve as the onset region of earthquakes that grow into different but selective magnitudes. Our joint analysis of hypocenter and centroid locations enables quick estimations of rupture propagation directions without the need to perform slip inversion analyses. We estimate the location uncertainties via bootstrapping, with the 95% confidence intervals yielding sub‐100 m horizontal uncertainties in both the centroid and hypocenter locations with the best station coverage and event availability. These results point to limited but existing correspondence between the hypocenter and centroid locations and the earthquake magnitudes, as well as complexity in the hypocenter distributions, even within repeating earthquake sequences whose rupture areas largely overlap. Plain Language Summary: Earthquake locations have attracted considerable research attention because of their importance in better understanding earthquake processes. Conventional methods can better determine the centroid (i.e., the center of mass of the earthquake rupture) locations by, for instance, applying cross‐correlation‐based methods that analyze the long‐period component of the earthquake waveforms. However, hypocenter (i.e., the initial rupture point of earthquakes) locations are relatively poorly determined since the most commonly used approach is to pick the onsets of the earthquake waveforms from seismic records that are often noisy. Here, we present a new cross‐correlation‐based method, which is traditionally used to better determine centroid locations, to refine the hypocenter locations. We obtain high‐resolution relative locations among the hypocenters and centroids, and then perform a joint analysis of the centroids and hypocenters, which enables us to compare their distribution relative to each other under the same unified coordinate. We present three case studies, each of which illuminates concentrated patches where earthquakes initiate and sometimes grow into different‐sized earthquakes. The existence of these patches suggests that, at least for these earthquakes, we will have difficulties in predicting their final sizes since the earthquakes that initiate from these patches can grow into different‐sized earthquakes. Key Points: A new cross‐correlation‐based hypocenter relocation method to refine hypocenter locations is proposedA joint inversion is performed to determine the relative centroid and hypocenter locations simultaneously and in a unified coordinateHypocenter hotspots, which host hypocenters of earthquakes that can grow into different‐magnitude events, are illuminated [ABSTRACT FROM AUTHOR]

Earthquakes in subduction zones occur in the slab mantle, in the subducting crust, on the subduction plate interface, and, in some cases, in the mantle wedge–regions that are separated by strong seismic discontinuities. These discontinuities are typically imaged with techniques using teleseismic waves, while local earthquakes are located based on arrival times. While this combination of imaging and earthquake location provides a good initial overview of where the earthquakes are located, the uncertainties associated with the two approaches are too large (i.e., few kilometers) to robustly identify on which side of a discontinuity (with thickness ∼100 m) the earthquakes occurred. Here we investigate how the waveforms of local earthquakes, which contain secondary phases arising from wave scattering at discontinuities, can be exploited to determine the source region of subduction zone earthquakes more robustly. Our investigation involves a three‐step approach and includes an application to data from western Greece. First, to identify characteristic secondary phases, we analyzed synthetic seismograms from a generic 2‐D subduction zone. Second, to enhance the visibility of secondary phases in field data, we implemented a workflow to process three‐component seismograms. Third, to identify individual secondary phases in the data, we matched their timing to arrivals computed in a 3‐D velocity model. We identified on average two to three secondary arrivals per station. These include P‐ and S‐reflections from the plate interface which indicate hypocenters in the mantle wedge, and P‐reflections from the slab Moho which indicate hypocenters on the plate interface and in the subducting crust. Plain Language Summary: In subduction zones, two tectonic plates collide, with one plunging into the Earth's mantle. During this process, many earthquakes occur—some in the crust and the mantle of the lower plate, and some in the crust and mantle of the upper plate—four regions with highly different mechanical properties due to differences in pressure (due to depth), temperature, and rock composition. As earthquakes occur, they generate seismic waves that can be recorded at seismic stations as seismograms. The timing of first arriving waves measured at multiple stations is commonly used to compute the earthquake's location, but the location uncertainty is often too large to clearly state in which of the four regions the earthquake occurred. But as the seismic wave travels across the boundary between two regions, a secondary wavefront is generated, both through reflection of the wave and conversion between two compressional and shear waves. Here we investigate which reflected and converted waves can be recognized in seismograms, by analyzing data generated synthetically, and by analyzing data recorded in Greece. We identify on average two to three reflected/converted waves at each station, allowing us to confirm the earthquake's relative location above or below the boundaries. Key Points: Secondary arrivals arise from wave scattering at major seismic discontinuities in subduction zonesThe arrival's timing and visibility is controlled by the earthquake's location relative to the discontinuitiesA data example from Greece shows arrivals for earthquakes in the mantle wedge, on the plate interface, and in the slab crust [ABSTRACT FROM AUTHOR]

