| Summary: | 博士 === 國立中央大學 === 地球物理研究所 === 98 === The Taiwan Chelungpu-fault Drilling Project (TCDP) drilled a 2-km-deep research borehole in Dakeng, Taiwan. Geophysical logs of the TCDP were carried out over depths of 500–1900 m in two boreholes. In order to identify the shear zones, a shear zone at a depth of 1110 meters is interpreted to be the Chelungpu fault, located within the Chunshui shale. Stress-induced borehole breakouts were observed over nearly the entire length of the wellbore from image logs (FMI). These data show an overall stress direction (~N115°E) that is essentially parallel to the regional stress field and parallel to the tectonic stress direction. The Dipole Sonic logs (DSI) also analyzed in this study, the data shows that the most dislocation of fast shear azimuth is close to the depth 1110 meters and consistence with the borehole breakout rotation.
The logging data show that near the fault, the azimuth of the maximum horizontal principal stress (SHMAX) changes by about 90o from the regional tectonic stress direction (N130oE). Hydraulic fracturing tests were used to determine the magnitude of the minimum principal stress (S3) at multiple depths. Through dislocation modeling, we simulated the abrupt stress rotation observed in the image logs at the depth of the Chelungpu Fault. In addition, the modeling indicates that the magnitudes of the minimum horizontal principal stress Shmin changed markedly during the earthquake. In order for the co-seismic stress changes to result in a ~90o stress rotation near the fault, the state of stress prior to the earthquake had to have been a reverse faulting stress regime (as expected), but with SHMAX ≈ Shmin >>Sv. The modeling in the stresses with low frictional coefficient and a near complete stressdrop after the earthquake suggests a weak Chelungpu fault, at least in the northern part of the fault.
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