Seismic structure and seismogenesis off Sanriku region, northeastern Japan

Abstract

S U M M A R YIn 1997, a seismic experiment using an airgun array and ocean bottom seismographs (OBSs) was performed in the forearc region of the northeastern Japan (NEJ) arc. The objectives of this experiment were to clarify seismic structures of the forearc region off Sanriku, Japan using airgun-OBS data and to understand the nature of the Japan trench seismogenic zone. Fundamental features of the structure are as follows:(i) The subduction angle becomes steeper from east to west, from 3 to 8 to 11 • , with the top of the plate located at depths of 11 km (40 km west from the trench), 21 km (120 km west from the trench) and 28 km (140 km west from the trench), respectively. (ii) The NEJ Moho beneath the forearc is defined at a depth of approximately 20 km. (iii) P-wave velocity of the island arc upper mantle is approximately 8 km s −1 . (iv) The P-wave velocity of the Cretaceous sedimentary layer exhibits significant lateral variability (4.0-5.5 km s −1 ) in the E-W direction.(v) The upper crust of NEJ forearc also has the significant lateral heterogeneity (5.0-6.3 km s −1 ) in the E-W direction, but not in the N-S direction. (vi) The middle crust defined on land extends in some locations 40 km east of the coastline. (vii) Lateral variability in the P-wave velocity of the lower crust (6.7-7.0 km s −1 ) is small compared with that of the upper crust.According to these results, it is suggested that the rupture areas of large earthquakes (>M7) in this region are limited within the contact zone between the NEJ arc crust and the subducting oceanic crust. Although a thin layer with low velocity is probably present within this contact zone, the serpentinization of uppermost mantle wedge of the NEJ arc by reaction between the mantle materials and the water from the thin layer beneath forearc region is limited. This might indicate that a significant volume of water and sediments subduct with the slab to depths of over 20 km.

P-wave velocity structure in the southernmost source region of the 2011 Tohoku earthquakes, off the Boso Peninsula, deduced by an ocean bottom seismographic survey

“…According to several studies, the contact zone between the island arc crust and the oceanic crust can be the rupture areas of a great earthquake with magnitude >7 (e.g. Nakanishi et al, 2004;Takahashi et al, 2004). Depths of the Moho in the oceanic crust of the PAC is 24-26 km below sea level in the northern part of the profile.…”

“…Since the 1980s, many seismic experiments using controlled sources and Ocean Bottom Seismometers (OBSs) have been conducted to study the seismic structure beneath the landward slope of the JT (e.g. Suyehiro et al, 1985;Takahashi et al, 2004;Mochizuki et al, 2008). The precise structure of the forearc region in the NEJ arc, including the subducting oceanic crust, has been obtained from these studies.…”

Simulation of Dynamic Earthquake Ruptures in Complex Geometries Using High-Order Finite Difference Methods

“…The problem is motivated by the recent magnitude 9.0 Tohoku-Oki, Japan, megathrust earthquake and the resulting tsunami. The specific geometry we consider is shown in Figure 6, and is loosely based on the subduction zone structure in the vicinity of the Japan trench (Tsuru et al, 2000;Takahashi et al, 2004). The Pacific Plate is being subducted to the west beneath the North American / Okhotsk Plate, with relative motion across the plate interface (the fault) occurring during megathrust earthquakes.…”

Along-trench structural variation and seismic coupling in the northern Japan subduction zone

“…von Huene et al (1982) inferred that the Paleogene rocks on the Cretaceous sediments had been uplifted by the end of the Paleogene and then subsided in the Neogene (during the opening of the Japan Sea), creating the prominent unconformity. The Cretaceous metamorphosed sedimentary rocks below the unconformity, which are considered to be the old Cretaceous forearc basin and accretionary prism (Finn, 1994), have P-wave velocities of 4-5.5 km/s according to previous seismic structure studies (Ito et al, 2004(Ito et al, , 2005Takahashi et al, 2004).…”

“…The thick red lines labeled NS1 and NS2 are our wide-angle seismic survey transects, and the white circles on these lines represent OBSs. The thick gray lines labeled EW2000, EW1999, EW1997, and NS1999 are past wide-angle seismic survey profiles (Ito et al, 2004(Ito et al, , 2005Takahashi et al, 2004;Miura et al, 2005;Fujie et al, 2006). Gray contours show coseismic slip distributions of large interplate earthquakes (more than M 7) since 1930; the contour interval is 0.5 m, and the areas with values within half the maximum slip are shaded in yellow (Yamanaka and Kikuchi, 2004).…”