The 2011 Mw 9.0 Tohoku‐oki earthquake is one of the world's best‐recorded ruptures. In the aftermath of this devastating event, it is important to learn from the complete record. We describe the state of knowledge of the megathrust earthquake generation process before the earthquake, and what has been learned in the decade since the historic event. Prior to 2011, there were a number of studies suggesting the potential of a great megathrust earthquake in NE Japan from geodesy, geology, seismology, geomorphology, and paleoseismology, but results from each field were not enough to enable a consensus assessment of the hazard. A transient unfastening of interplate coupling and increased seismicity were recognized before the earthquake, but did not lead to alerts. Since the mainshock, follow‐up studies have (1) documented that the rupture occurred in an area with a large interplate slip deficit, (2) established large near‐trench coseismic slip, (3) examined structural anomalies and fault‐zone materials correlated with the coseismic slip, (4) clarified the historical and paleoseismic recurrence of M∼9 earthquakes, and (5) identified various kinds of possible precursors. The studies have also illuminated the heterogeneous distribution of coseismic rupture, aftershocks, slow earthquakes and aseismic afterslip, and the enduring viscoelastic response, which together make up the complex megathrust earthquake cycle. Given these scientific advances, the enhanced seismic hazard of an impending great earthquake can now be more accurately established, although we do not believe such an event could be predicted with confidence. Plain Language Summary: The Mw 9 Tohoku‐oki earthquake was one of the most disastrous earthquakes in recent history. In this review, we first clarify the knowledge of the earthquake and tsunami potential before the earthquake. Pre‐Tohoku‐oki studies partly recognized the potential of Mw 8 or larger earthquakes. However, the knowledge based on different types of observations was incomplete and the occurrence of such a great event was not considered in the official earthquake probabilities. The improved understanding of earthquake‐cycle and rupture processes since the Tohoku‐oki earthquake advanced the leading edge of efforts to characterize megathrust earthquake hazards. We can summarize the lessons as follows. (1) The incorporation of interdisciplinary research is essential to advance our understanding of the processes underlying the occurrence of earthquakes. (2) The recognition of earthquake potential informed by geologic evidence extending beyond available instrumental records is essential for assessing the largest possible earthquake in a subduction zone. (3) The development of advanced scientific infrastructure, especially ocean‐bottom observations is necessary to evaluate earthquake potential and monitor dynamic megathrust fault‐zone processes. (4) Although post‐Tohoku‐oki studies have better characterized the hazard and a number of possible precursors have been identified, the confident prediction of such events appears impossible in the near future. Key Points: The lessons learned in the last decade highlight more realistic estimation of seismic hazard and importance of interdisciplinary studyPre‐2011 studies based on a variety of evidence did not result in a consensus assessment of the great‐earthquake hazardDespite the precursory foreshocks and slow slip and improved monitoring capabilities, prediction of such events still appears impossible [ABSTRACT FROM AUTHOR]

• A weak LAB beneath the slab affects deformation following megathrust earthquakes. • Stress relaxation in the low-viscosity LAB enhances landward motion beyond rupture. • Observed deformation can be explained by a thin LAB of low viscosity (<1 × 1017 Pa s). • The preferred LAB thickness of 10 km or less agrees with seismological inferences. • Mechanical effect of LAB with heterogeneous melt distribution deserves further study. The rheology of the seismically imaged Lithosphere-Asthenosphere Boundary (LAB) is an important new question in plate tectonics. If the LAB is associated with ponding of partial melts as inferred from many seismic studies, it should have a low viscosity that minimizes resistance to plate motion and subduction. Here, we propose that recently observed postseismic enhanced landward motion (ELM) in coastal areas outside the rupture zone of large subduction earthquakes provides the first geodetic evidence for a low-viscosity LAB beneath subducting oceanic lithosphere. Using three-dimensional viscoelastic finite element models, we demonstrate that a low-viscosity LAB readily leads to postseismic ELM over broad coastal areas far beyond the rupture zone. If the LAB thickness is assumed to be 10 km, key spatiotemporal characteristics of the observed ELM can be well explained by an LAB viscosity of 5 × 1016 Pa s, orders of magnitude lower than typical asthenospheric viscosities. A thicker LAB such as 20 km with a higher viscosity may also explain the postseismic ELM but is incompatible with existing, albeit limited, near-field vertical deformation observations. The small thickness and the low viscosity of the model LAB are consistent with inferences from seismological observations and support the notion of ponding melts. Our preliminary weak-LAB models also predict diagnostic deformation features that can be tested by future onshore and offshore geodetic observations. Whether a weak LAB is ubiquitously present before and after plate subduction and how its presence and spatial heterogeneity impact postseismic and plate-scale deformation are important research targets for future investigations. [ABSTRACT FROM AUTHOR]

