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Earthquakes occur by overcoming fault friction; therefore, quantifying fault resistance is central to earthquake physics. Values for both static and dynamic friction are required, and the latter is especially difficult to determine on natural faults. However, large earthquakes provide signals that can determine friction in situ. The Japan Trench Fast Drilling Project (JFAST), an Integrated Ocean Discovery Program expedition, determined stresses by collecting data directly from the fault 1–2 years after the 2011 Mw 9.1 Tohoku earthquake. Geological, rheological, and geophysical data record stress before, during, and after the earthquake. Together, the observations imply that the shear strength during the earthquake was substantially below that predicted by the traditional Byerlee's law. Locally the stress drop appears near total, and stress reversal is plausible. Most solutions to the energy balance require off-fault deformation to account for dissipation during rupture. These observations make extreme coseismic weakening the preferred model for fault behavior. ▪ Determining the friction during an earthquake is required to understand when and where earthquakes occur. ▪ Drilling into the Tohoku fault showed that friction during the earthquake was low. ▪ Dynamic friction during the earthquake was lower than static friction. ▪ Complete stress drop is possible, and stress reversal is plausible. [ABSTRACT FROM AUTHOR]

Ocean bottom pressure (OBP) observations are a powerful tool for determining vertical crustal displacements, especially due to earthquakes and slow earthquakes, with centimeter‐level resolution. In these studies, removal of oceanographic noise (tens of centimeters) is required to identify centimeter‐level crustal deformation. We undertake barotropic modeling to remove oceanographic signals from data from an OBP array deployed offshore New Zealand in 2014/2015. We show that removing the nontidal component calculated from a barotropic ocean model reduces the variance in the data by about 66% and provides a feasible means to resolve pressure changes due to crustal deformation during the slow slip events. We also discuss the vertical displacements from slow slip events that occurred in late September to mid‐October 2014, and we outline our procedure for processing OBP data. Plain Language Summary: We developed a new method for determining pressure changes due to slow slip events (SSEs) on offshore subduction plate boundaries by using information from the ocean bottom pressure records and numerical simulation. We use an oceanographic model to correct the seafloor pressure data for oceanographic signals, so that centimeter‐level vertical deformation of the seafloor during the SSEs can be isolated. We show that this method can be used to identify SSEs that occurred off the coast of New Zealand in 2014. Our results indicate that our ocean model can be a useful tool to use ocean bottom absolute pressure gauge data to resolve crustal deformation. Key Points: The oceanographic corrections from our model help to reduce noise in ocean bottom pressure data recorded during slow slip eventWe show a barotropic oceanographic model can be used to reduce the variance in seafloor pressure measurements by about 66%Our oceanographic model is particularly valuable for the shallower‐water sites [ABSTRACT FROM AUTHOR]

We applied tomographic inversion and receiver function analysis to seismic data from ocean-bottom seismometers and land-based stations to understand the structure and its relationship with slow slip events off Boso, Japan. First, we delineated the upper boundary of the Philippine Sea Plate based on both the velocity structure and the locations of the low-angle thrust-faulting earthquakes. The upper boundary of the Philippine Sea Plate is distorted upward by a few kilometers between 140.5 and 141.0°E. We also determined the eastern edge of the Philippine Sea Plate based on the delineated upper boundary and the results of the receiver function analysis. The eastern edge has a northwest–southeast trend between the triple junction and 141.6°E, which changes to a north–south trend north of 34.7°N. The change in the subduction direction at 1–3 Ma might have resulted in the inflection of the eastern edge of the subducted Philippine Sea Plate. Second, we compared the subduction zone structure and hypocenter locations and the area of the Boso slow slip events. Most of the low-angle thrust-faulting earthquakes identified in this study occurred outside the areas of recurrent Boso slow slip events, which indicates that the slow slip area and regular low-angle thrust earthquakes are spatially separated in the offshore area. In addition, the slow slip areas are located only at the contact zone between the crustal parts of the North American Plate and the subducting Philippine Sea Plate. The localization of the slow slip events in the crust–crust contact zone off Boso is examined for the first time in this study. Finally, we detected a relatively low-velocity region in the mantle of the Philippine Sea Plate. The low-velocity mantle can be interpreted as serpentinized peridotite, which is also found in the Philippine Sea Plate prior to subduction. The serpentinized peridotite zone remains after the subduction of the Philippine Sea Plate and is likely distributed over a wide area along the subducted slab. [ABSTRACT FROM AUTHOR]

Abstract: The March 2011 9.0 Tohoku-Oki earthquake activated shallow aftershocks with normal faulting focal mechanisms near the trench axis. To investigate stress state in the shallow subduction zone near the trench axis, we used ocean bottom seismographs (OBSs) to record earthquakes from August to October 2011. We estimated hypocenter locations and focal mechanisms of the recorded earthquakes by using a grid-search method in a 2-D velocity model based on an active seismic survey. The results show that most of the earthquakes occurred within both the overriding and subducting plates, with very few on the plate interface and none in the most seaward 45 km of the overriding plate (aseismic wedge). The low seismicity along the plate interface is consistent with the nearly complete stress drop on the megathrust fault in the 9.0 earthquake. Focal mechanisms indicate that trench-normal tension is pre-dominant in both plates and extends to a depth of about 30 km at least. On the other hand, several trench-normal compressional earthquakes were located at the landward end of the aseismic wedge. These earthquakes suggest horizontal compression of the overriding prism that may be caused by a strengthening of the shallowest part of the megathrust. [Copyright &y& Elsevier]

