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To assess the risk of tsunamis from outer‐rise earthquakes, we carried out tsunami simulations using 33 simple rectangular fault models with 60° dip angles based on marine seismic observations and surveys of the Japan Trench. The largest tsunami resulting from these models, produced by a Mw 8.7 normal‐faulting event on a fault 332 km long, had a maximum height of 27.0 m. We tested variations of the predictions due to the uncertainties in the assumed parameters. Because the actual dip angles of the Japan Trench outer‐rise faults range from 45° to 75°, we calculated tsunamis from earthquakes on fault models with 45°, 60°, and 75° dip angles. We also tested a compound fault model with 75° dip in the upper half and 45° dip in the lower half. Rake angles were varied by ±15°. We also tested models consisting of small subfaults with dimensions of about 60 km, models using other earthquake scaling laws, models with heterogeneous slips, and models incorporating dispersive tsunami effects. Predicted tsunami heights changed by 10–15% for heterogeneous slips, up to 10% for varying dip angles, about 5–10% from considering tsunami dispersion, about 2% from varying rake angles, and about 1% from using the model with small subfaults. The use of different earthquake scaling laws changed predicted tsunami heights by about 50% on average for the 33 fault models. We emphasize that the earthquake scaling law used in tsunami predictions for outer‐rise earthquakes should be chosen with great care. Key Points: Tsunamis were accurately predicted using well‐mapped outer‐rise normal faults along the Japan TrenchIt is necessary to carefully select the earthquake scaling law when predicting tsunamis from outer‐rise earthquakes.Heterogeneous slips have a large impact on tsunami predictions for outer‐rise earthquakes. [ABSTRACT FROM AUTHOR]

The strong ground motions, large crustal deformation, and tsunami generated by the 2011 Tohoku-oki earthquake (Mw 9.1) reveal that a large coseismic slip likely propagated to shallow depth in the Japan Trench. Although data acquired by onshore networks cannot resolve the slip behavior of the updip fault rupture, marine geophysical and geological studies provide direct evidence of coseismic slip to the trench. Differential bathymetry data show ∼50 m of coseismic seafloor displacement extending to the central Japan Trench (38–39.2°N). Seismic data show that coseismic slip ruptured the seafloor within the trench. Pelagic clays may have promoted slip propagation to shallow depths, whereas disturbed/metamorphosed clays may have restricted slip to the main rupture zone. Those observations imply that a smooth, broadly distributed, weak, clay-rich sediment in a shallow part of a subduction zone is a characteristic factor that can foster a large coseismic slip to the trench and, consequently, the generation of a large tsunami. ▪ During the 2011 Tohoku-oki earthquake (Mw 9.1), more than ∼50 m of slip occurred on a fault that ruptured the seafloor in the central Japan Trench. ▪ The fault rupture reaching the seafloor caused a large tsunami. ▪ Marine geophysical explorations revealed that a clay-rich sediment in the subduction zone was one factor fostering the large fault slip. ▪ Understanding of slip behavior in the shallow portion of a subduction zone will help us prepare for future large tsunamis along the Japan-Kuril Trench. [ABSTRACT FROM AUTHOR]

We conducted multibeam bathymetric surveys crossing the northern Japan Trench offshore Sanriku and examined seafloor displacement by means of the difference in bathymetry before and after the 11 March 2011 Tohoku-oki earthquake. The seafloor displacements were less than several meters in vertical displacement and less than 20 m in trench-normal horizontal displacement; consequently, lower amplitude seafloor displacements were evaluated. Comparatively, the landward lower slope near the trench was uplifted, the middle slope was subsided, and the midslope terrace was depressed after the earthquake. Therefore, localized very large fault slip or very large submarine landslide is unlikely. Instead, smaller uplift of the lower slope near the trench extending broader along the trench seems to be plausible explanation for the devastating tsunami source in the northern Japan Trench area. [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]