Abstract In several examples of subduction zones, we compared pairs of quartz veins formed either at the lower temperatures of the seismogenic zone (260°C or below), or at the higher temperatures of its downdip limit (∼330°C). All the veins analyzed here are mode I cracks that formed contemporaneously with the host‐rock main stage of deformation at peak burial conditions. Lower‐temperature veins show examples of quartz crystals with euhedral shapes and growth rims, while higher‐temperature veins contain crack‐seal microstructures. In the lower‐temperature realm, quartz growth rims have alternatingly either: (1) high cathodoluminescence (CL), CL‐blue color and high concentration in trace elements and fluid inclusions, or (2) low luminescence, CL‐brown color and low concentration in trace elements and fluid inclusions. In contrast, the quartz from higher‐temperature samples is homogeneously low luminescent and CL‐brown, except for very restricted domains of the crack‐seal microstructures where patches of CL‐blue quartz are present. The highly luminescent quartz contains high concentrations of aluminum and lithium, up to 3,000 and 400 ppm, respectively. Variations in Al and Li correlate well, so that Li appears as the main charge‐compensating cation for Al. We propose that the incorporation of Al and Li reflects the amplitude of the fluid pressure variations, which control crystal growth rates. Quartz geochemistry might therefore unravel the contrast between the seismogenic zone, where large fluid pressure variations are present, and its downdip limit, where fluid pressure variations are much more limited in amplitude.

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 Cascadia subduction zone (CSZ) is an exceptional geologic environment for recording evidence of land-level changes, tsunamis, and ground motion that reveals at least 19 great megathrust earthquakes over the past 10 kyr. Such earthquakes are among the most impactful natural hazards on Earth, transcend national boundaries, and can have global impact.Reducing the societal impacts of future events in the US Pacific Northwest and coastal British Columbia, Canada, requires improved scientific understanding of megathrust earthquake rupture, recurrence, and corresponding hazards. Despite substantial knowledge gained from decades of research, large uncertainties remain about the characteristics and frequencies of past CSZ earthquakes. In this review, we summarize geological, geophysical, and instrumental evidence relevant to understanding megathrust earthquakes along the CSZ and associated uncertainties. We discuss how the evidence constrains various models of great megathrust earthquake recurrence in Cascadia and identify potential paths forward for the earthquake science community. Despite outstanding geologic records of past megathrust events, large uncertainty of the magnitude and frequency of CSZ earthquakes remains. This review outlines current knowledge and promising future directions to address outstanding questions on CSZ rupture characteristics and recurrence. Integration of diverse data sets with attention to the geologic processes that create different records has potential to lead to major progress. [ABSTRACT FROM AUTHOR]

