Inverse Method for Static Stress Drop and Application to the 2011 Mw9.0 Tohoku‐Oki Earthquake

Abstract Seismic stress drop is a fundamental parameter for the investigation of earthquake mechanism. In general, it is indirectly predicted by fault slip based on the dislocation source model or seismic moment; inversion for the stress change on faults has not received the deserved attention. In this study, we propose a finite element method to invert the stress drop on fault, constrained by the observed coseismic deformation. Rupture termination and displacement on fault are automatically predicted from the model. Applying the method to the 2011 Mw9.0 Tohoku‐Oki earthquake, we find that the fault consists of two asperities with maximum shear stress drops of 11.7 and 10.1 MPa, respectively. The predicted maximum horizontal and vertical displacements on the hanging wall at the Japan Trench are 55.2 and 10.8 m, respectively, in good agreement with observation. The predicted total static moments of the mainshock and the Mw7.9 aftershock, 29 min after the mainshock, are 4.48 × 1022 and 1.46 × 1021 Nm, corresponding to moment magnitudes of Mw9.0 and Mw8.0, respectively, again in excellent agreement with the observationally determined Mw9.0 and Mw7.9 by the U.S. Geological Survey.