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Bulletin of Volcanology

, Volume 68, Issue 1, pp 47–56 | Cite as

Deep low-frequency earthquake swarm in the mid crust beneath Mount Fuji (Japan) in 2000 and 2001

  • Motoo UkawaEmail author
Research Article

Abstract

Beneath Mount Fuji, the highest active volcano in Japan, deep low-frequency (DLF) earthquake activity has been monitored since the early 1980s. The DLF earthquakes occurred in the mid-crustal depth range, and burst-type activity lasting from several minutes to 30 min was detected 10 to 20 times in an ordinary year. The DLF earthquake activity increased sharply in the period from October 2000 to May 2001, showing swarm-like activity. The occurrence rate during the DLF earthquake swarm was approximately 20 times higher than the usual activity, and the wave energy released during the swarm period was twice as high as the total wave energy during the past 20 years. The DLF earthquakes in the period from 1987 to 2001 were relocated by estimating station corrections in order to reduce the effect of the change of seismic station distribution. The epicenters of most DLF earthquakes occurred in an elongated region with a long axis of about 5 km, whose center is located 2–3 km NE from the summit. A few percent of the DLF earthquakes, however, occurred around the summit area, significantly apart from the main epicenter region. The focal depths of well-located DLF events range from 10 to 20 km. During the high activity period in 2000 and 2001, most DLF events occurred within this main hypocenter area. The sharp increase of DLF earthquake activity at Mount Fuji started immediately after magma discharge and intrusion events in the Miyake-jima and Kozu-shima regions in July and August 2000. The tectonic and volcanic activity changes around the area suggest that the DLF earthquake swarm at Mount Fuji was triggered by the change of state of the deep magmatic system around Mount Fuji.

Keywords

Deep low-frequency earthquakes Mount Fuji Seismicity Earthquake swarm 

Notes

Acknowledgements

I would like to thank my colleagues in NIED. Eisuke Fujita helped with data processing of the Mount Fuji observations, Hideki Ueda provided programs to calculate crustal stress changes and Masae Kikuchi provided programs to plot hypocenters. Haruhisa Nakamichi and Sadato Ueki kindly permitted me to use their program for plotting a diamond diagram. Masakazu Ohtake, Yoshimitu Okada, Fumio Yamamizu, Shin-ichi Noguchi and Kazushige Obara helped to construct the Mount Fuji observation network during the initial stage in 1980s. I also wish to thank John Stix, Randall A. White and Minoru Takeo for their helpful comments and suggestions

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Copyright information

© Springer-Verlag 2005

Authors and Affiliations

  1. 1.National Research Institute for Earth Science and Disaster PreventionTsukuba-shiJapan

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