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We extract sequences of small repeating earthquakes to clarify inter‐plate coupling of subducting plates over a large area of the Japanese Islands. As a result, many sequences are detected at the Philippine Sea plate subducting from the Ryukyu trench and Pacific plate subducting from the Kuril‐Japan trench. The average slip‐rates and standard deviations estimated from the sequences show substantial spatial changes of inter‐plate coupling. The large deviations of slip‐rates correspond to the occurrence of episodic slips such as after‐slips following large earthquakes. Constant slip‐rates approaching the relative plate motion indicate weak coupling areas. Slip deficits and sparse distributions of repeating groups suggest locked areas. In the Nankai trough, deep low‐frequency earthquakes in the transition zone and burst‐type repeating sequences within plates have not been located in the downdip direction of groups with slow slip‐rates. This suggests that the space‐time characteristics of inter‐plate coupling affected these seismic events.

Co‐seismic fault slip distributions of the 2011 Tohoku‐Oki earthquake and inter‐seismic couplings prior to the earthquake were estimated from surface 3‐dimensional displacements measured by the GPS Earth Observation Network (GEONET). The deduced co‐seismic slip was up to 30 m near the epicenter, and greater than 5 m in a 400 × 200‐km region extending along the Japan Trench. The moment magnitude of the slip was estimated to beMw9.0. The area of co‐seismic slip greater than 10 m overlaps with four previously known M7‐class asperities. We researched inter‐plate coupling on the fault for 15 years prior to the earthquake using daily GPS data. We determined that the plate boundary was locked during at least the last 15 years in a limited area of a few 10 s of km, approximately coinciding with the maximum co‐seismic slip area, suggesting that this small area had been persistently locked and that the rupture of this last patch played an important role to cause the mega‐thrust earthquake. The low seismic‐coupling which is ratio of seismic slip to slip deficit accumulation of M7‐class earthquakes in this area, which were reported by a study using seismic wave analysis over the past 80 years, can be explained by the influence of this locked area which may have dragged the hanging wall and suppressed the effective slip deficit in surrounding area. Key Points A small patch around the epicenter of M9 earthquake had been persistently locked Low seismic coupling around the focal area is explained by a strong asperity Monitoring real seismic coupling is important

Earth's subducting plates move 3–4 times faster than its overriding plates, but it remains unclear whether these contrasting plate speeds are caused by additional pull from subducting slabs or by increased viscous drag on the lithosphere‐asthenosphere boundary beneath deeply‐protruding continental roots. To investigate the relative importance of plausible controls, we predicted global patterns of plate motions using numerical models that incorporate the influence of subducting slabs, convective mantle flow, and continental roots. From the mantle convection models, we computed a set of dynamically consistent plate velocities by balancing forces that drive and resist the motion of each plate. When deep continental roots anchor to the sub‐asthenospheric upper mantle, the calculated patterns of plate motions are close to the observations if only ∼20% of (excess) upper mantle slab weight contributes to the slab pull force. However, this small contribution causes plates to move too slowly on average unless mantle viscosity is a factor of ∼2 lower than expected from post‐glacial rebound. In contrast, we show that predicted and observed plate motions are more easily reconciled if even the deepest continental roots are underlain by a low‐viscosity layer and at least half of (excess) upper mantle slab weight contributes to the slab pull force. This preferred scenario agrees with recent seismological evidence for a global asthenosphere and previously proposed mechanisms for partial decoupling of slabs from surface plates. Key Points Even the deepest continental roots are underlain by a low‐viscosity layer About half of upper mantle slab weight needs to contribute to slab pull forces With anchoring roots, predicted plate motions differ from the observations