The Japan Trench is a plate convergent zone where the Pacific Plate is subducting below the Japanese islands. Many earthquakes occur associated with plate convergence, and the hypocenter distribution is variable along the Japan Trench. In order to investigate the detailed structure in the southern Japan Trench and to understand the variation of seismicity around the Japan Trench, a wide-angle seismic survey was conducted in the southern Japan Trench fore-arc region in 1998. Ocean bottom seismometers (15) were deployed on two seismic lines: one parallel to the trench axis and one perpendicular. Velocity structures along two seismic lines were determined by velocity modeling of travel time ray-tracing method. Results from the experiment show that the island arc Moho is 18–20 km in depth and consists of four layers: Tertiary and Cretaceous sedimentary rocks, island arc upper and lower crust. The uppermost mantle of the island arc (mantle wedge) extends to 110 km landward of the trench axis. The P-wave velocity of the mantle wedge is laterally heterogeneous: 7.4 km/s at the tip of the mantle wedge and 7.9 km/s below the coastline. An interplate layer is constrained in the subducting oceanic crust. The thickness of the interplate layer is about 1 km for a velocity of 4 km/s. Interplate layer at the plate boundary may cause weak interplate coupling and low seismicity near the trench axis. Low P-wave velocity mantle wedge is also consistent with weak interplate coupling. Thick interplate layer and heterogeneous P-wave velocity of mantle wedge may be associated with the variation of seismic activity. [Copyright &y& Elsevier]

The coseismic slip during the 2011 Tohoku earthquake has been revealed to reach the Japan trench axis, but the detailed slip behaviors in different zones along the trench remain an unsolved problem. To investigate the along-trench structures that directly affect the coseismic activities, we collected densely distributed seismic reflection data south of the major coseismic slip zone in the region of 36–37.5°N. Our seismic data document numerous local structural variations along different segments of the study zone: varying thicknesses of incoming sediments; chaotic structures in the north and fold-and-thrust belts in the south at the wedge front; different morphologies of the subducting plate, correlating with the physical properties above the plate interface and influencing, in turn, shallow megathrust slip and tsunami genesis. The southern limit of the 2011 event large coseismic rupture zone at ∼37°N works as a transition zone, which appears to correlate with the landward extension of a subducting channel in the deep places, and the shallower detachment fault structures in the graben beneath the trench axis that are less developed than those in the large slip zone further north. It also appears to correlate with the different elastic properties along the basal layer of the overriding plate caused by regionally various thickness and lithologies. • The seismic data show the structural heterogeneities across the 2011 Tohoku earthquake southern boundary slip zone. • Variations of sediments thickness, prism deformation and subducting plate morphologies lead to different seismic behaviors. • Coseismic propagation weakening may relate to a transition zone without well-developed thrust faults in the trench. • The subcuting graben-related duplex channel features affect the physical properties of the plate interface. [ABSTRACT FROM AUTHOR]

Abstract: Many destructive earthquakes have occurred in the northern Japan trench region owing to the subduction of the Pacific plate. Two plate bending points, where the dip of the subducting plate changes markedly, have been found, and correlations between the bending points and the updip limits of large earthquake rupture zones have been previously found in the regions off Aomori ( N) and off Miyagi ( N). In this study, we investigate whether sharp plate bending is also present in the region off Iwate ( N), located approximately halfway between the off-Aomori and off-Miyagi regions. We analysed the existing off-Iwate wide-angle seismic experimental data by applying a new method, in which observed reflection phases are directly projected onto the depth–distance domain from the time–distance domain. As a result, we found a plate bending point in the region off Iwate; this is the first time such a point has been found there. Distances between the three bending points and the trench axis are almost uniform and sharp plate bending is a common feature in the northern Japan trench subduction zone. The bending axis, which is defined by connecting bending points, approximately coincides with the updip limit of the rupture zones of large interplate earthquakes. Seismic swarm activities have been observed inside the subducting oceanic crust along the bending axis, and we infer that some of these activities are caused by bending of the subducting plate. In addition, P-wave velocities of the upper plate vary across the bending axis, implying that the position of the bending axis is closely related to physical properties of the upper plate and to seismic activities. [Copyright &y& Elsevier]

