3D velocity structure of Mt. Fuji and the south Fossa Magna, Japan

HR: 1340h
AN: T43A-1361 [Abstracts]
TI: 3D velocity structure of Mt. Fuji and the south Fossa Magna, Japan
AU: * Nakamichi, H
EM: hnakamichi@usgs.gov
AF: Volcano Hazards Team, U. S. Geological Survey, 345 Middlefield Rd. MS910, Menlo Park, CA 94025 United States
AB: We present models of the velocity structure beneath the south Fossa Magna and Mt. Fuji, Japan, using local earthquake tomography and new data from a dense seismic network. The seismic network includes 28 temporary and 138 permanent 3-component seismic stations. The stations are deployed 2 - 10 km apart with a gradual increase in station spacing away from Mt. Fuji and are also densely distributed in the direction NE-SW across Mt. Fuji. We inverted 63,287 P- and 59,558 S-wave arrival times from 1087 local earthquakes to obtain the 3D P- (V_p) and S-wave velocity (V_s) and their ratio (V_p/V_s) structure. There is a high correlation between the V_p and V_s structures. At 5-15 km depths both velocity structures show low velocity anomalies trending northward in the west along the Fujikawa river, from Suruga Bay to northwest of Mt. Fuji. North of the Izu Peninsula, there is a broad high-velocity zone. These results correlate with Bouguer gravity anomalies previously observed. A shallowing high velocity anomaly is seen above sea level beneath Mt. Fuji. A low-V_p, V_s and V_p/V_s (1.5-1.6) anomaly is seen at depths of 7 - 17 km beneath Mt. Fuji, corresponding to locations of low-frequency (LF) earthquakes. Volatile fluids (CO₂-H₂O) could be abundant in the low V_p/V_s region and play an important role in generating the LF earthquakes. A low-V_p, V_s and high-V_p/V_s (1.8-1.9) anomaly is seen at depths of 15 - 25 km beneath Mt. Fuji and is interpreted as partial melt (basaltic magma). P-wave velocities of 6.0 - 6.5 km/s in the Izu collision zone beneath the Tanzawa Mountains indicate a plutonic body of tonalite within an accreted crustal slice of the Philippine Sea plate.
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7270 Tomography (6982, 8180)
DE: 7280 Volcano seismology (8419)
DE: 8178 Tectonics and magmatism
DE: 8499 General or miscellaneous
SC: Tectonophysics [T]
MN: Fall Meeting 2005