HR: 1330h
AN: V52B-0428    [PDF]
TI: Precise Hypocenter Location of Low- and High-Frequency Earthquakes Beneath Mt. Fuji, Japan
AU: * Nakamichi, H
EM: nakamit@eri.u-tokyo.ac.jp
AF: National Research Institute for Earth Science and Disaster Prevention, Tennodai 3-1, Tsukuba, 305-0006 Japan
AU: * Nakamichi, H
EM: nakamit@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Ukawa, M
AF: National Research Institute for Earth Science and Disaster Prevention, Tennodai 3-1, Tsukuba, 305-0006 Japan
AU: Sakai, S
AF: Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Takagi, A
AF: Metrological Research Institute, Nagamine 1-1, Tsukuba, 305-0052 Japan
AB: Low- and high-frequency earthquakes (LFs and HFs) have been observed at the seismic networks of ERI, NIED and JMA around Mt. Fuji. Many LFs occurred in two periods September-November 2000 and April-May 2001. In this period, the LF occurred at a rate of 20 - 70 per month, which is about ten times higher than those of background level. Generally, the LFs have little energy above 5 Hz, emergent P arrivals and strong S arrivals unlike those of shallow LFs. P and S waves of the LFs are often contaminated by high-frequency ($>$5 Hz) waves at close stations. They frequently occur as bursts of overlapping events. The duration of LF bursts is usually few minutes but occasionally 20-30 minutes. HFs intensively occurred in the period form January to April 2001. We combined data of the three organizations, picked up arrival times of LFs and HFs with careful identification of phases and determined their hypocenter precisely. We located hypocenters for the 75 LFs and 58 HFs with good S/N during 1998-2002 by using the DD method. The magnitudes of the located LFs and HFs were up to 2.2 and 2.7, respectively. The LF hypocenters define an ellipsoidal volume some 5 km in diameter ranging from 11 to 16 km in focal depth beneath the northeast flank. Consulting the location errors, the extent of the volume is significant. This volume is centered at 3 km northeast of the summit and its long axis directs SE-NW. The HF hypocenters distribute between 6 and 12 km in depth along a sub-vertical plane beneath the south flank, which also directs SE-NW. The SE-NW direction coincides with the major axis direction of tectonic compression around Mt. Fuji. The locations of LFs in 2000-2001 shift 1 km northwest from those in 1998-1999 and 2002. While the focal depths of the LFs in 1998-1999 are at depths of 13-16 km, the LFs located apparently shallow with time at depths of 11-16 km in September 2000 - December 2001. Although no definite source mechanism of the LFs have been revealed, the spatial extent and migration of the LF and HF hypocenters give a key to understanding the relation between the generation of LFs and magmatic fluid.
DE: 7215 Earthquake parameters
DE: 7230 Seismicity and seismotectonics
DE: 7280 Volcano seismology (8419)
DE: 8434 Magma migration
DE: 9320 Asia
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting