Source mechanisms of volcanic tremor and low-frequency earthquakes associated with the 1988-89 eruptive activity of Mt Tokachi, Hokkaido, Japan

Abstract

Summary During the 1988–89 eruptive activity of Mt Tokachi, Hokkaido, Japan, volcanic tremor and many low-frequency earthquakes were observed by the National University Team at three stations close to the active crater. We calculated the spectral ratios of the tremor and the low-frequency events which were associated with the ash emission from the active crater, and showed that the ratios of both these events have similar features, indicating similar source mechanisms and focal depths. We further resolved the source mechanism and the focal depth of these events by fitting the theoretical spectral ratios to the observed ones, assuming three source models that are consistent with the eruptive activity. The single force model, which represents a counter force of eruption, can explain well both the observed spectral ratios and the distributions of ballistic rocks emitted from the active crater. On the other hand, though the solutions of the tensile crack model and the cylindrical model, which represent movements of a crack or a conduit due to the ash emissions, can explain the observed spectral ratios as well as the single force model, they cannot explain the surface configurations of the volcano and the direction of the ash emissions simultaneously. The focal depths were estimated at less than 0.5 km. As a result, we concluded that the tremor and the low-frequency events were excited by single force applied to the shallow part of the crater. We further calculated the spectral ratios of the explosion earthquakes observed at Mt Tokachi, and identified similarities of the source mechanisms to those of the tremor and the low-frequency events. This suggests that source mechanisms of volcanic earthquakes and tremors associated with eruptions (emissions of volcanic materials from craters) are similar while these events have different source time functions and magnitudes.