2000年有珠山噴火活動に伴う低周波地震(<特集>2000年有珠山噴火 (3))

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Usu volcano has erupted at 13:07 JST on 31st of March, 2000, about 23 years after the 1977 eruption. Many precursory earthquakes occurred since March 27, as in the case of the former eruption. In order to reveal characteristics of these precursory earthquakes, we performed temporal strong-motion observations at two stations near Usu volcano for the period of March 29 to April 1. Wide band and wide dynamic range accelerometers were installed and more than 300 earthquakes (M〓4) were obtained by them. Low frequency earthquakes are picked out based on frequency contents of seismic waves. The number of the low frequency earthquakes is 51, that is much less compared with that of the high frequency earthquakes. The low frequency earthquakes having a dominant frequency of 0.6 Hz show a complex waveform and long duration in comparison with the volcanotectonic earthquakes. At least eight earthquake families are found among the low frequency earthquakes. Although it is somewhat difficult to identify P phases of these earthquakes, hypocenters of about half of them are determined based on 1D velocity structure and a few P-wave arrival times. Most of the epicentres are scattered around the new craters, Nishi-yama and Kompira-yama, but some of them are located around Meiji-Shinzan, east of the craters. The focal depths are less than 3 km. Activity of these earthquakes makes a peak in and around the eruption in contrast to the peak activity of volcanotectonic earthquakes on 30th of March, one day before the eruption. We performed the moment tensor inversion to determine the source mechanism of a part of earthquake families. It seems that the result of the inversion can be explained by a tensile-share crack model and that the source process is complicated.
2002-11-29

2000年有珠山噴火活動に伴う低周波地震(<特集>2000年有珠山噴火 (3))

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