Hypocenter Distribution in the Japan Trench Region, off Sanriku, Northeast Japan, Determined by an Ocean-Bottom Seismometer Array Observation
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An OBS array observation (seven stations and one month) was. performed in June and July, 1981 off-Sanriku, Japan Trench region, Northeast Japan. The array covered the area of both the island-arc side and the ocean basin side of the trench axis. In order to improve the resolution capability of focal depth estimation, we developed a new earthquake location algorithm, which is characterized by a ray initializer technique for the three-dimensional heterogeneous velocity structure, an addition of the origin time constraint in the least-squareiteration procedure, and another addition of an examining procedure for the effect of the initial trial-focal-depth. Three distinct groups. were revealed in the hypocenter distribution in the Japan Trench region. (1) One group is a long and narrow zonal distribution along the bathymetric contour of a depth of about 6km in the outer-slope region. The depths of these earthquakes are shallow within the ocean crust. They appear to correspond to the graben structures in this region representing the bending motion of the ocean crust. (2) Another group is the earthquakes in clusters near the trench axis in the outer-slope region. The depths of these earthquakes extend deep into the uppermost mantle beneath the Moho discontinuity from the shallow crust along a nearly vertical plane representing a lithospheric faulting motion. This place corresponds to the locatin of the main shock of the 1933 large off-Sanriku earthquake. This seismicity appears to be associated with the subsidence and a sharp bending de formation of the subducting ocean crust under the inner-slope region after crossing the trech axis. (3) The third group is the swarm earthquakes in the inner-slope region at a water depth of 2-3km. The depths of these earthquakes are nearly at the interface between. the overlying landside rock mass and the underlying ocean crust. This seismic activity would represent either a slip or a thrust faulting motion along the interface.海底地震計群列観測を,1981年夏,三陸沖日本海溝域において行い,7観測点で,6月11日から7月12日に至る33日間の同時観測のデータを得た.群列観測網は海溝軸を挟んで海盆側と島弧側と両方に設けた.震源深度の分解能を高めるために,新しく震源決定アルゴリズムを開発した.それは(1)3次元不均質構造を取扱うRay Initializerを用いたこと,(2)最小二乗法反復手順に震源発震時に対する拘束条件を加えたこと,(3)初期試行震源深度の影響を吟味する手順を加えたことなどに特徴づけられる.その結果,日本海溝域の震源分布について,3つの群が見出された.それらの特徴は次の通りである.(1)1つの群は日本海溝外側斜面域において水深約6kmの等深線に沿って,長くかつ狭い幅の線状傾向を示す帯状分布である.これらの地震の震源深度は浅く,海洋地殻内に起っている.これらは海溝外側斜面域に発達する地溝構造に対応しているようであり,海洋地殻の曲げ変形運動を表わしているものと思われる.(2)2つめの群は海溝軸の近傍,外側斜面下に房状のまとまりを示す地震である.その場所は1933年三陸沖大地震(M=8.3)の主震の震央に対応している.これらの地震の震源深度は深く,海洋地殻からモホ面下,最上部マントル内部約45kmにわたって起つており,その分布はほぼ垂直に近い面に沿っており,海洋リソスフィヤの断層運動を示しているようである.この最上部マントル内の地震活動は海溝軸を越えて海溝内側斜面下へ沈み込む海洋地殻の鋭い曲げ変形と沈降とに関連しているもののようである.(3)3つめの群は海溝内側斜面下水深2-3kmの地域に起った群発地震である.これらの地震の震源深度は海洋地殻と上盤の陸側岩体との境界付近にあたっている.この地震活動は境界面に沿うすべり或は断層運動を表わしているものであろう.
- 東京大学地震研究所,Earthquake Research Institute, University of Tokyo,Earthquake Research Institute, University of Tokyoの論文
- 1985-03-30
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