HR: 0800h
AN: NG41A-0147    [Abstracts]
TI: Time-Evolution of Later Phases in the Transfer Functions Obtained by a Permanent Seismic Source and Receivers (ACROSS)
AU: * Watanabe, T
EM: watanabe@seis.nagoya-u.ac.jp
AF: RSVD, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 4648601, Japan
AU: Furukawa, T
EM: furukawa@seis.nagoya-u.ac.jp
AF: RSVD, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 4648601, Japan
AU: Hasada, Y
EM: hasada@seis.nagoya-u.ac.jp
AF: RSVD, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 4648601, Japan
AU: Ikuta, R
EM: ryoya@eri.u-tokyo.ac.jp
AF: ERI, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 1130032, Japan
AU: Yamaoka, K
EM: kyamaoka@seis.nagoya-u.ac.jp
AF: RSVD, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 4648601, Japan
AB: Repeated transmission of a controlled seismic signal and a long-term continuous observation of the seismic waves enable precise monitoring of time-evolving physical properties of the Earth's interior. An observation using the ACROSS (Accurately-Controlled, Routinely-Operated Signal System) source was conducted to establish a method to monitor the state of coupling of the plate boundary and to understand the time-evolving phenomena such as slow-slips and deep low-frequency tremors (DLFTs). We carried out a long-distance seismic monitoring experiment using ACROSS for 10 months in the Tokai distinct, Japan. Our attempt was to detect reflected phases from the top of the subducting Philippine Sea plate and to detect their time evolution. The results have been presented in the past AGU meetings (Yamaoka et al., 2004, Soma et al., 2005, Watanabe et al., 2005, Ikuta et al., 2006.) A seismic array composed of 12 seismometers was deployed at 55 km away from the source. Stacking and deconvolution of continuous data retrieved transfer functions, which correspond to time-domain waveform, with improved S/N ratio. We applied an array analysis to identify the later phases that are coherent among the array elements. By comparing the synthetic waveform and the coherent phases extracted from the semblance analysis, we concluded that the later phases included the reflected waves from the top of Philippine Sea plate, Moho discontinuity and other layer boundaries. Using borehole seismometer (Hi-net) data that were free from environmental effects, the time-evolutional changes of each phase found in the transfer function were examined. The first arrival showed small variation, on the other hand, the later phases show larger variation. The time evolving changes indicate the existence of time- variant scatterers in deep crust or the effect of multiple scattering. An interesting feature is that the time-variation of the later phases shows a correlation with the episodic activity of the DLFTs in this area. This implies the possibility of seismic monitoring of seismogenic plate-subduction zone using ACROSS.
DE: 0935 Seismic methods (3025, 7294)
DE: 0994 Instruments and techniques
DE: 3270 Time series analysis (1872, 4277, 4475)
DE: 3285 Wave propagation (0689, 2487, 4275, 4455, 6934)
DE: 7240 Subduction zones (1207, 1219, 1240)
SC: Nonlinear Geophysics [NG]
MN: 2007 Fall Meeting