Abstract

The low-frequency flat levels and corner frequencies of the aftershocks of the Kobe earthquake, together with the site effects of the recording stations, are inverted by using genetic algorithms. All records were corrected by the quality factor, Q, of propagation-path, and for each aftershock, a different omega-squared model was assumed such that the low-frequency flat levels and corner frequencies would minimize the standard deviation of the site effects at all stations. It was observed that site effects for rock sites showed significant amplification at high frequencies. This amplification was found to correlate with spectral ratios of records located at 70-m depth and at the surface for one of the stations. After estimating the seismic moment and the stress drop, it was observed that the relation of the seismic moment, M0, versus the corner frequency, fo, followed the scaling law M0 α fo–3. Large values of stress drop were found around asperities on the fault zone and at depth.

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