High-frequency radiation process during earthquake faulting - Envelope inversion of acceleration seismograms from the 1993 Hokkaido-Nansei-Oki, Japan, Earthquake

We investigate the process of high-frequency (1 to 10 Hz) radiation on the fault plane of the 1993 Hokkaido-Nansei-Oki, Japan, earthquake (M-w = 7.5) from the envelope inversion of strong-motion acceleration seismograms. For the analysis, empirical Green's functions are used because theoretical approach is not available for such high frequencies. The source is modeled with two-fault planes with different strike angles. The rupture process of this earthquake is very complex in terms of high-frequency wave generation. The rupture, which started on the northern fault plane, had a delay of about 10 sec or propagated very slowly between the northern and southern fault planes. High-frequency wave radiation is large at the northern and southern edges of the source region. Deceleration of rupture is also observed there. This is interpreted to be associated with stopping of rupture. Another high-frequency wave radiation area is found at the center of the northern fault plane, where discontinuity in the depth distribution of aftershocks suggests an existence of a barrier. The areas of high- and low-frequency wave radiation are not correlated. This is considered to result from the complexity of rupture process. We cannot distinguish between westward and eastward dip of the southern fault plane because of one-sided station distribution.