We derived a tectonic stress field model in the vicinity of the source area of the Mw9.0 Northeastern Japan Earthquake from modeling seismicity and analyzing earthquake focal mechanisms before and after the mainshock. There are 5 large earthquakes (Mj>7.0) occurred in the source region of Mw9.0 Tohoku-Oki Earthquake from 2003 to 2011. We performed temporal and spatial analysis to the stresses before and after the mainshock from static stress change and focal mechanism solutions. The results show that the stress field in source region barely had any change before and after each large earthquake prior to the Mw9.0 mainshock, and it might indicate that the background stress had been accumulating and a series of progressive failure processes happened prior to the mainshock. The stress state did not change in the places where are distant away from the rupture center of the Mw9.0 mainshock during its occurrence, with the azimuth of the maximum principal stress to be 290" and dip angle to be 19°. In comparison, the static stress has significant change in the central part of the source region after mainshock, with the occurrence of numerous normal fault type events. It turns out that there exists an area with negative stress change in the source area of the Mw9.0 earthquake after the mainshock, with the P axis rotated from horizontal direction to vertical direction, which triggered the occurrence of numerous normal faulting events, where was dominated by thrust faulting prior to the mainshock. This pattern indicates a complete stress and strain release or locally stresses regime change. Our results provide a constrain to the spatial distribution of the stress release pattern spatially and temporally for the source region of 2011 Tohoku-Oki earthquake.
The 2011 Mw9.0 Tohoku-Oki earthquake occurred along the plate boundary between the subducting Pacific plate and the overlying Okhotsk plate. This earthquake is the largest one even recorded in Japan Trench Subduction Zone (JTSZ). The dynamic background and mechanics of this event is one of the hottest research topics in geodynamics and seismology currently.