Shear‐wave splitting observations can provide insight into mantle flow, due to the link between the deformation of mantle rocks and their direction‐dependent seismic wave velocities. We identify anisotropy in the Cook Inlet segment of the Alaska subduction zone by analyzing splitting parameters of S waves from local intraslab earthquakes between 50 and 200 km depths, recorded from 2015–2017 and emphasizing stations from the Southern Alaska Lithosphere and Mantle Observation Network experiment. We classify 678 high‐quality local shear‐wave splitting observations into four regions, from northwest to southeast: (L1b) splitting measurements parallel to Pacific plate motion, (L1a) arc‐perpendicular splitting pattern, (L2) sharp transition to arc‐parallel splitting, and (L3) splitting parallel to Pacific plate motion. Forward modeling of splitting from various mantle fabrics shows that no one simple model fully explains the observed splitting patterns. An A‐type olivine fabric with fast direction dipping 45° to the northwest (300°)—aligned with the dipping slab—predicts fast directions that fit L1a observations well, but not L2. The inability of the forward model fabrics to fit all the observed splitting patterns suggests that the anisotropy variations are not due to variable ray angles, but require distinct differences in the anisotropy regime below the arc, forearc, and subducting plate. Plain Language Summary: Mantle flow can cause seismic wave velocity to become directionally dependent (seismic anisotropy) and is related to the flow direction. Using a dense temporary seismic transect along the Cook Inlet segment of the Alaska subduction zone, we identify four distinct regions of anisotropy. This includes a sharp transition from arc‐parallel fast directions in the forearc to arc‐perpendicular fast directions in the backarc. Forward modeling of various mantle fabrics shows that no one simple model fully explains the anisotropy and requires distinct differences in the anisotropy regimes below the arc, forearc, and subducting plate. Key Points: The study measures local splitting recorded by Southern Alaska Lithosphere and Mantle Observation Network network (ZE) stationsThere is a sharp transition from trench‐perpendicular splitting pattern in the backarc to trench‐parallel in the forearcVariations in splitting requires distinct anisotropy regimes below the arc, forearc, and subducting plate [ABSTRACT FROM AUTHOR]

In several examples of subduction zones, we compared pairs of quartz veins formed either at the lower temperatures of the seismogenic zone (260°C or below), or at the higher temperatures of its downdip limit (∼330°C). All the veins analyzed here are mode I cracks that formed contemporaneously with the host‐rock main stage of deformation at peak burial conditions. Lower‐temperature veins show examples of quartz crystals with euhedral shapes and growth rims, while higher‐temperature veins contain crack‐seal microstructures. In the lower‐temperature realm, quartz growth rims have alternatingly either: (1) high cathodoluminescence (CL), CL‐blue color and high concentration in trace elements and fluid inclusions, or (2) low luminescence, CL‐brown color and low concentration in trace elements and fluid inclusions. In contrast, the quartz from higher‐temperature samples is homogeneously low luminescent and CL‐brown, except for very restricted domains of the crack‐seal microstructures where patches of CL‐blue quartz are present. The highly luminescent quartz contains high concentrations of aluminum and lithium, up to 3,000 and 400 ppm, respectively. Variations in Al and Li correlate well, so that Li appears as the main charge‐compensating cation for Al. We propose that the incorporation of Al and Li reflects the amplitude of the fluid pressure variations, which control crystal growth rates. Quartz geochemistry might therefore unravel the contrast between the seismogenic zone, where large fluid pressure variations are present, and its downdip limit, where fluid pressure variations are much more limited in amplitude. Plain Language Summary: We have examined several examples of veins that formed at large depths in collision and subduction zones. These veins formed as a result of the opening of a crack‐shaped cavity and were filled by the precipitation of quartz from dissolved silica. These veins thus have characteristics that reflect the water that was present in the pores and cracks of the rock at depth. The samples we have analyzed come from either the seismogenic zone (for temperatures of the order of ∼260°C or lower), i.e., the depth domain where large earthquakes are generated, or its downdip limit (for temperatures of the order of ∼330°C), i.e., the depth domain below which deformation is principally non seismic. The chemistry of quartz varies strongly between these two depth domains. At temperatures of the order of ∼260°C, the quartz contains either a large concentration in impurities, or is relatively pure. At higher temperatures, the quartz is essentially pure. We interpret the domains of quartz rich in impurities as having formed as a result of earthquakes, by rapid growth in disequilibrium conditions, when the pressure of the fluid dropped strongly and the fluid became suddenly highly oversaturated in dissolved silica, which then precipitated. Key Points: Variations in Al and Li concentration in hydrothermal quartz span more than two orders of magnitude and are up to 3,000 and 400 ppmGrowth rims with large Al and Li concentrations are present only at seismogenic depth and might reflect large fluid pressure dropsCrack‐seal microstructures formed at the downdip limit of the seismogenic zone might be associated with smaller fluid pressure variations [ABSTRACT FROM AUTHOR]

