HR: 1340h
AN: S13D-1095    [Abstracts]
TI: Moho Depth Variation Beneath Southwest Japan Revealed From Inverted Velocity Structure Based on Receiver Functions
AU: * Shiomi, K
EM: shiomi@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1 Tennodai, Tsukuba-Shi, 305-0006 Japan
AU: Obara, K
EM: obara@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1 Tennodai, Tsukuba-Shi, 305-0006 Japan
AU: Sato, H
EM: sato@zisin.geophys.tohoku.ac.jp
AF: Department of Geophysics, Tohoku University, 6-3 Aramaki Aoba, Aoba-Ku, Sendai-Shi, 980-8578 Japan
AB: We determine the depth variation of the Moho discontinuity beneath Chugoku-Shikoku region, southwest Japan. We apply the receiver function analysis to teleseismic waveforms from more than 250 earthquakes with magnitude 5.5 or larger recorded by the High Sensitivity Seismograph Network (Hi-net). Integrating estimated receiver functions into six groups according to the back azimuth of each station, we estimate the seismic velocity structure for every group of the receiver functions by using the improved linearized time-domain waveform inversion method. This improved method adopts a weighting function to determine the shallow structure well and estimate both S and P wave velocity, simultaneously. We detect a clear velocity discontinuity corresponding to the Moho across which the S wave velocity changes to 4.5 km/s from 3.7 km/s. The depth of the discontinuity is about 30 km beneath northern (the Japan Sea) and southern (the Pacific) coastlines and more than 40 km beneath central part of the study region. In the central part, a low velocity layer (LVL) with 10 km thickness exists under the Moho. The depth of the upper boundary of the LVL is 45 to 50 km. The Philippine Sea plate (PHS) is subducting toward the northwest from the Nankai Trough beneath the Chugoku-Shikoku region where both the continental and the oceanic Moho exist. The LVL corresponds to the subducting oceanic crust of the PHS and the oceanic Moho is the bottom of the oceanic crust. The continental Moho of the Eurasian plate lies above the low velocity oceanic crust. However, at stations in the northern and southern part of the study region, we find only one major velocity discontinuity. We read the depth of these clear discontinuities from the inverted velocity models and map the Moho depth at the conversion point. By interpolating the results, we separately draw the depth contour of the continental and the oceanic Moho beneath Chugoku-Shikoku region under the assumptions: (1) the Moho of the Pacific side and the Japan sea side are the oceanic and the continental Moho, respectively; and (2) each type of the Moho is continuous. We found that the oceanic Moho continues down to northwest and shows a complicated configuration with ridges and valleys. The corresponding depth is about 25 km beneath the Pacific seashore and 45 km beneath the central part of the study region. The oceanic Moho extends to the 34.5$^\circ$N at least. The depth of the continental Moho is relatively shallower at northern coastlines and shows a major depression with more than 40 km in depth at around 34.7$^\circ$N and 133.5$^\circ$E. In the south of 34$^\circ$N, we found only one discontinuity indicating the oceanic crust of the PHS is attached to the continental lower crust. The geometry of these Moho discontinuities has a high correlation with the seismicity in the crust and the Bouguer gravity anomaly.
DE: 9320 Asia
DE: 7220 Oceanic crust
DE: 7230 Seismicity and seismotectonics
DE: 7205 Continental crust (1242)
DE: 7218 Lithosphere and upper mantle
SC: Seismology [S]
MN: 2004 AGU Fall Meeting