Abstract: Large earthquakes have repeatedly occurred in the area off southeastern Hokkaido Island, Japan, as the Pacific Plate subducts beneath the island, which is on the North American Plate. The most recent large earthquake in this area, the 2003 Tokachi-oki earthquake (Mw=8.0), occurred on September 26, 2003. In order to investigate aftershock activity in the rupture area, 47 Ocean Bottom Seismometers (OBSs) were quickly deployed after the main shock. In the present study, we simultaneously estimate the hypocenters and 3-D seismic velocity models from the P- and S-wave arrivals of the aftershocks recorded by OBSs. The subducting plate is clearly imaged as a northwest dipping zone in which Vp is greater than 7 km/s, and the relocated hypocenters also show the subducting Pacific Plate. The aftershock distribution reveals that the dip angle of the plate boundary increases abruptly around 90 km from the Kuril Trench. The bending of the subducting plate corresponds to the southeastern edge of the rupture area. The island arc crust on the overriding plate has P-wave velocities of 6–7 km/s and a Vp/Vs of 1.73. A region of Vp/Vs greater than 1.88 was found north of the epicenter of the main shock. The depth of the high Vp/Vs region extends about 10 km upward from the plate interface. The plate boundary just below the high Vp/Vs region has the largest slip at the main rupture. A high Vp anomaly (~7.5 km/s) is found in the island arc crust in northeast part of the study area, which we interpret as a structural boundary related to the arc–arc collisional tectonics of the Hokkaido region, as the rupture of the main shock terminated at this high Vp region. We suggest that the plate interface geometry and the trench-parallel velocity heterogeneity in the landward plate are principal factors in controlling the rupture area of the main shock. [Copyright &y& Elsevier]

• 3D traveltime mapping shows two strong reflection areas from the top of the PHS. • One is near the main slip area of the Boso SSEs and the other is about 60 km to the east. • The reflections from the former area are generated by a thin low-velocity layer. • The latter area contains a high-velocity structure (HVS) near the top of the PHS. • This HVS may represent boninite or partially serpentinized peridotite or gabbro. The region off the Boso Peninsula, Japan, is a tectonically complex area where the Pacific plate is subducting beneath both the landward plate and the Philippine Sea plate (PHS) from the Japan trench and the Izu-Bonin trench as the PHS is subducting under the landward plate from the Sagami trough. It is important to better determine the structure of this region to deepen our understanding of its seismicity. Previous seismic reflection studies have shown that reflections from the upper surface of the PHS vary with depth, being stronger in the main slip area of the slow slip events beneath the Boso Peninsula (Boso SSEs). However, the spatial relationship between the reflective area and the SSEs is poorly constrained. This study mapped the distribution of the reflective area using data recorded by ocean bottom seismometers during an active-source seismic experiment. We constructed a 3D P-wave velocity structure by using traveltimes of first arrivals from 18 ocean bottom seismometer records. We also adapted the traveltime mapping method to reflection traveltimes, projecting them to the depth–distance domain, to map the 2D distribution of strong reflections from the top of the PHS. These reflections were concentrated in two areas, one near the main slip area of the Boso SSEs and the other about 60 km to the east. In the first area, the absence of strong velocity contrasts near the top of the PHS suggests that the reflections were generated by a thin low-velocity layer. In contrast, the structure of the second area has a convex shape of high velocity with a high velocity gradient near the top of the PHS. This structure may represent boninitic material of the outer-arc high, partially serpentinized peridotite, or gabbro displaced by intraoceanic reverse faults. [ABSTRACT FROM AUTHOR]

Many previous studies constructed coseismic slip distribution models of the 2011 Tohoku-oki earthquake (M9.0) using the daily global navigation satellite systems (GNSS) site coordinate time series, seafloor GNSS-Acoustic (GNSS-A), and/or ocean bottom pressure (OBP) data to estimate the coseismic displacements. Studies for these several years suggested that the coseismic displacements must include early postseismic deformation due to a low-viscosity zone beneath the Pacific plate that is necessary to explain terrestrial and seafloor postseismic crustal deformations. This low-viscosity zone may not be persistent but could result in huge changes in stress resulting from the coseismic slip with reflecting non-linear viscoelastic response in the asthenosphere. Because the effect of non-linear rheology is great immediately after a main shock, therefore, coseismic displacements based on the daily site coordinate time series and GNSS-A measurements cannot help including early postseismic deformation. Thus, we derived "pure" coseismic displacements based on 1-Hz site coordinate time series at GNSS sites of the Geospatial Information Authority of Japan estimated by utilizing kinematic PPP (precise point positioning) analysis and based on 1-min average seafloor level time series at OBP sites. The residual displacements were obtained by subtracting the pure coseismic displacement from that based on the daily site coordinate time series. The data showed subsidence and trenchward motions in the entire Tohoku district. These broad deformations could not be explained solely by aftershocks nor by afterslip on the plate interface. Viscoelastic deformation that occurs immediately after the main shock is required to explain the displacement field. We present the results of our investigations into coseismic slip distribution and early postseismic deformation by applying Green's function, which is calculated by considering the shapes of surface terrain and subducting slabs as well as heterogeneous thermal structures and power-law rheology. [ABSTRACT FROM AUTHOR]