Increase in Seismic Activity in 1996 at Kuchierabujima Volcano

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  • 1996年口永良部島火山の地震活動の活発化について
  • 1996ネン クチノエラブジマ カザン ノ ジシン カツドウ ノ カッパツカ

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Abstract

Volcanic earthquakes at Kuchierabujima volcano are classified into 5 types; A-type: tectonic-like earthquake with clear P- and S-phases, HF-type: dominated by high frequency, Hybrid-type: mixed with high and low components, LF-type: dominated by lower frequency, and Monochromatic event. Less than 10 per month HF-type events were observed before 1995. After occurrence of A-type earthquakes and LF events in early January 1996, the number of HF events suddenly increased and Hybrid and LF with lower frequency content and Monochromatic events frequently appeared. Temporary seismic observation was conducted in May and June 1996, installing 6 seismic stations around the summit crater and a small network with 4 seismometers on the western flank. Assuming Vp = 3.3 kim/s estimated from artificial explosions, locations of the earthquakes were determined. Two A-type earthquakes were located northeast off the volcano and the other types were concentrated beneath the summit crater at depth of sea level. P-wave first motions of LF and Monochromatic event were dilatation at all the station. The mechanism of the events may not be double-couple.

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