Focal Mechanisms Statistics in Spatiotemporal Vicinity of the 2011 Tohoku Catastrophic Earthquake, Japan

Abstract

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 Mw = 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.