HR: 1340h
AN: T43B-1331    [Abstracts]
TI: Analysis of Magnetic and Grain Fabrics across Fault Gouge in the Chelungpu Fault of Taiwan
AU: Yeh, E
EM: enchaoyeh@ntu.edu.tw
AF: Department of Geosciences, National Taiwan University, Roosevelt Rd, Sec 4, No1, Taipei, 106, Taiwan
AU: Lee, T
AF: Institute of Earth Sciences, Academia Sinica, P.O.Box 1-55, Nankang, Taipei, 115, Taiwan
AU: Lin, Y
AF: Department of Geosciences, National Taiwan University, Roosevelt Rd, Sec 4, No1, Taipei, 106, Taiwan
AU: Chen, T
AF: Institute of Earth Sciences, Academia Sinica, P.O.Box 1-55, Nankang, Taipei, 115, Taiwan
AU: * Chou, Y
AF: Department of Geosciences, National Taiwan University, Roosevelt Rd, Sec 4, No1, Taipei, 106, Taiwan
AU: Lu, C
AF: Department of Geosciences, National Taiwan University, Roosevelt Rd, Sec 4, No1, Taipei, 106, Taiwan
AB: Behaviors of gouge zones in the fault core of the 1999 Chi-Chi earthquake is a key to understanding its slip mechanism. Fault zone samples from two outcrops closed to the Chelungpu fault of the Chi-Chi rupture were densely collected and subjected to analysis of magnetic and grain fabrics. Symmetric elements, hanging wall, black gouge, foliated gouge, black gouge, and footwall, are consisted in the fault architecture. Spatial distributions of magnetic/grain lineation L, magnetic/grain foliation F and anisotropy degree show obvious drops in foliated gouges. Close to black gouges, L and F show different patterns: lineation L directly decreased, but foliation F increased dramatically in black gouges, then dropped in the foliated gouge. Ellipsoid shapes of magnetic susceptibility and grain change from prolate to oblate with increasing anisotropy from wall rocks to gouges. Directional plots of the magnetic susceptibility anisotropy show different types within and outside the fault zones. For wall rock, three principal axes are well clustered individually. However, in fault zones, only Kmin axes are well clustered at the area closed to the horizontal plane, the other two axes are scatteredly distributed along a girdle, especially for black gouges. Compression stress direction inferred from the magnetic fabric is similar to the regional compression orientation. Also, magnetic foliation planes of gouge samples show a 20 degree thrust and 20 degree clockwise rotation, compared with that of wall rocks. Orientation plots of grain ellipsoids across the fault zone are similar to the magnetic fabric analysis but have a systematic difference. The long-axis orientation of grains in wall rocks is parallel to the orientation of regional compression stress but rotates 29 degree clockwisely in gouge zones. Fabric analysis results elucidate that a strong relationship holds for grain and magnetic fabrics and the characteristic movement across the foliated fault zone should be tightly related to the formation of foliated gouges and black gouge of the Chi-Chi earthquake.
DE: 1518 Magnetic fabrics and anisotropy
DE: 5109 Magnetic and electrical properties (0925)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 8004 Dynamics and mechanics of faulting (8118)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting