Frictional and transport properties of the Chelungpu fault fromshallow borehole data and their correlation with seismic behaviorduring the 1999 Chi-Chi earthquake

摘要

We carried out low- and high-velocity friction tests on fault rock samples fromshallow boreholes on the Taiwan Chelungpu fault and measured their fluid transportproperties under high pressure, with the objective of explaining the different seismicbehaviors in the northern and southern sections of the fault during the 1999 Chi-Chiearthquake. Our results of low-velocity friction tests demonstrate that fault gouge from thesouthern section of the fault exhibits velocity-weakening frictional behavior, whereasgouge from the northern section exhibits velocity-strengthening friction. Friction in thenorthern gouge decreased strongly with increasing wetness, whereas friction in southerngouge samples was not affected by wetness. A rapid reduction of friction was observedimmediately 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, although permeability inthe northern fault zone was lower than that in the south. Numerical modeling indicated thatthermal pressurization in the northern fault zone promoted stress reduction and faultinstability during slip, whereas it did not in the south. This contrasting seismic behaviorbetween north and south is caused mainly by differences in fluid transport properties of theslip zones. More efficient thermal pressurization in the north explains the large slipdisplacement there. The results of our low-velocity friction tests are consistent withnucleation of the Chi-Chi earthquake in the south and propagation of the rupture from southto north.