Abstract

We have constructed a short‐period source model for the 2011 Mw 9.0 Off the Pacific Coast of Tohoku earthquake using strong‐motion records at stations near the source fault. The observed strong motions contain five wavepackets that correspond to specific strong‐motion generation areas (SMGAs). The origins of the wavepackets were retrieved from the original seismograms using a semblance analysis. We determine the locations of the SMGAs based on sources extracted from the corresponding wavepackets. The short‐period source model consists of five SMGAs (SMGA1–5), which are located west of the hypocenter and along the down‐dip edge of the source fault. SMGA1 is located in the Miyagi‐Oki source region west of the hypocenter, SMGA2 in the middle Sanriku‐Oki source region north of the hypocenter, and SMGA3 in the southern Sanriku‐Oki source region west of the hypocenter. SMGA4 and SMGA5 are located near the down‐dip edge of the mainshock source fault, extending from offshore Fukushima prefecture to offshore Ibaraki prefecture. At some stations near the source fault, impulsive waves are also seen on the recorded seismograms. Ground motions corresponding to these impulsive waves cannot be accounted for using the conventional uniform SMGA model. We attempt instead to simulate the observed ground motions at the Onagawa Nuclear Power Plant at a depth of 128 m very near the source fault, using a heterogeneous source model. The impulsive waves are well simulated using the heterogeneous model with higher stress parameters within a small subarea inside the SMGAs.

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