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Title: 1996年口永良部島火山の地震活動の活発化について
Other Titles: Increase in Seismic Activity in 1996 at Kuchierabujima Volcano
Authors: 山本, 圭吾  kyouindb  KAKEN_id
井口, 正人  kyouindb  KAKEN_id
高山, 鐵朗  KAKEN_name
石原, 和弘  KAKEN_name
Author's alias: YAMAMOTO, Keigo
IGUCHI, Masato
ISHIHARA, Kazuhiro
Keywords: 口永良部島
Kuchierabujima volcano
volcanic earthquake
hypocentral distribution
focal mechanism
Issue Date: 1-Apr-1997
Publisher: 京都大学防災研究所
Journal title: 京都大学防災研究所年報. B = Disaster Prevention Research Institute Annuals. B
Volume: 40
Issue: B-1
Start page: 39
End page: 47
Abstract: 口永良部島火山に発生する火山性地震は, P波, S波の明瞭なA型, 高周波成分の卓越するHF型, 高周波成分と低周波成分の混在するHybrid型, 低周波成分の卓越するLF型および単一の周波数にスペクトルのピークを持つMonochromatic型に分類される。1995年以前は, 1ヶ月に10個以下のHF型地震が観測されていたが, 1996年1月初めの比較的規模の大きいA型地震とLF型地震の発生以後, HF型の発生頻度が高くなっただけでなく, 低周波成分の含まれるHybrid型, LF型およびMonochromatic型が発生するようになった。1996年5月から6月に, 西側山腹に4点, 山麓に6点地震計を設置し, 臨時地震観測を行った。島内における砕石発破を利用してP波速度構造を求めた。Vp=3.3㎞/sの半無限均質構造を仮定して震源決定を行ったところ, A型地震は口永良部島北東部に, その他の種類の地震は, 新岳の火口直下海抜Om付近に集中していることがわかった。LF型およびMonochromatic型の初動は, 全観測点において引きであり, 4象限型の発震機構で説明するのは困難である。
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.
Appears in Collections:No.40 B-1

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