Estimation of high-frequency wave radiation areas on the fault plane by the envelope inversion of acceleration seismograms

We have developed an inversion method to estimate high-frequency wave radiation areas on the fault plane. It is very difficult to perform a source inversion for seismic waveforms whose frequencies are higher than 1 Hz because of two problems: one is the difficulty in obtaining Green's functions that are accurate enough for such high frequencies, and the other is that involved in doing a waveform inversion for such oscillatory data. We overcame these problems by using aftershock records as empirical Green's functions, and by using the envelopes of acceleration seismograms as the inversion data instead of waveforms. Envelopes were calculated by smoothing the acceleration seismograms with a running root-mean-square time window. We applied this method to the 1993 Kushiro-Oki, Japan, earthquake (M(W) = 7.6), which occurred on a horizontal fault plane inside the Pacific Plate subducting beneath the North American Plate. We adopted the M(JMA) 4.9 aftershock as the empirical Green's function event. We bandpass-filtered (2.0-10.0 Hz) the strong-motion acceleration seismograms at three stations and calculated the envelopes by smoothing with a 5.0 s window length. The 40 km x 60 km fault plane was divided into 4 x 6 meshes, and the acceleration radiation intensity and rupture time of each mesh were obtained by the inversion. The results are as follows. (1) High-frequency waves radiated weakly around the rupture starting point, but radiated mainly near the periphery of the fault plane. An area with particularly strong radiation intensity of high-frequency waves is seen at the edge of the fault plane about 30 km southwest of the rupture starting point; its size is about 20 km x 20 km. (2) The distribution of high-frequency radiation intensity and slip distribution are complementary to each other. That is, the radiation of high-frequency waves was strong in those areas where that of low-frequency waves was weak.