Characteristics of Volcanic Earthquakes at Kuchierabujima Volcano-Geophysical and Geochemical Joint Observation 2000-

IGUCHI, Masato, YAMAMOTO, Keigo, TAKAYAMA, Tetsuro, MAEKAWA, Tokumitsu, NISHIMURA, Takeshi, HASHINO, Hironori, YAKIHARA, Hiroshi, HIRANO, Shuichiro

A seismic observation was conducted at Kuchierabujima volcano, south off Kyushu, during the period from November 27, 2000 to March 18, 2001. We installed 5 seismic stations with 3-component seismometers and 3 seismic stations with short-period vertical seismometer (2Hz) around Shindake crater at the summit. Most of volcanic earthquakes observed at the volcano were high-frequency events with the spectra of 6-30Hz. The high- frequency events occurred at the western part of the Shindake crater with the depth of l00-400m. The epicentral area corresponds to the geothermal area at the western part of Shindake crater. The focal mechanisms of the high-frequency events were of normal fault type with east-west extension. These facts suggest that the high-frequency events were generated by shear fracture of rock in the geothermal area. Monochromatic events have a dominant spectrum peak at 8.0 Hz and have some sub-peaks at 1.1, 3.0, 4.8, 6.2 and some peaks higher than 10Hz. The amplitudes at the peaks of 1.1, 3.0, 4.8 and 6.2 Hz were attenuated slowly with lapse time from the initial motion. The energy at the peaks higher than 8 Hz was concentrated in the initial part of the events. It is inferred that higher frequency components of the events were reflected by repetition of high-frequency events, which acted as trigger to resonance of fluid body generating seismic waves with some peak frequencies lower than 7 Hz.

Characteristics of Volcanic Earthquakes at Kuchierabujima Volcano-Geophysical and Geochemical Joint Observation 2000-

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