Precise hypocenter locations of midcrustal low-frequency earthquakes beneath Mt. Fuji, Japan

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.