HR: 1340h
AN: NG33B-0177    [Abstracts]
TI: Detection of Reflected Waves from Plate Boundary Using ACROSS Source and Seismic Array
AU: * Soma, T
EM: soma@eps.nagoya-u.ac.jp
AF: Nagoya University, Furocho, Chikusa, Nagoya, 468-8602 Japan
AU: Watanabe, T
EM: watanabe@seis.nagoya-u.ac.jp
AF: Nagoya University, Furocho, Chikusa, Nagoya, 468-8602 Japan
AU: Ikuta, R
EM: ryoya@seis.nagoya-u.ac.jp
AF: Nagoya University, Furocho, Chikusa, Nagoya, 468-8602 Japan
AU: Saiga, A
EM: saiga@eps.nagoya-u.ac.jp
AF: Nagoya University, Furocho, Chikusa, Nagoya, 468-8602 Japan
AU: Miyajima, R
EM: miyajima@seis.nagoya-u.ac.jp
AF: Nagoya University, Furocho, Chikusa, Nagoya, 468-8602 Japan
AU: Yamaoka, K
EM: yamaoka@eri.u-tokyo.ac.jp
AF: University of Tokyo, Yayoi 1-1-1, Bunkyo, Tokyo, 113-0032 Japan
AU: Tsuruga, K
EM: tsuru@tono.jnc.go.jp
AF: Japan Nuclear Cycle Development Institte, Yamanouchi 1-63, Akeyo, Mizunami, 509-5132 Japan
AU: Kunitomo, T
EM: kunitomo@tono.jnc.go.jp
AF: Japan Nuclear Cycle Development Institte, Yamanouchi 1-63, Akeyo, Mizunami, 509-5132 Japan
AU: Hasada, Y
EM: hasada@tono.jnc.go.jp
AF: Japan Nuclear Cycle Development Institte, Yamanouchi 1-63, Akeyo, Mizunami, 509-5132 Japan
AU: Kasahara, J
EM: kasahara.junzo@jnc.go.jp
AF: Japan Nuclear Cycle Development Institte, Yamanouchi 1-63, Akeyo, Mizunami, 509-5132 Japan
AU: Satomura, M
EM: semsato@ipc.shizuoka.ac.jp
AF: Shizuoka University, Ooya 836, Suruga, Shizuoka, 422-8629 Japan
AU: Kumazawa, M
EM: mkz@tono.jnc.go.jp
AF: Japan Nuclear Cycle Development Institte, Yamanouchi 1-63, Akeyo, Mizunami, 509-5132 Japan
AU: Fujii, N
EM: fujii@seis.nagoya-u.ac.jp
AF: Nagoya University, Furocho, Chikusa, Nagoya, 468-8602 Japan
AB: ACROSS (Accurately Controlled and Routinely Operated Signal System) is effective in monitoring temporary changes of Earth's interior. A long-term operation experiment near Nojima fault [Ikuta et al.,2004] detected small temporary changes of travel time of P and S waves at tele-seismic events. Toward Tokai monitoring plan to detect the reflected phases from the top of Philippine Sea Plate and monitor its temporal changes, a mid-term continuous experiment was conducted using ACROSS source and a seismic array. The experiment was operated for the period from Dec. 2004 to Sep.2005 in the Tokai area, Pacific side of the central part of Japan. In this region, the expected Tokai earthquake is a serious concern. In addition, slow slip events and low-frequency tremors are observed in this area. A strong reflected phase from the plate boundary was found by the seismic observation using artificial sources [Iidaka et al.,2003]. The purpose of the experiment is to establish a method to detect and monitor the reflection from the plate boundary using ACROSS. The ACROSS source is located in Toki city and operated by Tono Geoscience Center. The ACROSS source continuously transmits precisely-controlled frequency-modulated signals whose frequency band ranges from 10 to 20 Hz with an interval of 50 seconds. We deployed a short-span seismic array at the distance of 55 km from the ACROSS source. The cross-shaped seismic array spanning 2 km consists of 12 seismometers equipped with an offline data logger, amplifier and solarpanel. We stacked the received signal for a month with an interval of 200 seconds in order to improve signal noise ratio. We extracted a series of line spectrum of ACROSS signal. Transfer function can be obtained by dividing spectrum by the source. Applying inverse Fourier transform, we can obtain the transfer function in time-domain. We identified direct P and S phases by comparing with the standard travel time table by JMA. We also found some coherent later phases. By comparing the travel-time of these phases with the travel-time calculated by ray-tracing with a given structure model, these phases may include the reflection waves from the top of Philippine Sea plate, the Mohorovicic discontinuity and some other physical discontinuities. We calculated the semblance of the later phases to estimate the slowness and the incident angle. A later phase with a low slowness can be found after the direct P phase. We also try to evaluate the temporal change of travel time and waveform. The detail results will be presented in the conference.
DE: 7219 Seismic monitoring and test-ban treaty verification
DE: 7299 General or miscellaneous
SC: Nonlinear Geophysics [NG]
MN: Fall Meeting 2005