Based on manually analyzed waveforms recorded by the permanent Ecuadorian network and our large aftershock deployment installed after the Pedernales earthquake, we derive three‐dimensional Vp and Vp/Vs structures and earthquake locations for central coastal Ecuador using local earthquake tomography. Images highlight the features in the subducting and overriding plates down to 35 km depth. Vp anomalies (∼4.5–7.5 km/s) show the roughness of the incoming oceanic crust (OC). Vp/Vs varies from ∼1.75 to ∼1.94, averaging a value of 1.82 consistent with terranes of oceanic nature. We identify a low Vp (∼5.5 km/s) region extending along strike, in the marine forearc. To the North, we relate this low Vp and Vp/Vs (<1.80) region to a subducted seamount that might be part of the Carnegie Ridge (CR). To the South, the low Vp region is associated with high Vp/Vs (>1.85) which we interpret as deeply fractured, probably hydrated OC caused by the CR being subducted. These features play an important role in controlling the seismic behavior of the margin. While subducted seamounts might contribute to the nucleation of intermediate megathrust earthquakes in the northern segment, the CR seems to be the main feature controlling the seismicity in the region by promoting creeping and slow slip events offshore that can be linked to the updip limit of large megathrust earthquakes in the northern segment and the absence of them in the southern region over the instrumental period. Plain Language Summary: Using seismic data recorded by the permanent Ecuadorian network and the large emergency installation after the 2016 Pedernales earthquake, we obtained the seismic velocity structure together with precise earthquake locations for the coastal Ecuadorian margin. Our images highlight the heterogeneities of the subduction zone affected by seamounts and ridges comprising the oceanic crust. These features play an important role in controlling the seismic behavior of the margin. While seamounts can contribute to the occurrence of intermediate (M ∼ 7–7.5) megathrust earthquakes in the north, the Carnegie Ridge seems to be the main feature controlling the seismicity in the region by promoting creeping and slow slip events offshore that can be linked to the updip limit of large megathrust earthquakes in the northern segment and the absence of them in the southern region. Key Points: 3D Vp and Vp/Vs models were calculated using local earthquake tomography in the region affected by the 2016 Pedernales, Ecuador earthquakeTomographic images highlight the heterogeneities of the margin affected by seamounts and ridges comprising the oceanic crustCarnegie Ridge seems the main feature controlling the seismic activity and the offshore extent of large megathrust earthquakes in the region [ABSTRACT FROM AUTHOR]

We image seismic attenuation near the Hikurangi trench offshore New Zealand, using ocean bottom and land‐based seismometers, revealing high attenuation above a recurring shallow slow‐slip event and within the subducting Hikurangi Plateau. The Hikurangi subduction margin east of the North Island, New Zealand is the site of frequent shallow slow slip events. Overpressured fluids are hypothesized to lead to slow slip at shallow depths close to the oceanic trench. Seismic attenuation, energy loss of seismic waves, can be used to detect high temperatures, melt, the presence of fluids, and fractures. We use local earthquake P‐ and S‐waves from 180 earthquakes to invert for t*, and subsequently invert for Qp and Qs, offshore the North Island directly above the area of slow slip. We image Qp and Qs to ∼25 km depth, increasing resolution of previously identified coastal low Q (100–300), and finding a new region of even higher attenuation (Qp and Qs < 50–100) directly above the shallow slow slip event of 2014–2015, beneath the offshore seismic array. This highest attenuation is downdip of a subducting seamount, and is spatially correlated with a high seismic reflectivity zone and Vp/Vs > 1.85, all of which provide evidence for the presence of fluids. The Qp and Qs is low at the trench (<50–100) and in the subducting plate (100–200), suggesting that seismic wave scattering due to faults, fractures, and the inherent heterogeneous composition of the Hikurangi Plateau, a large igneous province, plays a role in seismic attenuation. Plain Language Summary: The Hikurangi Margin is a major subduction zone which has produced earthquakes that lead to tsunamis, which could endanger people living on the North Island of New Zealand and the fishing and shipping industry. Mysterious slow earthquakes, or slow slip events, happen near the shore where large earthquakes have happened in the past. Scientists do not yet understand why these events occur, but it may be because of fluids on the plate that is subducting beneath the North Island. These fluids can come from chemical reactions or the water content of the sediment on the incoming plate. Our study finds evidence of fluids in the area of the plate interface by measuring energy loss from earthquakes offshore of the North Island, which we use to create a 3D image of where energy is lost. As seismic waves travel through rock, they lose a lot of energy when they encounter fluids, like water, and as they pass through layered, volcanic rocks. We combine our study with other studies to show where pockets of fluids are likely located near the subducting plate, which will help scientists in future field studies and also help us understand why slow slip events occur at the Hikurangi Margin. Key Points: Low Qp and Qs (<50–100) coincides with high Vp/Vs and are interpreted as fluid near the plate interface, downdip of a subducted seamountLow Qp (100–200) in the Hikurangi Plateau large igneous province suggests a thicker (~23 km) oceanic crustNear‐trench low Qp and Qs (<50–100) may indicate seismic wave scattering from faults in the Hikurangi Basement and the deformed upper plate [ABSTRACT FROM AUTHOR]

