IDENTIFY AND CHARACTERIZE THE SLIP ZONE OF 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 provides geoscientific communities an opportunity to examine the behavior and mechanisms of large displacement faulting. The purpose 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 how mechanism of large displacement seismic faulting occurs. We have finished drilling the TCDP hole-A which it has the cuttings from 0 to 431.34 m and cores from 431.34 to 2003.26 m deep in the end of 2004. Meanwhile, the Hole-B with a branch was cored between 950 m to 1350 m in May, 2005. At least six fault zones have been identified in the cores. They are located around the depths of 1111 m, 1153 m, 1222 m, 1580 m, 1712 m and 1818 m. Based on the surface trail of rupture, regional geology, shallow seismic reflection, well logging, thermal measurement and core examinations, the fault zone of 1999 Chi-Chi earthquake may be occurred in the depth around 1111 m. It is composed of black and 12-cm thick ultracataclasites with slickenline as fault core, and asymmetric damage zone with 5 m thick in the hanging wall and 1 m thick in the footwall. Closed examination under the optical, scanning electron (SEM) and transmitted electron microscopies (TEM) on the black ultracataclasites shows less deformed structures with scarcity of microcracks and fresh glasses (pseudotachylyte). Those lines of evidence suggest that the slip zone of earthquake event was located in the depth of 1111.29 about 2 centimeters in thickness. Comparing the glassy morphology with the laboratory experiments on clay decomposition it suggests that the friction temperature may be up to 1100 °C during the seismic faulting. Furthermore, the frictional energy during the faulting of this event can be calculated.