HR: 1330h
AN: T32A-0906    [PDF]
TI: Long Term Seismic Observation in Mariana by OBSs : Activity of Deep Earthquakes
AU: * Shiobara, H
EM: shio@eri.u-tokyo.ac.jp
AF: ERI, Univ. Tokyo, Yayoi 1-1-1, Bunkyo, Tokyo, 113-0032 Japan
AU: Mochizuki, K
EM: kimi@eri.u-tokyo.ac.jp
AF: ERI, Univ. Tokyo, Yayoi 1-1-1, Bunkyo, Tokyo, 113-0032 Japan
AU: Ohki, S
EM: toko@eri.u-tokyo.ac.jp
AF: ERI, Univ. Tokyo, Yayoi 1-1-1, Bunkyo, Tokyo, 113-0032 Japan
AU: Kanazawa, T
EM: kanazawa@eri.u-tokyo.ac.jp
AF: ERI, Univ. Tokyo, Yayoi 1-1-1, Bunkyo, Tokyo, 113-0032 Japan
AU: Fukao, Y
EM: fukao@eri.u-tokyo.ac.jp
AF: ERI, Univ. Tokyo, Yayoi 1-1-1, Bunkyo, Tokyo, 113-0032 Japan
AU: Sugioka, H
EM: hikari@jamstec.go.jp
AF: IFREE, Jamstec, Natsushima 2-15, Yokosuka, 237-0061 Japan
AU: Suyehiro, K
EM: suyehiro@jamstec.go.jp
AF: DSRD, Jamstec, Natsushima 2-15, Yokosuka, 237-0061 Japan
AB: In order to obtain the deep arc structural image of Mariana, a large-scale seismic observation by using 58 long-term ocean bottom seismometers (LTOBS) has been started since June 2003 for about one year. It is a part of the MARGINS program (US-JAPAN COLLABORATIVE RESEARCH: MULTI-SCALE SEISMIC IMAGING OF THE MARIANA SUBDUCTION FACTORY), and the aim of this observation is the crustal and mantle structure modeling by using passive and active seismic sources. The 50 and 8 LTOBSs are owned by LDEO and ERI, respectively, and they were deployed during the cruise of R/V Kaiyo (Jamstec), KY03-06. Prior to this experiment, we made a pilot long-term seismic array observation in the same area by using 10 LTOBSs, deployed in Oct. 2001 by R/V Yokosuka (Jamstec) and recovered in Feb. 2003 by R/V Kaiyo. This LTOBS has been developed by ERI, which has the PMD sensor (WB2023LP) and a titanium sphere housing (D=50cm) and was already used in several long-term observations (ex. trans-PHS array observation presented at the AGU fall meeting, 2000, S51B-02). Two of 10 LTOBSs could not be recovered due to malfunction of the releasing system, and one recovered had a trouble in the sensor control unit. But, seven others have obtained more than 11 months long data continuously. As passive source studies of these observations use characteristic deep earthquakes in this area, the activity of them will be introduced in this presentation, from the data obtained just above them. At the first step, difference of hypocenters of known events, listed on the PDE catalog, is examined. There are 59 events of epicenters within a circular area centered at $19\deg$N, $145\deg$E with radius of 1000km from the catalog during the observation. P and S arrivals are picked by using the WIN system, and the iasp91 model (only {$V_P$} with {${V_P}/{V_S}=1.732$}) is used for the hypocenter determination. Station corrections are applied only for the sediment layer, estimated from several arrival time data of P and P-S converted phase at the sediment-basement interface below the OBS. The major result is a systematic shift of epicenters with about 200km depth to SE direction below the island chain, and those relocated hypocenters show concentration.
DE: 7218 Lithosphere and upper mantle
DE: 7230 Seismicity and seismotectonics
SC: Tectonophysics [T]
MN: 2003 Fall Meeting