HR: 1340h
AN: S23A-1111    [Abstracts]
TI: Evaluation of P-S conversion in the oceanic crust and Vp/Vs estimation
AU: * Tsuruga, K
EM: tsuruga.k@tairikudana.com
AF: Japan Continental Shelf Survey Co. Ltd., NTC bldg. 3F, Kyobashi 1-11-2, Chuo-ku, Tokyo, 104-0031, Japan
AU: Kasahara, J
EM: kasahara.j@tairikudana.com
AF: Japan Continental Shelf Survey Co. Ltd., NTC bldg. 3F, Kyobashi 1-11-2, Chuo-ku, Tokyo, 104-0031, Japan
AU: Nishiyama, E
EM: nishiyamae@kge.co.jp
AF: Kawasaki Geological Engineering Co. Ltd., Mita-Kawasaki bldg. 6F, Mita 2-11-15, Minato- ku, Tokyo, 108-8337, Japan
AU: Murase, K
EM: murasek@kge.co.jp
AF: Kawasaki Geological Engineering Co. Ltd., Mita-Kawasaki bldg. 6F, Mita 2-11-15, Minato- ku, Tokyo, 108-8337, Japan
AU: Kubota, R
EM: kubotar@kge.co.jp
AF: Kawasaki Geological Engineering Co. Ltd., Mita-Kawasaki bldg. 6F, Mita 2-11-15, Minato- ku, Tokyo, 108-8337, Japan
AU: Tanaka, T
EM: t_tanaka@jgi.co.jp
AF: JGI, Inc., Meikei Building, Otsuka 1-5-21, Bunkyo-Ku, Tokyo, 112-0012, Japan
AU: Nishizawa, A
EM: azusa@jodc.go.jp
AF: Japan Coast Guard, Tsukiji 5-3-1, Chuo-ku, Tokyo, 104-0045, Japan
AB: Recently, in order to determine crustal structure in the oceanic region, a huge amount of seismic data has been collected using a set of digital OBSs and a large volume tuned airgun-array. In the OBS-airgun records, P to S and/or S to P converted phases have been frequently observed by horizontal and vertical seismometers/hydrophone, respectively. Using such converted phases, we can estimate S wave structure in the crust and the mantle. In order to evaluate S wave velocities, the precise estimation of P and S velocity structures in the sediments is important. In this paper, we evaluated the P to S and S to P conversions to obtain S wave structure, and applied to the S wave structures in the crust and the mantle around Japan and the adjacent region. In the oceanic region, S(V) waves observed in horizontal components are converted from P to S(V) at interfaces with large impedance contrast. We evaluated a total energy flux of P-S conversion waves on 1) efficiencies of transmission and conversion from P to S and/or S to V through the ocean bottom/sediments/hard-rock interfaces and 2) relative converted P or S wave square amplitudes at OBS relative to the incident P waves penetrated into the ocean bottom. The conversion from P to S occurs at (a) sediments/hard-rock interface and/or (b) seawater/bare rock interface. The case (a) corresponds to the presence of thin unconsolidated sediment layer (P-wave velocity, Vp < 2.2km/s, S-wave velocity, Vs< 1.0 km/s) more than tens meters underlined by sedimentary rocks or hard rock layer (Vp>2.5km/s, Vp/Vs ratio ~ 1.78). The case (b) corresponds to bare rock layer exposed at the ocean bottom. We assume that the conversion occurs only at the ocean bottom and sediment layer /hard-rock interface and calculate the conversion rates in terms of relative energies. Among possible seven phases, large conversions are expected for (i) sediments/hard-rock interface at the incident side, (ii) at the ocean bottom of incident side, and (iii) at the sediments/hard-rock interface just beneath an OBS. Both of (i) and (ii) travel through whole crust as S wave. In the case of (iii), P wave converted to S wave only at the sediments/hard-rock interface just beneath an OBS. We examined real OBS-arigun data and found that most of conversions are observed on horizontal seismographs. Comparing P-wave velocity crustal structure, the observed records fit to the cases 1) and 3) for most of OBS records. Vp/Vs are 3 ~ 20 and Vs values are consistent with those measured by Hamilton (1976, 1979). In the region around Japan and the western Pacific, we obtained fairly constant Vp/Vs for the crust, namely 1.78. On the other hand, the Vp/Vs varied from 1.60 to 1.80 in the upper most mantle. The reason for this wide variation of Vp/Vs in the mantle is partly due to serpentinization, and partly due to S-wave anisotropy.
DE: 3025 Marine seismics (0935, 7294)
DE: 3060 Subduction zone processes (1031, 3613, 8170, 8413)
DE: 7208 Mantle (1212, 1213, 8124)
DE: 7220 Oceanic crust
DE: 7240 Subduction zones (1207, 1219, 1240)
SC: Seismology [S]
MN: 2007 Fall Meeting