HR: 0800h
AN: S31A-0198    [Abstracts]
TI: Fault Zone Behavior Observed from Crossing Fault 7-level TCDP Boreholes Seismometers
AU: * Ma, K
EM: fong@earth.ncu.edu.tw
AF: Institute of Geophysics, National Central University, Chung_li, 32054, Taiwan
AU: Tanaka, H
EM: tanaka@eps.s.u-tokyo.ac.jp
AF: Department of Earth and Planetary Science, University of Tokyo, Tokyo, 113, Japan
AU: Lin, Y
EM: nycticorax@eqkc.earth.ncu.edu.tw
AF: Institute of Geophysics, National Central University, Chung_li, 32054, Taiwan
AB: We investigated the waveforms recorded by 7-level borehole seismometers (Taiwan Chelunpu-Fault Drilling Project, TCDP 7-level BHS), which were placed from the depth of about 950m to 1300m crossing the fault zone associated with the 1999 Chi-Chi earthquake. The seismometers were installed over the depth range of hanging wall and footwall with the depth interval of about 50-60m, and a seismometer at the depth of 1111km as identified to be the fresh slip zone for the 1999 Chi-Chi earthquake. The TCDP 7-level BHS recorded nice waveforms for micro events with magnitude down to -0.5. The detail velocity structure in the boreholes was constructed from logging data. Even though the 7-level seismometers are only 50 m apart in depth, the seismometers located above the low velocity zone (LVZ), which might be related to a shear zone, show distinct different in waveform compared to other seismometers. The shear zone is about 20m in thickness, which contribute to a Stoneley waves like wavelet in horizontal component. These waves might be related to the fault zone permeability or viscosity. In addition to the observations for regular events, a cross fault experiment of fluid injection test (FIT) was carried out after the drilling and the completed installation of the seismometers to understand the in-situ hydraulic behavior of the fault zone. A high pressure fluid (~4MPa) was injected in hole-B with chemical and gas observations and monitoring in hole-A. We observed distinct feature in the seismograms, which only showing a triangle source type P-wave without S-wave, recorded by TCDP BHS during FIT. Whether this observation was in an association of new open cracks after high pressure Fluid Injection Test (FIT) is carrying on. With the high resolution TCDP 7-level BHS and FIT in an active fault zone, we try to understand dynamics of the fault zone and the estimation on in-situ permeability from FIT related triggered events. The observations would also give hint to fault zone healing process through time.
DE: 7200 SEISMOLOGY
DE: 7209 Earthquake dynamics (1242)
SC: Seismology [S]
MN: 2007 Fall Meeting