HR: 10:20h
AN: NG32A-01 INVITED     [Abstracts]
TI: A proposal of active monitoring of deep crustal reflection beneath active faults and volcanic region in central Japan using ACROSS
AU: * Kasahara, J
EM: kasahara.junzo@jnc.go.jp
AF: Tono Geoscience Center, JNC, Yamanouchi 1-63, Akeyocho, Mizunami, 509-6132 Japan
AU: Tsuruga, K
EM: tsuru@tono.jnc.go.jp
AF: Tono Geoscience Center, JNC, Yamanouchi 1-63, Akeyocho, Mizunami, 509-6132 Japan
AU: Fujii, N
EM: fujii@seis.nagoya-u.ac.jp
AF: Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8602 Japan
AU: Yamaoka, K
EM: yamaoka@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1 Bunkyo, Tokyo, 113-0032 Japan
AB: ACROSS (Accurately-Controlled Routinely-Operated Signal System) is a method to investigate any change of physical state in the Earth's interior using accurately controlled seismic and/or electromagnetic source. The seismic ACROSS system can continuously transmit sweep sinusoidal waves, e.g., 10 Hz to 20 Hz. Although the signal level is not so high, the stacking of data in time domain or frequency domain can largely enhance S/N. During past two years, we have continuously transmitted seismic signals from Toki city, Japan and we confirmed refracted and/or reflected P & S arrivals up to nearly 75 km distance using one-month long stacking data. We also made an experimental transmission of electromagnetic waves around 1 Hz for a half year and identified the signals up to 30 km distance when we used similar one-month long stacking data.
The seismic reflection experiment across the central Japan revealed the strong PxP reflection phase from 30-40 km deep of the Philippine Sea Plate boundary around the Tokai Region, Japan (Iidaka et al., 2003). From the November last year, we started an ACROSS experiment to detect seismic reflection phase from the surface of the Philippine Sea Plate (Kasahara et al., 2005, Tsuruga et al., 2005). We preliminary obtained Pg, Sg, and possible plate boundary reflected PxP phase using one-moth long stacking data.
The same seismic exploration line crossed the area where many active faults and volcanoes distribute. One of the big issues for revealing the loading process for active faults is to examine the process in the lower crust around active faults. We examined the seismic exploration records obtained by Iidaka et al.(2003), and noticed the presence of strong reflection phases. They can be explained as PP reflection from the plane at 20km deep around Atera fault, which is one of the most active faults in Japan. The phases are identified by a number of explosions on the survey line from north to south of this region. The high velocity contrast can explain such PxP reflection phase. One of possibilities for the 20km reflector is the detachment in the lower crust. Another possibility for the reflector in 20 km depth is magma sources beneath On-take and Yake-dake Volcanoes in the central Japan volcanic range. We evaluated the possibility of observing such phase by the ACROSS transmission from the present source located in Toki city and may conclude us to be able to observe such phase and its temporal change, if the seismometers are placed 50-100km distance north of this region. We also generated synthetic seismograms to confirm the detection. In addition to seismic ACROSS, electromagnetic ACROSS method may identify the deep crustal reflector and its temporal change using resistivity analysis around the Tokai Region, Japan. This approach can give strong possibility to watch the change of physical state beneath the active faults and it may contribute for the earthquake forecasting.
Acknowledgements: We appreciate the permission to use the seismic waveform data provided by the Research Group for Seismic Expedition in Central Japan.
DE: 7200 SEISMOLOGY
DE: 7218 Lithosphere (1236)
DE: 7240 Subduction zones (1207, 1219, 1240)
SC: Nonlinear Geophysics [NG]
MN: Fall Meeting 2005