Estimates of Fault Strength, Frictional Heat, and Energy for the Chi-Chi, Taiwan Earthquake

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The main purpose of this project was to look for a temperature anomaly associated with faulting from the 1999 Chi-Chi, Taiwan earthquake (Mw7.6) on the Chelungpu fault. Any temperature change across the fault would give an estimate of the frictional heat produced at the time of the earthquake. Determining the level of frictional heat adds valuable information to a long-standing problem in seismology about the absolute stress levels and strengths of faults during large earthquakes. For the temperature measurements we able to use the Taiwan Chelungpu fault Drilling Project (TCDP) borehole which penetrated the fault at a depth of about 1100 meters in a region of large slip.Using a high-resolution borehole instrument developed in this project, we measured continuous temperature profiles in the borehole in September 2005. The results, which were obtained 6 years after the earthquake, showed a small increase of temperature across the fault. This small temperature signal is interpreted as the residual frictional heat from the time of the earthquake. The modeled temperature data showed that a shear stress level of 0.8 to 1.7 MPa on the fault during the rupture. This value corresponds to an apparent coefficient of friction of 0.08 to 0.1, which is a very low value. If this is a representative value for the area of large faulting, significant low friction mechanisms are needed to explain the faulting.Material properties, such as variations in heat conductivity, may affect the interpretation of the temperature signal. Investigating these effects is the purpose of a follow-up project. Other studies in the project investigated physical properties of the fault zone from the borehole cores and a water pumping test was done to determine the fluid flow properties.