Three-Dimensional P-Wave Velocity Structure Of Bandai Volcano In Northeastern Japan Inferred From Active Seismic Survey

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Journal Article: Three-Dimensional P-Wave Velocity Structure Of Bandai Volcano In Northeastern Japan Inferred From Active Seismic Survey

Abstract
The three-dimensional P-wave velocity structure of the Bandai volcano has been revealed by tomographic inversion using approximately 2200 travel-time data collected during an active seismic survey comprising 298 temporary seismic stations and eight artificial shots. The key result of this study is the delineation of a high-velocity anomaly (Vp>4.6 km/s at sea-level) immediately below the summit peak. This feature extends to depths of 1-2 km below sea-level. The near-surface horizontal position of the high-velocity anomaly coincides well with that of a positive Bouguer gravity anomaly. Geological data demonstrate that sector collapses have occurred in all directions from the summit and that the summit crater has been repeatedly refilled with magmatic material. These observations suggest that the high-velocity region revealed in this study is a manifestation of an almost-solidified magmatic plumbing system. We have also noted that a near-surface low-velocity region (Vp<3.0 km/s at sea-level) on the southern foot of the volcano corresponds to the position of volcanic sediments including ash and debris avalanche material. In addition, we have made use of the tomographic results to recompute the hypocenters of earthquake occurring during seismic swarms beneath the summit in 1988 and 2000. Relocating the earthquakes using the three-dimensional velocity model clearly indicates that they predominantly occurred on two steeply dipping planes. Low-frequency earthquakes observed during the swarms in 2000 occurred in the seismic gap between the two clusters. The hypocentral regions of the seismic swarms and the low-frequency earthquakes are close to the higher-velocity zone beneath the volcano's summit. These observations suggest that the recent seismic activity beneath the summit is likely associated with thermal energy being released within the solidifying magmatic plumbing system.

Authors 
Teruo Yamawaki, Satoru Tanaka, Sadato Ueki, Hiroyuki Hamaguchi, Haruhisa Nakamichi, Takeshi Nishimura, Jun Oikawa, Tomoki Tsutsui, Kiyoshi Nishi, Hiroshi Shimizu, Sosuke Yamaguchi, Hiroki Miyamachi, Hitoshi Yamasato and Yutaka Hayashi








Published Journal 
Journal of Volcanology and Geothermal Research, 2004





DOI 
Not Provided
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Citation

Teruo Yamawaki,Satoru Tanaka,Sadato Ueki,Hiroyuki Hamaguchi,Haruhisa Nakamichi,Takeshi Nishimura,Jun Oikawa,Tomoki Tsutsui,Kiyoshi Nishi,Hiroshi Shimizu,Sosuke Yamaguchi,Hiroki Miyamachi,Hitoshi Yamasato,Yutaka Hayashi. 2004. Three-Dimensional P-Wave Velocity Structure Of Bandai Volcano In Northeastern Japan Inferred From Active Seismic Survey. Journal of Volcanology and Geothermal Research. (!) .