Temporary seismic observations were carried out for about 1 month at Miyakejima Island on the Izu-Bonin arc after an eruption on October 3, 1983. More than 3000 seismic events were observed and they are classified into 4 different categories: short-period earthquakes, long-period earthquakes, isolated tremors and continuous tremors. Two active source regions of the short-period earthquakes were located along the old caldera rim of the Miyakejima volcano. One region was very close to the eruptive fissures. Another was about 1-2 km northwest of the fissures. Almost all the earthquakes in the former region showed dilatant first motions of P-waves at all the stations in the island. On the other hand, almost all events in the latter region showed compressive first motions of P-waves at all the stations. The first motions of P-waves for these earthquakes cannot be explained by the quadrant-type pattern due to a double-couple force system. Nucleation of a tensile crack coupled with a shear crack (a tensile-shear crack model) is proposed as a source mechanism of the short-period earthquakes. Moment tensor elements of tensile and shear cracks have been determined by the least squares method using observed P- and S-wave data. Results suggest that tensile cracking (opening or closing) is dominant for generating those earthquakes in comparison with shear cracking. The strikes of the tensile cracks for the earthquakes are shown to be nearly parallel to the strike of the fissures. The earthquakes that occurred in the northwestern region of the fissures are considered to be generated by a sudden opening of tensile-shear cracks due to the excess pressure of intrusive magma. In contrast, the earthquakes occurring beneath the fissures are probably generated by a sudden closing of tensile-shear cracks, and this suggests that the magmatic pressure under the fissures rapidly decreased after the eruption. Moment tensor analysis of the tensile-shear crack model in this study demonstrates the non double-couple source mechanism for volcanic earthquakes related to magmatic activity.
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