HR: 0800h
AN: T51A-1319    [Abstracts]
TI: Anisotropy of Magnetic Susceptibility and P-Wave Velocity in Core Samples From the Taiwan Chelungpu-Fault Drilling Project (TCDP)
AU: * Louis, L
EM: llouis@notes.cc.sunysb.edu
AF: Dept of Geosciences, SUNY Stony Brook, Stony Brook University, Stony Brook, NY 11794 United States
AU: David, C
EM: christian.david@geol.u-cergy.fr
AF: D‚partement des Sciences de la Terre et de l'Environnement, Universit‚ de Cergy-Pontoise, Bƒtiment Neuville 3.1, 5, mail Gay-Lussac, Neuville-sur-Oise, Cergy-Pontoise, 95031 France
AU: Wong, T
EM: Teng-fong.Wong@stonybrook.edu
AF: Dept of Geosciences, SUNY Stony Brook, Stony Brook University, Stony Brook, NY 11794 United States
AB: The Chelungpu fault which ruptured during the Mw 7.6 Chi-Chi earthquake in 1999 is the focus of TCDP. We investigated both magnetic susceptibility (AMS) and P-wave velocity (APV) anisotropies in 15 triplets of relatively intact samples retrieved from the country rock and damage zones at depths ranging from 600 to 1400 m. Data from bench-top measurements on dry and saturated samples were analyzed using an inversion procedure developed by Louis et al. (2004), which assumes that the P-wave anisotropy can be approximated by a symmetric, second rank tensor similar to the low (<300 A/m) field magnetic susceptibility. Two sedimentary facies (9 shaly siltstone and 6 sandstone samples) were analyzed. The AMS results are almost identical for both facies and in every location: fabrics are triaxial with minimum and maximum susceptibility axes parallel to the bedding pole and bedding azimuth, respectively. This type of fabric is often considered as resulting from layer parallel shortening predating and/or persisting over the structure formation. In contrast with AMS, APV fabrics display different patterns depending on the facies investigated. In siltstone, APV appears to be closely related to the bedding with a minimum velocity axis close to the pole to bedding and no clear positioning of the maximum and intermediate axes inside it (sedimentary fabric). Interestingly, the control exerted by the bedding over sonic properties in the siltstone agrees well with hydromechanical data obtained in the same facies where failure mode and permeability anisotropy show strong geometrical dependence on bedding orientation. In sandstone, APV fabrics are very different: the minimum velocity axis is located inside the bedding along its azimuth while the maximum velocities are measured parallel to the maximum regional stress. This fabric suggests the presence of preferentially oriented damage features independent from the bedding and rather related to regional tectonics. Consistently, the failure modes in TCDP sandstone samples were observed to be primarily controlled by the applied stress field and weakly dependent on bedding. The microstructural origin of these contrasting anisotropy patterns is currently under investigation, as potential interplay of an original sedimentary fabric with syntectonic processes such as solid grain rotation, newly crystallized phases or stress-induced microcracking needs to be elucidated. Nevertheless these anisotropy measurements on magnetic susceptibility and P-wave velocity provide useful constraints on the structural context in which the Chi Chi earthquake occurred and the timing and amount of strain accumulated interseismically in an earthquake cycle. Louis, L., Robion, P., David, C., 2004. A single method for the inversion of anisotropic data sets with application to structural studies. J. Struct. Geol. 26, 2065-2072.
DE: 5102 Acoustic properties
DE: 5109 Magnetic and electrical properties (0925)
DE: 8099 General or miscellaneous
SC: Tectonophysics [T]
MN: Fall Meeting 2005