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Report No.


Ijima, Takashi; Irie, Tsutomu

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 ans/or electromagnetic sources. We preliminary obtained Pg, Sg, and possible plate boundary reflected PxP phase using one-moth long stacking data up to nearly 70 km distance. 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). The same seismic exploration line crossed the area where many active faults 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. To examine the seismic exploration records obtained by Iidaka et al. (2003), we noticed the presence of strong phases, which can be explained as PxP reflection from the plane at 20 km deep around the 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. One of high possibilities for the 20km reflector is the detachment in the lower crust. The high velocity contrast can explain such PxP reflection phase. We evaluated the possibility to observe such phase by the ACROSS transmission from the present source located in Toki city and may conclude 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, electrometric ACROSS method may identify the strong-reflection source and its temporal change using resistively analysis around the Tokai Region, Japan. This approach can give high possibility to watch the change of physical state beneath the active faults and it may contribute for the earthquake forecasting.



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