Laboratory Characterization of Permeability and Its Anisotropy of Chelungpu Fault Rocks

In Taiwan an international project to drill into the Chelungpu fault (TCDP) was initiated after the M-w 7.6 Chi-Chi earthquake in 1999. At Takeng, two vertical holes (A and B) to depths of about 2 km have been drilled through the northern portion of the Chelungpu fault system. In this study, we conducted systematic hydromechanical tests on TCDP drillcores collected from Hole-A at various depths above and below the major slip zone of the Chelungpu fault. We focus on the measurements of permeability as function of pressure and the brittle failure behavior. Evolution of permeability as a function of pressure and porosity was measured using either steady-state flow or a pulse transient technique. When subjected to an effective pressure reaching 100 MPa, permeability values of shaly siltstone samples range from 10(-16) to 10(-19) m(2). In comparison, permeability values of porous sandstones are at least an order of magnitude higher, ranging from 10(-14) to 10(-18) m(2). To characterize permeability anisotropy associated with the bedding structure of the rocks of the Chelungpu fault, cylindrical samples were taken from the TCDP drillcores along three orthogonal directions, denoted X, Y and Z respectively. Direction Z is parallel to the TCDP core axis, and the other two directions are perpendicular to the core axis, with X (N105A degrees E) perpendicular and Y (N15A degrees E) parallel to the strike of the bedding. In shaly siltstones, permeability values of samples cored along the strike of bedding (direction Y) can be up to 1 order of magnitude higher than those cored perpendicular to the strike of bedding (direction X). These observations indicate that permeability anisotropy is controlled by the spatial distribution of bedding in Chelungpu fault host rocks. Permeability evolution of fault rocks plays an important role in dynamic weakening processes, which are particularly pertinent to large earthquakes such as the Chi-Chi earthquake. Our experimental data on permeability and its anisotropy of TCDP core samples provide necessary constraints on fault models and proposed weakening mechanisms.