In-situ stress at the northern portion of the Chelungpu fault, Taiwan, estimated on boring cores recovered from a 2-km-deep hole of TCDP

We have investigated the depth variation in the stress state at the northern part of the Chelungpu fault, Taiwan, which slipped during the 1999 Chi-Chi earthquake (M_w 7.6). In-situ stress around the fault was estimated based on the stress memory of rocks recovered from five depths (739-1316 m) of a 2-km-deep-hole. The borehole intersects a fault zone at a depth of 1111 m (FZ 1111), which is the best candidate for the Chelungpu fault. Our main results are: (1) the S_Hmax direction is parallel to the slip for the Chi-Chi earthquake and agrees with those of the local, regional, or tectonic scales estimated from various stress indicators; (2) significant aspects of the stress field are identical to those of paleostress; (3) the horizontal differential stress was significantly reduced just above FZ1111, which may be attributable to the existence of a significantly deformable zone at this depth.