Dynamic Processes of Fault Creep along the Chihshang Fault, Taiwan

Active fault creep along the Chihshang fault in Taiwan has been observed for the past 20 years (Lee et al., 2004). The Chihshang Creepmeter experiment was set up across this most active segment along the Longitudinal Valley fault system, the present-day plate suture between the Eurasian and the Philippine Sea plates in eastern Taiwan. The daily creep data revealed an annual shortening rate ranging from 13 to 18 mm across the fault. The data exhibit a distinct seasonal variation, with the fault creeping steadily in the rainy season and remaining locked during the dry season of the year. Similar observations were also observed in other places of the world, for example, the Parkfield section of the San Andreas Fault. Based on the water level observation in a nearby monitoring well, we calculated the Coulomb failure stress changes induced by the underground water level change. Contributions to the Coulomb failure stress changes are from: (1) the pore water pressure change as a result of the change in ground water table (0.3 bars); (2) loading on the footwall induced by the difference in porosities between the footwall and hanging wall materials (0.1 bars). We found that the loading in the shallow surface layer due to recharge of its aquifer system in the wet-season has increased the Coulomb failure stress and results in periodic creeping activity along the Chihshang fault. For a rate and state dependent friction process, we found the parameter (A-B) is around 10e-3. We constrained the friction parameter (A-B) using observations of the postseismic creep triggered by the Coulomb stress changes of the M6.5 Chengkung, Taiwan earthquake that occurred on the same fault segment in 2003. We re-examined the Coulomb failure stress change for the seasonal induced creep process and found the value is between that with and without the maximum effect of pore water pressure, suggesting a partial hydraulic interconnection of the fault system to its surrounding structures.