Thermal and pore fluid pressure history on the Chelungpu fault at a depth of 1111 m during the 1999 Chi‐Chi, Taiwan, earthquake

On 20 September 1999 the M s 7.6 Chi‐Chi earthquake ruptured the Chelungpu fault in central Taiwan. In 2005 the Taiwan Chelungpu Fault Drilling Project was launched to drill two deep holes (holes A and B) cutting across the fault plane. The heat strength (= 7.0°C m), within a heated layer of ∼5 mm, on the fault at hole A due to frictional faulting is evaluated from the values of shear stress and thermal and mechanical parameters measured from the core samples. Based on a 1‐D heat conduction equation and 2‐D faulting model, with the values of thermal diffusivity evaluated within a representative temperature range, the thermal and pore fluid pressure history at depths 1110.37–1111.34 m in hole A is constructed. Results show that the peak temperature at the center of the heated layer could have been higher than 1100°C during faulting, and temperature rise decreased with increasing distance and time. The possible relationships between the temperatures and chemical reactions of clay minerals are discussed in detail. In the heated layer, pseudotachylites have been formed and quartz plasticity might also have been operative during faulting. Outside this slip zone, the temperature rise was low and thus clay minerals were stable during faulting. The evaluated pore fluid pressure is 22.5 MPa. This suggests the existence of a suprahydrostatic state in the fault zone during the earthquake. Indirect evidence of the existence of fluids in the fault zone during faulting is described. Additionally, rock physics is applied to interpret the observations.