The coupling at the interface between tectonic plates is a key geophysical parameter to capture the frictional locking across plate boundaries and provides a means to estimate where tectonic strain is accumulating through time. Here, we use both interferometric radar (InSAR) and Global Navigation Satellite System (GNSS) data to investigate the plate coupling of the Hikurangi subduction zone beneath the North Island of New Zealand, where multiple slow slip cycles are superimposed on the long‐term loading. We estimate the plate coupling across the subduction zone over three multi‐year observational periods targeting different stages of the slow slip cycle. Our results highlight the importance of the observational time period when interpreting coupling maps, emphasizing the temporal variability of plate coupling. Leveraging multiple geodetic data sets, we demonstrate how InSAR provides powerful constraints on the spatial resolution of both plate coupling and slow fault slip, even in a region where a dense GNSS network exists. Plain Language Summary Plate coupling as a concept describes to what degree the boundaries between tectonic plates are locked and building up stress. Such accumulated stress (over hundreds to thousands of years) will eventually be released in earthquakes, and therefore provides important information about the potential for future earthquakes. Our study uses satellite data to investigate how coupling between tectonic plates along the Hikurangi subduction zone (New Zealand's largest and most dangerous plate boundary fault) changes with time. We analyzed Interferometric Synthetic Aperture Radar and Global Navigation Satellite System data to map the areas where the plates are stuck together (coupled) and where they move past each other (uncoupled). We show that plate coupling varies significantly in space over 2, 4, and 10‐year time scales, highlighting the importance of carefully considering the observational time period when interpreting coupling maps. Key Points Integration of high‐resolution displacement maps from radar imagery captures plate coupling at fine scales Estimates of plate coupling depend strongly on the time period over which surface velocities are measured Temporal variations in plate coupling highlight when and where slow slip dominates the slip budget

We model interseismic plate coupling distribution on the Main Himalayan Thrust (MHT) in Nepal through the inversion of secular GNSS velocity data. To complement previously published data, we compile velocity data from ten additional continuous stations to improve spatial resolution in central and mid-western Nepal. A regional non-planar structural model is adopted to reproduce the MHT fault plane. In general, the coupling pattern seems nearly binary, indicating that transition from full coupling to decoupling is occurring sharply in very narrow zones. In eastern Nepal (> 84.5ºE), plate coupling is very strong from the surface to intermediate depths, including the source region of the 2015 Mw 7.8 Gorkha earthquake. Geodetically estimated slip deficit rates are consistent with the rupture history of great earthquakes in the east revealed by geomorphological observations. In the west (< 83.0ºE), a weakly coupled zone extends laterally at intermediate depths, whereas the coupling in the shallower part remains very strong. Although the slip deficit rate in the west is significantly smaller than that in the east, seismic moment accumulated, since the last complete rupture in 1505 may be capable of generating a future great event. In central Nepal, estimated slip deficit rates are comparable with those in the east, and no great event has been documented over the past several centuries. Thus, the seismic risk may be most urgent in central Nepal. The development of local seismicity and crustal deformation should be carefully monitored. Graphical Abstract

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

The long‐term state of stress in the subduction forearc depends on the balance between margin‐normal compression due to the plate‐coupling force and the margin‐normal tension due to the gravitational force on the margin topography. In most subduction margins, the outer forearc is largely in margin‐normal compression due to the dominance of the plate‐coupling force. The inner forearc's state of stress varies within and among subduction zones, but what gives rise to this variation is unclear. We examine the state of stress in the forearc region of nine subduction zones by inverting focal mechanism solutions for shallow forearc crustal earthquakes for five zones and inferring the previous inversion results for the other four. The results indicate that the inner forearc stress state is characterized by margin‐normal horizontal deviatoric tension in parts of Nankai, Hikurangi, and southern Mexico. The vertical and margin‐normal horizontal stresses are similar in magnitudes in northern Cascadia as previously reported and are in a neutral stress state. The inner forearc stress state in the rest of the study regions is characterized by margin‐normal horizontal deviatoric compression. Tension in the inner forearc tends to occur where plate coupling is shallow. A larger width of the forearc also promotes inner‐forearc tension. However, regional tectonics may overshadow or accentuate the background stress state in the inner forearc, such as in Hikurangi. Plain Language Summary The state of stress in the overriding plate between the trench and the volcanic arc of subduction zones depends on frictional coupling between the overriding and subducting plates and gravitational force, which causes lateral compression and tension, respectively. The trench‐ward portion of this so‐called “forearc” region is generally in compression due to the dominant effect of plate coupling, but for the arc‐ward portion, the relative importance of the two forces varies spatially. We constrain the state of stress in the forearc using earthquake data for five subduction zones and inferring results from previous studies for four other subduction zones. The results indicate that the forearc stress state seems to correlate with the downdip depth of plate coupling and the width of the forearc. A relatively shallow downdip extent of coupling in a wide forearc tends to have the arc‐ward portion in tension or neutral stress state as observed in parts of Nankai, Hikurangi, Mexico, and Cascadia although this tendency is impacted by the local tectonic settings. Key Points Focal mechanism inversion results indicate the correlation of inner forearc stress state with the downdip depth of plate coupling Margin‐normal horizontal deviatoric tension in the inner forearc tends to occur where plate coupling is shallow and the forearc is wide The variation in the inner forearc stress state does not require a variation in the subduction fault strength

