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

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 M(w)9.0 Tohoku-Oki earthquake, we find that the fault consists of two asperities with maximum shear stress drops of 11.7 and 10.1MPa, respectively. The predicted maximum horizontal and vertical displacements on the hanging wall at the Japan Trench are 55.2 and 10.8m, respectively, in good agreement with observation. The predicted total static moments of the mainshock and the M(w)7.9 aftershock, 29min after the mainshock, are 4.48x10(22) and 1.46x10(21)Nm, corresponding to moment magnitudes of M(w)9.0 and M(w)8.0, respectively, again in excellent agreement with the observationally determined M(w)9.0 and M(w)7.9 by the U.S. Geological Survey. Plain Language Summary This paper provides a method to invert for the static stress drop induced by earthquake based on an earthquake stress model. The model is supposed to consist of two elastic blocks connected by interface or fault, across which normal displacement is continuous and stress is the same in magnitude and opposite in direction on its two sides. We use the method and earthquake stress model to invert for the stress drop of the 2011 giant Tohoku-Oki earthquake. From the stress drop we found two asperities in the fault rupture area which is enclosed by a zero shear stress drop line. The maximum fault slip is at the trench, not at the place of the maximum stress drop.