Precise Hypocenter Determination for Volcanic Earthquakes Observed at Asama Volcano, Japan

DOI Web Site 23 References
  • OIKAWA Jun
    Earthquake Research Institute, University of Tokyo
  • IDA Yoshiaki
    Graduate School of Life Science, University of Hyogo
  • TSUJI Hiroshi
    Komoro Observatory of Volcano-Chemistry, Earthquake Research Institute, University of Tokyo

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Other Title
  • 浅間山で発生する火山性地震の精密震源決定
  • アサマヤマ デ ハッセイ スル カザンセイ ジシン ノ セイミツ シンゲン ケッテイ

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Description

Hypocenters of volcanic earthquakes in Asama volcano were reexamined and precisely determined using additional four seismic stations that had been installed close to the summit crater from 1996 to 1999. The most significant result obtained in the present study is that the groups of shallow earthquakes beneath the summit crater (B-type and N-type earthquakes), which show broad distribution within the depth of 2.5 km according to the old calculation without the four summit stations, are compactly concentrated into a shallower range within the depth of 600m underneath the west side of the crater. In addition to the conventional method of the hypocenter determination, the double difference method was applied but its application turned out not to have a significant influence on the result. The summit stations were also quite effective in constraining the seismic velocity structure and the P-wave velocity of Asama volcano was determined to be 3.5 km/s so as to minimize the variance between the observed and calculated travel times. Most volcano-tectonic earthquakes (A-type earthquakes) are distributed near the sea level under the west flank of the volcano. The precise hypocenter determination has revealed that there is a seismic gap between the shallow and flank earthquake groups. It is inferred that this seismic gap may represent a magma reservoir because the gap agrees with the location of the pressure source that was estimated from a previous study of the ground deformation.

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