Temporal change in attenuation intensity before and after the Eastern Yamanashi Earthquake of 1983 in central Japan

Abstract

A crutal earthquake of magnitude 6.0 took place on August 8, 1983, in the middle of the Kanto‐Tokai microearthquake observation network of the National Research Center for Disaster Prevention in central Japan. By analyzing vertical seismograms of microearthquakes occurring in a small volume in the aftershock region, we measured temporal change in attenuation intensity for a period from 22 months before to 9 months after the main shock. Attenuation change was estimated by using two different kinds of analyses. In the first analysis, station magnitude calculated from the maximum amplitude at each station was compared with the average of station magnitudes. Deviations of station magnitude from the average magnitude at five stations in the vicinity of the aftershock area are found to have increased after the main shock compared with that before. In the second analysis, S coda duration from the origin time was measured and was compared with the average magnitude. The increase of S coda duration for the same average magnitude is found after the main shock at two stations close to the main shock epicenter. The simplest interpretation of the results is that the attenuation intensity in the upper crust in the close vicinity of the focal region decreased after the main shock for frequencies f from 2 to 20 Hz: ΔQs−1 = −1.4×10−2f−1. Since other precursors appeared in the preparation stage, it is possible to interpret the change in Qs−1 as a precursor. The attenuation intensity would have been anomalously large before the main shock.