Anisotropy of magnetic susceptibility and P-wave velocity in core samples from the Taiwan Chelungpu-Fault Drilling Project (TCDP)

The anisotropy of magnetic susceptibility (AMS) and anisotropy of P-wave velocity (APV) were characterized in core samples retrieved from the TCDP Hole A at depths ranging from 600m to 1400m, thus cutting across the thrust fault that ruptured during the 1999 Chi-chi earthquake. Our data show that the AMS was similar for all the samples regardless of the lithologic contrasts, with a magnetic fabric characteristic of weakly deformed sedimentary rocks that had undergone layer parallel shortening. This robust observation applies to both the siltstones and sandstones at all depths investigated. In contrast the APV data reflect fundamental differences between the elastic fabrics in the two facies. While APV of the siltstone samples is controlled by a preferred orientation of the matrix minerals, the APV of the sandstones is greatly influenced by microcrack porosity with a preferred orientation parallel to the maximum principal direction of paleostress and tectonic deformation. These AMS and APV data obtained in TCDP Hole A at locations not directly impacted by the coseismic rupture provide a first order petrofabric framework for the characterization of hydromechanical anisotropy in the Chelungpu fault system.