Source parameters of the 1983 Japan Sea earthquake sequence.

Abstract

The source parameters of the main shock and the aftershocks of the 1983 Japan Sea earthquake MJMA 7.7 are determined by using seismic records obtained at stations with the hypocentral distances from 70 to 250 km. First, we attempt to separate the source spectrum, the propagation path effect, and the site amplification by applying a linear inversion method to the logarithm of the observed spectra. Using the propagation path and the site effects determined by the inversion, we can estimate the source spectra of events which are recorded at even a single station, by removing the propagation and the site effects from the observed spectra. Next, we estimate the source parameters and examine the relation between those parameters. Also we test the applicability of the ω-2 model to the source spectrum with two methods: the first compares the dynamic stress drop to the static stress drop; the second compares the seismic moment and the corner frequency determined from Andrews' automated objective method based on the ω-2 model to those from the hand-reading method. Most of the events obey the ω-2 model, but several events with seismic moment greater than 3×1022 dyn·cm, especially the main shock and the largest aftershock, cannot be explained by the ω-2 model, but can be explained by the multi-crack model or the asperity model, because the dynamic stress drop of these earthquakes is larger than the static stress drop.