Frictional and transport properties of the Chelungpu fault from shallow borehole data and their correlation with seismic behavior during the 1999 Chi-Chi earthquake

We carried out low- and high-velocity friction tests on fault rock samples from shallow boreholes on the Taiwan Chelungpu fault and measured their fluid transport properties under high pressure with the objective of explaining the different seismic behavior in northern and southern sections of the fault during the 1999 Chi-chi earthquake. Our results of low-velocity friction tests demonstrate that fault gouge from the southern section of the fault exhibits velocity-weakening frictional behavior, whereas gouge from the northern section exhibits velocity-strengthening friction. Friction in the northern gouge decreased strongly with increasing wetness, whereas friction in southern gouge samples was not affected by wetness. A rapid reduction of friction was observed immediately after the onset of slip in high-velocity friction tests. The results of high-velocity friction tests were similar for all fault gouge samples tested, though permeability in the northern fault zone was lower than that in the south. Numerical modeling indicated that thermal pressurization in the northern fault zone promoted stress reduction and fault instability during slip, whereas it did not in the south. This contrasting seismic behavior between north and south is caused mainly by differences in fluid transport properties of the slip zones. More efficient thermal pressurization in the north explains the large slip displacement there. The results of our low-velocity friction tests are consistent with nucleation of the Chi-Chi earthquake in the south and propagation of the rupture from south to north