HR: 1340h
AN: S33B-1098    [Abstracts]
TI: Feasibility of Active Monitoring for Plate Coupling Using ACROSS
AU: * Yamaoka, K
EM: yamaoka@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo, Tokyo, 113-0032 Japan
AU: Watanabe, T
EM: watanabe@seis.nagoya-u.ac.jp
AF: Graduate School of Environmental Studies, Nagoya University, 1 Furo-cho, Chikusa, Nagoya, 464-8601 Japan
AU: Ikuta, R
EM: ryoya@seis.nagoya-u.ac.jp
AF: Graduate School of Environmental Studies, Nagoya University, 1 Furo-cho, Chikusa, Nagoya, 464-8601 Japan
AB: Detectability of temporal changes in reflected wave from the boundary of subducting plates in Tokai district with active sources are studied. Based on rock experiments the change in the intensity of reflection wave can be caused by change in coupling between subducting and overriding plates. ACROSS (Accurately-Controlled Rountine-Operated Signal System) consists of sinusoidal vibration sources and receivers is proved to provide a data of excellent signal resolution. The following technical issues should be overcome to monitor the returned signal from boundaries of subducting plates. (1) Long term operation of the source. (2) Detection of temporal change. (3) Accurate estimation of source functions and their temporal change. First two issues have already overcome. We have already succeeded a long-term operation experiment with the ACROSS system in Awaji, Japan. The operation was carried out for 15 months with only minor troubles. Continuous signal during the experiment are successfully obtained. In the experiment we developed a technique to monitor the temporal change of travel time with a resolution of several tens of microseconds. The third issue is one of the most difficult problem for practical monitoring using artificial sources. In the 15-month experiment we correct the source function using the record of seismometers that were deployed around the source We also estimate the efficiency of the reflected wave detection using ACROSS system. We use a data of seismic exploration experiment by blasts that carried out above subducting plate in Tokai district. Clear reflection from the surface of the Philippine Sea plate is observed in the waveform. Assuming that the ACROSS source is installed at the same place of the blast source, the detectability of temporal variation of reflection wave can be estimated. As we have measured the variation of signal amplitude that depends on the distance from an ACROSS source, ground noise at seismic stations (receivers) provide us the signal-to-noise ratio for the signal from ACROSS. The resolution can be estimated only by the signal-to-noise ratio. We surveyed the noise level at the place where reflection from the boundary of subducting Philippine Sea Plate can be detected. The results show that the resolution will be better than 1% in amplitude and 0.1milisecond in travel time for the stacking of one week using three-unit source and ten-elements receiver arrays.
DE: 7294 Instruments and techniques
DE: 7200 SEISMOLOGY
DE: 7205 Continental crust (1242)
SC: Seismology [S]
MN: 2004 AGU Fall Meeting