Anomalously High Frequency Aftershocks and Source Characteristics of the Mid-Japan Sea Earthquake, May 26, 1983

Abstract

Frequency characteristics of the major aftershocks of the Mid-Japan Sea earthquake of May 26, 1983 (M=7.7) have been studied by use of the waveform records of the telemetered stations of the Abuyama Seismological Observatory and those of the temporary station at Fukaura. The major aftershocks during a month from immediately after the main shock are divided into three groups, type H, L and M by their characteristic frequencies. The shocks of type H are characterized by their anomalously high frequencies of 6-9Hz, whereas the shocks of type L, by comparatively low frequencies of 1-3Hz. The shocks of type M have both low and high frequencies and show rather flat velocity spectra of 1-10Hz. The frequency characteristics of the aftershocks are the same as those recorded at Fukaura where the epicentral distances (50-100km) are much shorter than those (630-810km) of the substations of the Abuyama Seismological Observatory. The aftershock area is divided by the frequency characteristics of the after-shocks into three parts which seem to correspond to the rupture characteristics of the main shock which consists of at least two big events. The aftershocks of type M are frequent in the southern part of the aftershock area where the first event of the main shock propagated from south end towards north-northeast and stopped at the northern end, while the aftershocks of type H occurred only at the central part of the aftershock area where the second event of the main shock began. Furthermore, the shocks of type L are frequent in the northern part where the second event propagated with a slower rupture velocity than the first event of the main shock and with a different rupture direction of south-southeast to north-northwest. This suggests that the frequency characteristics of aftershocks closely correlate with the rupture process of the main shock.