Subduction erosion is a commonly invoked model that is used to explain the tectonic subsidence of the Japan Trench forearc slope, although other models have explained the morphology and history of the margin. New multichannel seismic reflection and bathymetric data collected after the 2011 Tohoku earthquake provide the opportunity to investigate the detailed structure of the overriding plate near the earthquake epicenter and obtain new constraints on tectonic models. We use regional-residual separation of the local bathymetry to constrain fault scarp extents and local landward-dipping forearc basins. Seismic images of these basins clearly show landward-dipping horizons in the shallow section. The strata in these basins imply a different mechanism for formation than the surrounding forearc slope, and we propose that these basins formed from local uplift. A regional basal unconformity mapped ∼150 km along-trench has highly variable relief, indicating that forearc slope subsidence occurs at multiple wavelengths in response to multiple different sources. We characterize the upper to middle slope transition and propose that this region may be the landward limit of major subduction erosion and also the main region for large mass wasting. Normal faults found in this setting have maximum lengths of ∼20 km, limiting their role in margin processes. Our results place constraints on the extent of major subduction erosion at the Japan Trench margin, and indicate that subduction erosion should be revisited as the sole model of formation to include additional tectonic processes. [ABSTRACT FROM AUTHOR]

Abstract: The Japan Trench subduction zone, located east of NE Japan, has regional variation in seismicity. Many large earthquakes occurred in the northern part of Japan Trench, but few in the southern part. Off Miyagi region is in the middle of the Japan Trench, where the large earthquakes (M >7) with thrust mechanisms have occurred at an interval of about 40 years in two parts: inner trench slope and near land. A seismic experiment using 36 ocean bottom seismographs (OBS) and a 12,000 cu. in. airgun array was conducted to determine a detailed, 2D velocity structure in the forearc region off Miyagi. The depth to the Moho is 21 km, at 115 km from the trench axis, and becomes progressively deeper landward. The P-wave velocity of the mantle wedge is 7.9–8.1 km/s, which is typical velocity for uppermost mantle without large serpentinization. The dip angle of oceanic crust is increased from 5–6° near the trench axis to 23° 150 km landward from the trench axis. The P-wave velocity of the oceanic uppermost mantle is as small as 7.7 km/s. This low-velocity oceanic mantle seems to be caused by not a lateral anisotropy but some subduction process. By comparison with the seismicity off Miyagi, the subduction zone can be divided into four parts: 1) Seaward of the trench axis, the seismicity is low and normal fault-type earthquakes occur associated with the destruction of oceanic lithosphere. 2) Beneath the deformed zone landward of the trench axis, the plate boundary is characterized as a stable sliding fault plain. In case of earthquakes, this zone may be tsunamigenic. 3) Below forearc crust where P-wave velocity is almost 6 km/s and larger: this zone is the seismogenic zone below inner trench slope, which is a plate boundary between the forearc and oceanic crusts. 4) Below mantle wedge: the rupture zones of thrust large earthquakes near land (e.g. 1978 off Miyagi earthquake) are located beneath the mantle wedge. The depth of the rupture zones is 30–50 km below sea level. From the comparison, the rupture zones of large earthquakes off Miyagi are limited in two parts: plate boundary between the forearc and oceanic crusts and below mantle wedge. This limitation is a rare case for subduction zone. Although the seismogenic process beneath the mantle wedge is not fully clarified, our observation suggests the two possibilities: earthquake generation at the plate boundary overridden by the mantle wedge without serpentinization or that in the subducting slab. [Copyright &y& Elsevier]

We present the results of a seismic refraction experiment conducted off eastern Aomori, northeastern Japan forearc region, where the detailed crustal structure was poorly known. An objective analysis using high-quality survey data allowed us to obtain the seismic velocity structure to about 30 km depth, and in particular, to obtain a well-constrained geometry of the plate boundary in the source region of the 1994 Sanriku-oki earthquake. Our results show that the subducting Pacific oceanic plate increases its dip from about 5° to approximately 15° around the longitude of 143.5° E, 80 km landward of the trench axis near the latitude of 40° N. The location of this bend in the plate is 40 km east of the bending point previously proposed on the basis of hypocenter distribution. A cluster of aftershocks of the 1994 Sanriku-oki earthquake occurred around the bending point. This is because the bending of the subducting plate appears to affect the stress state in the vicinity of the plate boundary. Furthermore, the plate geometry is also considered to be related to the coupling or slip features along the plate boundary as the bending point correlates with the spatial distribution of repeating earthquakes. Our results will lead to more reliable determinations of asperity areas because an accurate dip angle of the fault plane is necessary for slip distribution studies. [Copyright &y& Elsevier]