HR: 0800h
AN: T51A-1317    [Abstracts]
TI: Stress Magnitudes and Orientations from Geophysical Logs and Leak-Off Tests in the Taiwan Chelungpu-Fault Drilling Project
AU: * Wu, H
EM: sonata@eqkc.earth.ncu.edu.tw
AF: Institute of Geophysics, National Central University, No. 300, Jhongda Rd , Jhongli, YW 320 Taiwan
AU: Ma, K
EM: fong@eqkc.earth.ncu.edu.tw
AF: Institute of Geophysics, National Central University, No. 300, Jhongda Rd , Jhongli, YW 320 Taiwan
AU: Ito, H
EM: hisaoito@jamstec.go.jp
AF: Center for Deep Earth Exploration,Japan Agency for Marine-Earth Science and Technology(JAMSTEC), Showacho 3173-25 Yokohama laboratory, Yokohama, TY 236-0001 Japan
AU: Hung, J
EM: jhung@earth.ncu.edu.tw
AF: Institute of Geophysics, National Central University, No. 300, Jhongda Rd , Jhongli, YW 320 Taiwan
AB: In Dakeng, Taiwan., the Taiwan Chelungpu-fault Drilling Project (TCDP) recently drilled a 2-km-deep hole to study the structure and mechanics of the Chelungpu Fault along the segment of the fault that ruptured in the 1999 M 7.5 Chi-Chi earthquake. Electrical image logs and other borehole geophysical logs together with 4 hydraulic fracturing stress measurements were carried out in this hole over the depth range of 500 m to 1850 m. Stress-induced borehole breakouts were observed throughout the image logs and indicate considerable local variability in stress directions at a variety of scales. At the scale of several hundred meters, dramatic changes in maximum horizontal principal stress (S_HMax) directions ranging from East-West to North-South are seen at depths of 950-1000 m, 1130 m, 1300 m, and 1700 m. Shorter-wavelength rotations in S_HMax are also seen in proximity to faults seen in the image logs. We surmise that both these types of stress variations are related to slip on secondary active faults in the hanging wall and foot wall of the Chelungpu Fault. Also, fast polarization directions of shear waves from dipole sonic logs show good agreement with some - but not all - of these S_HMax azimuths, suggesting that seismic anisotropy in these bedded shales is, in part, stress controlled. Furthermore, the zones over which S_HMax changes most rapidly are characterized by anomalously low seismic velocity and resistivity, which may provide further evidence of a causal relationship between these stress rotations and the presence of nearby active faults. Leak-off tests conducted at depths of 1000-1250 m were used to calculate the magnitudes of the minimum horizontal stress (S_hmin) as a function of depth. Near a depth of 1111 m, which corresponds to the location of a major shear zone seen in the core recovered from this well, the value of S_hmin is anomalously high and approximately equal to the calculated overburden stress. Major changes in S_HMax azimuth and fast polarization direction are also seen near this depth, suggesting that this shear zone may have a significant impact on both stress magnitudes and orientations. We are now analyzing the distribution of wellbore failure, in conjunction with estimates of the in-situ rock strength from well-logs, to try and ascertain the manner in which S_HMax might also change in proximity to this and other shear zones associated with the Chelungpu Fault.
DE: 8010 Fractures and faults
SC: Tectonophysics [T]
MN: Fall Meeting 2005