Nearly complete stress drop in the 2011 Mw 9.0 off the Pacific coast of Tohoku Earthquake

Temporal change in the stress field after the 2011 Tohoku earthquake was observed by stress tensor inversions of focal mechanisms of earthquakes near the source region. The maximum compressive stress (sigma(1)) axis before the earthquake has a direction toward the plate convergence, dipping oceanward at an angle of 25-30 degrees. Its dip angle significantly increased by 30-35 degrees after the earthquake, and sigma(1) axis came to intersect with the plate interface at a high angle of about 80 degrees. By using the observed rotation of sigma(1) axis, we estimated the ratio of mainshock stress drop to the background deviatoric stress Delta tau/tau to be 0.9-0.95. This shows that the deviatoric stress causing the M-w 9.0 earthquake was mostly released by the earthquake, or the stress drop during the earthquake was nearly complete. Adopting the average stress drop obtained by GPS observation data, the deviatoric stress magnitude is estimated to be 21-22 MPa. This suggests the plate interface is weak. The nearly complete stress drop caused a high dip angle of sigma(1) axis, which is the reason why not a small number of normal fault type aftershocks have occurred.