Source Process of the 2011 Great Tohoku earthquake Inferred from Waveform Inversion with Long-Period Strong-Motion Records and its Validation by Tsunami Simulation

We have investigated the rupture process of the 2011 off the Pacific coast of Tohoku earthquake by the multi-time-window linear waveform inversion method using the long-period strong-ground motion data. From the record section of the long-period motion of the 3 phases, it is indicated that the rupture process mainly consists of the 3 stages. We assumed a single planar fault model of 468 km long in strike and 228 km wide in dip. We used the strong-motion data obtained from 37 stations of F-net and KiK-net (NIED), and 1 station of Hokkaido University for the inversion. The seismic moment of this earthquake was estimated to be 4.3e22 Nm (Mw 9.0). The inverted slip distribution shows a large asperity with a maximum slip of about 47 m which is located on the shallower part of the fault plane (Fig. 1A). The rupture process is divided into three stages: first stage with moderate slip; second stage with large and long-duration slip in the shallow part of the fault; and third stage with relatively small and short-duration slip in the southern part of the fault. The feature of the ground motion suggested from the record section is well represented by the peak moment rate distributions (Fig. 1B), rather than the slip distributions. Rupture velocity is around 3.4 km/s in the first and third stages, while rupture progression suspended for a while before the rupture of the asperity. We validated this model with tsunami simulation using fully nonlinear Bousinessq type model of tsunami-wave propagation. We compared the tsunami simulation result with observation data and with results of simulation using other source models. Simulation results fit observed tsunami waves in the off-shore area, and inundation heights on the coast of Sendai and Fukushima. On the Sanriku coast simulated amplitudes are smaller than observed ones. Possible reason of the underestimated simulated amplitudes is a small resolution of used bathymetry data for this area.