In this article, we describe a method for predicting of the epicenter location region of Kamchatka and Commander earthquakes (EQs). It is based on the properties of the precursory atmospheric ULF/ELF (1–30 Hz) radiation and the spatial statistics of local EQs in relation to the Kuril-Kamchatka and Aleutian trenches. From this statistics, it follows that more than 90% of events with magnitude (ML) of more than 5 occur in the gap ~ 150 km west of the Kuril-Kamchatka trench and north-east of the Aleutian trench. Additionally, we obtain the approximate location of the epicenter by determining the position of the radiation source. We suppose that it is caused by gas eruption from the hearth of the EQ to the trench and has the nearest location to the EQ epicenter. Further, we obtain other parameters of supposed position of the epicenter from spatial statistics of EQs relative to the projection of radiation source, which approximately coincides with gas emanation area in the ocean surface. The main drawbacks of the method are the dependence of its accuracy on the industrial interferences and the ambiguity of determining the epicenter location in the case of single-point registration of radiation when the main lobe of the azimuthal distribution of radiation crosses both trenches. [ABSTRACT FROM AUTHOR]

Large earthquakes and slow slip events typically nucleate along plate boundaries near the depth limit of the seismogenic zone, which is also recognized as the brittle‐plastic transition zone (BPT). High Vp/Vs ratios are commonly observed at the BPT in subduction zones, indicating the presence of aqueous fluid in pore spaces. We conducted experiments to investigate the rheology of quartz with different fluid fractions at deformation conditions that cross the BPT. The strengths of quartz aggregates with fluid‐filled porosities of 5–25 vol% are significantly lower than predicted by wet quartzite flow laws, and decrease with increasing fluid fraction. Recovered samples deformed in the ductile regime exhibit S‐C´ mylonitic structures characterized by elongate grains, shear localization and fluid segregation. Variations in strength are explained by a combination of a constitutive law for dislocation creep that includes the geometric effects of fluid fraction, a friction law that includes the effect of fluid fraction through its role on the real area of contact, and an empirical function to describe the smooth brittle‐plastic transition. Our results indicate that the presence of fluid‐filled porosity promotes significant weakening in shear zones, and that variations in fluid fraction (together with temperature) can explain transitions in the spectrum of slip behaviors observed along plate boundaries. Key Points: The presence of fluid‐filled porosity promotes significant weakening in quartz shear zones across the brittle‐plastic transitionThe rheology of quartz shear zones is controlled by pore fluid pressure, fluid fraction, and fluid topologyVariations in fluid fraction can explain transitions in the spectrum of slip behaviors observed along plate boundaries [ABSTRACT FROM AUTHOR]