The presence of deeply penetrating continental roots may locally increase the magnitude of basal shear tractions by up to a factor of 4 compared to a layered viscosity structure. Here we examine how these increases in mantle‐lithosphere coupling influence stress patterns in the overlying elastic lithosphere. By coupling a mantle flow model to a model for the elastic lithosphere, we show that the amplification of mantle tractions beneath cratons increases elastic stress magnitudes by at most a factor of only 1.5 in a pattern not correlated to local basal traction changes. This disconnect is explained by the transmission of elastic stresses across large distances, which makes them sensitive to regionally‐averaged changes in basal tractions, but not local variations. Our results highlight the importance of regional variations in lithospheric strength, which could allow stress patterns to more closely match regional changes in basal shear.

A huge plate boundary earthquake eventuated between the Pacific plate and the landward plate in 2011 and was named the 2011 off the Pacific coast of Tohoku earthquake. Following the mainshock, many aftershocks were generated. It is essential to obtain spatiotemporal variation of aftershock activity for understanding of mechanism of the earthquake generation. Because extensive seafloor aftershock observations using many ocean bottom seismometers (OBSs) were performed following the mainshock until September 2011, a precise aftershock distribution just after the mainshock had been revealed. After the urgent OBS aftershock observation, we started seismic monitoring using long-term OBSs (LTOBSs) in the source area. Thirty-nine LTOBSs were deployed in September 2011, and the observation had been continued for approximately ten months. In September 2013, thirty LTOBSs were deployed in the northern source region and recovered one year later. From long-term observations, spatiotemporal variation of the aftershock activity in the northern source region of the mainshock was revealed. Because we had carried out a LTOBS observation in the study area before the mainshock, the seismicity before the mainshock and aftershock activities was compared. Source positions of the events were determined by P and S-wave arrival times, and focal mechanisms of them were obtained using polarities of vertical components of the first arrivals. Large slip areas have little seismicity during the whole period of the observations. It is deduced from a low seismicity area that the stress in the large slip region had not been recovered 3 years after the mainshock. In contrast, a seismic activity become high just after the mainshock comparing the seismicity before the mainshock in the region off Iwate prefecture. Although many earthquakes in the inside of the plates had normal and strike-slip focal mechanisms from 2011 to 2012, some thrust focal mechanism events also occurred. A ratio of earthquakes occurring near the plate boundary with thrust focal mechanism seems to increase in 2013 and 2014 compared to 2011 and 2012. We interpret the increase in thrust-type earthquakes as indicative of stress change starting at the edge of the large slip region of the mainshock.

The Ryukyu trench-arc-back arc system is part of the subduction margins of the Philippine Sea plate. Previous studies have indicated that several geophysical and geological characteristics reveal significant variations (including convergent rate, topography, subducting slab angle etc.) along this subduction system. In addition, the strength of plate coupling and the potential of large earthquake occurrence in the Ryukyu subduction zone have been major subjects of debate for decades. To gain new insights into the spatial variations in the crustal structure and strength of plate coupling along the Ryukyu subduction zone, in the present study, based on three P-wave seismic velocity profiles, we construct density models for 2-D gravity modeling. Then, we estimate the mantle lithosphere buoyancy (Hm) using these three density models to determine the strength of plate coupling between the subducting Philippine Sea plate and the overriding Eurasian plate, which could provide information for evaluating large earthquakes potential. 2-D gravity modeling results reveal that oceanic plateaus and/or submarine ridges with obviously less dense and thick oceanic crust are subducting in the northern and central parts of the Ryukyu Trench, which could increase the slab buoyancy in these regions. The Hm results indicate that the strength of plate coupling is almost weak in the north and is relatively strong in the central Ryukyu subduction zone.