Hydraulic properties of samples retrieved from the Wenchuan earthquake Fault Scientific Drilling Project Hole-1 (WFSD-1) and the surface rupture zone: Implications for coseismic slip weakening and fault healing

摘要

In this study, we report the hydraulic properties of samples recovered from the first borehole of the Wenchuan earthquake Fault Scientific Drilling and from outcrops associated with the surface rupture zone of the 2008 Wenchuan earthquake. Compositional and microstructural analyses have also been performed on selected samples. Using the pore pressure oscillation method, the permeability measurements show that (1) fault gouge samples have low permeabilities, decreasing from 2 x 10(-18) m(2) at an effective pressure (P-e) of 10 MPa (equivalent to an in situ depth of 600 m) to 9 x 10(-21) m(2) at 155 MPa. (2) Intact and cemented samples are impermeable with permeabilities less than 2 x 10(-20) m(2) at 10 MPa. (3) Fractured samples have variable permeabilities, ranging from 3 x 10(-15) to 1 x 10(-20) m(2) at 10 MPa, and are most insensitive to changes in the effective pressure. (4) Granitic cataclasites have a moderate permeability at low pressure (i.e., 10(-16) to 10(-17) m(2) at 10 MPa); which decreases rapidly with increasing P-e. Hydraulic conduction of the fault is believed to be influenced by the permeability of the fractures developed, which is controlled by the density, aperture, and/or connectivity of the fractures. Microstructural and compositional analyses of the samples indicate that the fault zone heals through chemically mediated fracture closure related to mineral precipitation, possibly assisted by pressure solution of stressed fracture asperities. Although other weakening mechanisms remain possible, our laboratory measurements combined with numerical modeling reveal that thermal/thermochemical pressurization, perhaps leading to gouge fluidization, played an important role in the dynamic weakening of the Wenchuan earthquake, at least in the study area.