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Abstract: We use recently acquired seismic reflection data and reprocessed sections to interpret the structure of the northern part of the NE Japan forearc. We identify a NNW-SSE trending deformation zone bounded by large transcurrent faults (trends T1–T4 from west to east) on the upper continental slope. The lower slope is characterized by a frontal prism and reverse faults. Bathymetric data reveal several en echelon topographic bulges with NW-SE elongation aligned parallel to the fault trends. The bulges are interpreted as a dextral wrench zone that has developed since the Neogene, based on structural contours of two late Cenozoic seismic horizons. Clockwise rotation associated the wrenching is indicated by the paleomagnetism of Paleogene marine sediments obtained from a drill hole within the faulted zone close to trend T1. A previous seismic refraction study shows remarkable contrast in P-wave velocity across trend T2, indicating a considerable rearrangement of crust associated with transcurrent motion along this fault. We propose a paleogeographic reconstruction assuming >200km southward (trenchward) lateral transportation along trend T2 and its northern extension. In our model, subduction erosion at the northern Japan Trench was enhanced by trenchward migration of the fore-arc sliver, but steadied following rearrangement of the sliver along the transect. The calculated variation in subduction erosion is consistent with subduction histories based on existing well data. [Copyright &y& Elsevier]

Abstract: Off the southern coast of Hokkaido the Hidaka-oki (offshore Hidaka) basin has developed on the western flank of a collision suture under the influence of long-standing compressional plate motion and provoked tectonic stresses around the northwestern Pacific rim throughout the late Cenozoic. The basin forming history of the Japan arc and Kuril arc collision zone is described on the basis of seismic reflection data interpretation. We identify two stages of basin formation: the older (late Oligocene–Miocene) faulted en echelon graben (pull-apart basin) and younger (Plio–Pleistocene) regional downwarping. Paleoenvironmental changes recorded within the fore-arc sediments indicate that the older basin filled up by the late Miocene. We inferred the volumes of the distinctive basins from the depth-conversion of seismic data, which suggest episodic uplifts and massive erosion of the Hidaka Mountains in the middle–late Miocene and the Plio–Pleistocene. Estimated sediment supply rates into the basins have a similar level for the both stages. Cause of an episodic uplift in the older stage is attributed to the delayed opening of the Japan Sea. The eastern Eurasian margin underwent N–S right-lateral faulting at 25Ma as a result of rifting of the Kuril back-arc basin. Formation of the Japan Sea back-arc basin since the early Miocene (ca. 20Ma) caused eastward motion of the western Hokkaido block and transpressive regime along the pre-existing N–S shear deformation zone. [Copyright &y& Elsevier]

We used recent seismic data and advanced techniques to investigate 3D fault geometry over the transition from the partially coupled to the fully coupled plate interface inboard of the Nankai Trough off the Kii Peninsula, Japan. We found that a gently dipping plate boundary décollement with a thick underthrust layer extends beneath the entire Kumano forearc basin. The 1 April 2016 Off-Mie earthquake (Mw6.0) and its aftershocks occurred, where the plate boundary décollement steps down close to the oceanic crust surface. This location also lies beneath the trenchward edge of an older accretionary prism (∼14 Ma) developed along the coast of the Kii peninsula. The strike of the 2016 rupture plane was similar to that of a formerly active splay fault system in the accretionary prism. Thus, the fault planes of the 2016 earthquake and its aftershocks were influenced by the geometry of the plate interface as well as splay faulting. The 2016 earthquake occurred within the rupture area of large interplate earthquakes such as the 1944 Tonankai earthquake (Mw8.1), although the 2016 rupture area was much smaller than that of the 1944 event. Whereas the hypocenter of the 2016 earthquake was around the underplating sequence beneath the younger accretionary prism (∼6 Ma), the 1944 great earthquake hypocenter was close to oceanic crust surface beneath the older accretionary prism. The variation of fault geometry and lithology may influence the degree of coupling along the plate interface, and such coupling variation could hinder slip propagation toward the deeper plate interface in the 2016 event. [ABSTRACT FROM AUTHOR]

Abstract: The southwestern Kuril Arc, which is not affected by a series of accretion and collision processes, is one of the most important experimental sites among the world arc–trench systems for investigating the evolution of arc crust. Published petrological and seismic studies have shown that, in south-central Hokkaido, the upper part of the Kuril Arc crust is thrusting upward toward the west while the lower part is descending downward in the same direction. Among several hypotheses for formation of continental crust, we discuss the role of delamination of the island arc lower crust based on geochemical and seismic velocity structure models. Seismic velocity structure is obtained from an onshore–offshore seismic survey across the southwestern Kuril Arc–Trench system. The structure in the back-arc area shows a thick accretionary complex formed by Paleogene accretion and collision. Based on published seismic results, this accretionary complex is considered to continue on land to the Cretaceous accretionary complex observed in the Hidaka Belt. The structure of the Kuril Arc appears characteristic of upper crustal andesitic rocks (6.0–6.2 km/s) and thick middle to lower crust (6.5–7.3 km/s). Based on published geochemical study, we consider a geochemical model of simple crustal evolution explaining the mechanism of generation of andesitic rocks on the Kuril Arc. Each volume of andesitic, basaltic, and mafic layers estimated from the geochemical model is consistent with those obtained from the seismic velocity model. Crustal composition estimated from the geochemical model and major-element chemistry of the volcanic rocks in the Kuril Arc indicate that mechanical delamination is insufficient to cause the oceanic island arc crust to evolve into continental crust. [Copyright &y& Elsevier]

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]

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]