HR: 0800h
AN: OS51A-0174    [Abstracts]
TI: Observations of stress orientation and magnitudes near the Chelungpu fault in TCDP hole-A
AU: * Wu, H
EM: sonata@eqkc.earth.ncu.edu.tw
AF: Institute of Geophysics, National Central University, N0. 300, Jhongda Rd., Jhongli, TWN 32001, Taiwan
AU: Ma, K
EM: fong@eqkc.earth.ncu.edu.tw
AF: Institute of Geophysics, National Central University, N0. 300, Jhongda Rd., Jhongli, TWN 32001, Taiwan
AU: Zoback, M
EM: zoback@pangea.stanford.edu
AF: Department of Geophysics, Stanford University, Mitchell Bldg. Room 347, Stanford, CA 94305, United States
AB: Borehole breakout and tensile fractures show the stress direction variations influenced by the Chelungpu fault. The breakout azimuth changes at the depth 1111m at the fault with the direction of maximum horizontal compressive stress (SHMax) changing by 83º. Leak-off test experiments determined the magnitudes of horizontal minimum stresses (Shmin) to be 20 MPa at this depth. The comprehensive suite of geophysical logs were collected in hole-A provided additional parameters to constrain the state of borehole after the Chi-Chi earthquake. Through modeling we attempt to estimate stress magnitudes before the earthquake and stress evolution on the fault during the earthquake. Simulation results show that at the depth of 1111 m, the magnitude of the maximum horizontal stress (SHMax) before and after the earthquake changed from 45 MPa to 20 MPa, which is smaller than the value of Shmin estimated from Leak-off test (LOT) on site. It suggests the exchanges of axes for the maximum and minimum stress directions after the earthquake. The Shmin data over the entire depth range shows a change of the stress regime from transitional reverse faulting to strike-slip faulting. This agrees with the observations of most of the strike-slip focal mechanisms of the aftershocks in the northern portion of the fault after the earthquake.
DE: 5102 Acoustic properties
DE: 8118 Dynamics and mechanics of faulting (8004)
DE: 8164 Stresses: crust and lithosphere
SC: Ocean Sciences [OS]
MN: 2007 Fall Meeting