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地震 第2輯
Vol. 57 (2004-2005) No. 2 P 171-185



A large intraslab earthquake (MJMA 7.1) with the focal depth of about 70km occurred in the Pacific slab below off Miyagi Prefecture, northeastern Japan on May 26, 2003. This earthquake source brought strong high-frequency ground motion to the near-source region. At first, a source model is estimated by the forward modeling through strong motion simulations using the empirical Green's function method. Strong ground motions are simulated in the broadband frequency range (0.3-10Hz). Four borehole station data of KiK-net are used. A simple source model having several rectangular “strong motion generation areas” (Miyake et al., 2003) is assumed. The fittings of acceleration, velocity, and displacement waveforms are fairly well. Next, strong ground motion simulation is conducted at all the stations of K-NET and KiK-net (surface) within about 100km distance from the epicenter to validate the obtained source model. The spatial distributions of peak horizontal velocity and acceleration are well reproduced, which gives an evidence that our obtained source model can describe principal characteristics of the actual source process of this earthquake. However, synthetic peak horizontal accelerations are much larger than observed ones at several stations, which may be caused by nonlinear effect of the superficial layers during the mainshock. The obtained source model contains three strong motion generation areas. The spatial distribution of the strong motion generation areas is almost corresponding to the large slip regions from the kinematic waveform inversion analysis. The total size of the strong motion generation areas against a given seismic moment follows the empirical relationship with source-depth for intraslab earthquakes by Asano et al. (2003). Each strong motion generation area of this source model has 105MPa of stress drop. The varieties and uncertainties in estimation of the stress drop value on strong motion generation areas are also discussed.

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