Change in Earthquake Spectrum before and after the Matsushiro Swarm

Abstract

Earthquake spectra before a n d after the Matsushiro Swarm, that were obtained at Matsushiro Station from local earthquakes of the same hypocentral region and of the same order of magnitude, showed a marked difference. Before the swarm, waves higher than 200 cps were abundantly recorded, whereas such high-frequency waves almost disappeared toward the end of the swarm. The present study revealed that the change in spctrum was due to the medium, which was highly fractured by an enormous number of earthquakes during the swarm, and that the attenuation of highfrequency energy in the seismic waves, which are propagated through the swarm region, is caused by scattering by many fissures.
It seems likely that the Matsushiro region of hig h heterogeneity was under strain, started breaking by many fractures, and was eventually released from the strain as MoGw's experiments suggested.