Abstract —Statistical analysis of the focal mechanisms of the earthquakes based on the ISC standard catalog for 2001 to 2017 is carried out in the spatiotemporal vicinity of epicentral zone of the catastrophic seismic event of 2011 with M w = 9.0 in Tohoku, Japan. It is established that during the 10-year period preceding this earthquake, more than 80% of the intermediate ( B ) axes of the focal mechanisms of the earthquakes in the depth interval from 0 to 65 km have a dip angle of at most 20° and are mainly oriented within the azimuthal sector from 190° to 210° enclosing the strike of the Japan Oceanic Trench. More than 75% of the pressure axes ( P ) are inclined to the horizon at 5°–35°, and more than 75° of the axes of tension ( T ) have a dip angle from 50° to 80°. This orientation of the principal axes of focal mechanisms unambiguously suggests the predominance of thrusting or underthrusting regime at these depths. This corresponds to the well-known tectonic hypothesis that the Pacific Ocean lithospheric plate subducts beneath the continental Okhotsk plate in the region of the Islands of Japan in the approximately sublatitudinal direction. The mega-thrust slip in the source of the catastrophic Tohoku earthquake also corresponds to this type of the focal mechanism. A similar type of the focal mechanism was also observed in the aftershock sequence of the earthquakes located in the vicinity of the surface separating the hanging and subducting edges of the lithospheric plates colliding here, exactly at the location of the catastrophic earthquake. The predominant depth range of these earthquakes is 30 to 55 km. At the same time, it is established that the aftershock sequence of this event at shallow depths (less than 20 km) within the coastal accretionary prism has a striking feature in the form of the earthquakes having a normal-fault mechanism, which are untypical of the subduction zones. These earthquakes make up about 70% of the total number of the aftershocks. It is hypothesized that the existence of aftershocks with a normal-fault type of focal mechanism at shallow depths within the accretionary prism is associated with the details in the internal structure of the prism and with the thickness of weak terrigenous sediments sucked into the interplate space by the subduction of the lithospheric plates. These details determine the observed pattern of relaxation of the accumulated tectonic stresses from a wide vicinity of the prism into a compact region of their almost complete release during the intense coseismic slip.

中文翻译：

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2011年日本东北大地震时空附近震源机制统计

摘要——在日本东北2011年M w = 9.0的特大地震事件震中带附近进行了基于ISC标准目录的2001-2017年地震震源机制的统计分析。确定在本次地震前10年期间，0～65km深度区间内80%以上的地震震源机制中轴（B）的倾角最大为20°并且主要位于 190° 到 210° 的方位角范围内，包围了日本大洋海沟的走向。超过 75% 的压力轴 ( P ) 与地平线倾斜 5°–35°，超过 75° 的张力轴 ( T ) 具有 50° 到 80° 的倾角。震源机制主轴的这种方向明确表明在这些深度处推力或下推力制度占主导地位。这对应于众所周知的构造假说，即太平洋岩石圈板块在近似亚纬度方向上俯冲到日本群岛地区的鄂霍次克大陆板块之下。东北大地震震源的超大推力滑移也对应于这种震源机制。在地震的余震序列中也观察到了类似类型的震源机制，这些地震的余震序列位于将在这里碰撞的岩石圈板块的悬挂边缘和俯冲边缘分开的地表附近，恰好在大地震的位置。这些地震的主要深度范围为 30 至 55 公里。同时证实，该事件在海岸增生棱柱内浅层（小于20km）的余震序列具有显着特征，即具有正断层机制的地震，这是非典型的地震。俯冲带。这些地震约占余震总数的70%。推测增生棱柱内浅层存在具有正常断层型震源机制的余震与棱柱内部结构的细节以及被吸入板间空间的弱陆源沉积物的厚度有关。岩石圈板块的俯冲。