Earth Planets Space, Vol. 56 (No. 11), pp. e37-e40, 2004E-LETTER
Haruhisa Nakamichi1,2, Motoo Ukawa1, Shinichi Sakai2
1National Research Institute for Earth Science and Disaster Prevention, 3-1 Tennodai, Tsukuba 305-0006, Japan
2Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan
(Received August 23, 2004; Revised October 11, 2004; Accepted October 28, 2004)
Midcrustal low-frequency earthquakes (MLFs) have been observed at seismic stations around Mt. Fuji, Japan. In September-December 2000 and April-May 2001, abnormally high numbers of MLFs occurred. We located hypocenters for the 80 MLFs during 1998-2003 by using the hypoDD earthquake location program (Waldhauser and Ellsworth, 2000). The MLF hypocenters define an ellipsoidal volume some 5 km in diameter ranging from 11 to 16 km in focal depth. This volume is centered 3 km northeast of the summit and its long axis is directed NW-SE. The direction of the axis coincides with the major axis of tectonic compression around Mt. Fuji. The center of the MLF epicenters gradually migrated upward and 2-3 km from southeast to northwest during 1998-2001. We interpret that the hypocentral migration of MLFs reflects magma movement associated with a NW-SE oriented dike beneath Mt. Fuji.
Key words: Low-frequency earthquake, hypocenter, focal mechanism, Fuji.