Slow slip events (SSEs) along subduction zones play an important role in accommodating relative plate motion. SSEs interplay with large megathrust earthquakes and other slow earthquakes, including low frequency and very low frequency earthquakes. The Kanto and Tokai regions of central Japan host frequent slow and large earthquakes, with significant differences in slip behavior along the subduction zones in the Suruga Trough, Sagami Trough, and Japan Trench. In this study, we conducted a systematic search to estimate the fault models and durations of short‐term SSEs using continuous Global Navigation Satellite System data collected from 1994 to 2020. We detected 176 potential SSEs with moment magnitudes of 5.3–7.0 and durations of 0–80 days from the time series. Along the Sagami Trough, two shallow regions at a depth of 10–20 km host Mw ≥ 6.5 SSEs off of the Boso Peninsula and accommodate most of the relative plate motion aseismically. Some SSEs also occur on the deep plate interface down to ∼50 km without tectonic tremors. Along the Japan Trench, the cumulative slip of the SSEs exhibits a bi‐modal depth distribution to avoid the large slip areas of past megathrust earthquakes at 30–40 km depth. The shallow SSEs are in the same depth range (10–30 km) as tectonic tremors, but are spatially separate from tremors along the trench. The detected SSEs have limited temporal correlations with other slow earthquakes and earthquake swarms, which suggests that many factors control the genesis of slow and regular earthquakes. Key Points: A systematic search using 25 years of Global Navigation Satellite System data detected 176 possible SSEs in central JapanSlow slip events (SSEs) along the Japan Trench are distributed updip and downdip of megathrust earthquakes and spatially complement tremors in the updipSynchronization of SSEs with tectonic tremors, very low frequencies, or seismic swarms is limited in central Japan [ABSTRACT FROM AUTHOR]

A model for the generation of the strongest earthquakes in subduction regions under conditions of the block structure of the medium is considered. This model represents a significant development of the mathematical keyboard model by L.I. Lobkovsky related to consideration of the discontinuities of not only the frontal but also the rear parts of the island arc. It is shown that the introduction of an additional structural element into the initial model leads to a more accurate assessment of a number of characteristics of the seismic process, since the approach proposed allows taking into account a wider range of geodynamic processes occurring in subduction zones at different stages of the seismic cycle, with the possibility of direct comparison of the theoretical calculations to the satellite geodetic data. [ABSTRACT FROM AUTHOR]

We analyzed continuous GPS data collected from 2002–2020 to characterize slow slip events (SSEs) in and near the Nicoya Peninsula, Costa Rica. These data are bisected by the 5 September 2012 Mw 7.6 earthquake. The displacement time series contain multiple signals, including plate convergence, plate interface locking, coseismic and postseismic deformation, seasonal oscillations, SSEs, and noise. GPS‐measured coseismic and postseismic displacements associated with the Mw 7.6 earthquake are modeled and removed by a step function plus multiple timescale relaxation processes with four characteristic times: 11, 94, 470, and 1,865 days. Seasonal oscillations are eliminated using a multichannel singular spectrum analysis (M‐SSA). Ten major SSEs (Mw > 6.6) are observed in the remaining time series, with a constant recurrence interval of 21.7 ± 2.6 months. SSEs occur in both shallow (~10 km) and deep (~35 km) portions of the plate interface, but the latter last longer and have larger magnitudes. There is minimum to no slow slip in the Mw 7.6 seismic rupture area and a persistent slow slip patch beneath the Nicoya Gulf entrance. Despite strong earthquake‐related stress perturbations, the inter‐SSE locking status on the megathrust is very similar between the late and early stages of the earthquake cycle and includes locked patches that ruptured in the 2012 earthquake or continue to rupture via SSEs. Some locked patches offshore south of the Nicoya Peninsula did not rupture in 2012, do not participate in SSEs, and may be indicative of supercycle behavior, that is, strain accumulation over several seismic cycles. These areas warrant heightened monitoring. Plain Language Summary: Slow slip events (SSEs) release strain energy in Earth's crust that accumulates due to plate motion and frictional locking on the boundaries between plates. SSEs are similar to earthquakes but are less damaging, since strain is released slowly, over weeks or months. We used GPS measurements with millimeter precision to study SSEs in Northwestern Costa Rica between 2002 and 2020 and compared them to the pattern of strain accumulation over the same period. We find that these events happened about every 22 months, and the repeat time is unchanged by the 5 September 2012 Mw 7.6 Costa Rica earthquake. Locking patterns also remain similar before and after this earthquake. However, some locked zones did not rupture in the 2012 earthquake or earthquakes in the last few decades and do not participate in SSEs. These areas may represent zones of higher seismic risk in the future. Key Points: Slow slip events >Mw 6.6 in northwestern Costa Rica occurred every 21.7 ± 2.6 months based on data from 33 GPS stations between 2002 to 2020Slow slip recurrence/distribution and inter‐transient strain accumulation patterns were unchanged by the 5 September 2012 Mw 7.6 earthquakeSome inter‐transient locked patches offshore did not rupture in 2012 and do not participate in slow slip events [ABSTRACT FROM AUTHOR]