Slip zone and Temperature of Faulting in the 1999 Chi-Chi Taiwan Earthquake

The activity of Chelungpu fault, induced the 1999 destructive Chi-Chi earthquake with magnitude Mw = 7.7, caused a total surface rupture of about 80-90 km long and the largest measured vertical offsets reaching as much as 5-9 m in west-central Taiwan. It, thus, provide us an opportunity to examine the fresh materials occurred by a large displacement faulting. The purpose, thus, of the Taiwan Chelungpu-fault Drilling Project (TCDP) is to obtain physical samples of the slip zone of 1999 Chi-Chi earthquake in the deep to make progress in understanding the weakening mechanism, energy budget and stress states of large displacement seismic faulting. We have finished drilling the TCDP hole-A and hole-B in the end of 2004 and May, 2005, respectively. At least six major fault zones have been identified in the cores. Based on several lines of evidence, including (1) the surface trail of rupture, regional geology and shallow seismic reflection; (2) thermal anomaly; (3) well logging data showing sharply decreasing resistivity and permeability, the lowest density, Vp and Vs, and high Vp/Vs and poisson¡¦s ratio; (4) principal stress rotations near the fault zone; (5) isotropic gouge layer in the fault core; and (6) pseudotachylyte found in the depth of 1111 m, the slip zone of 1999 Chi-Chi earthquake may be located in the black materials of fault zone FZ1111 around the depth of 1111.26 m. It is 2 cm in thickness which is located at 12 cm thick ultracataclasites with slickenline. The XRD and TEM analyses show that the black ultracataclasite consists of amorphous phase, pseudotachylyte and rich in smectite, but poor or scarce in other clay minerals, i.e. illite, chlorite and kaolinite. The decomposed experiments of thermal gravitational analyzer (TGA) on clay minerals indicate that the temperature of slip zone by frictional heating may reach as high as 950 ¢J during the faulting.