Initial science report of shallow drilling penetrating into the Chelungpu fault zone, Taiwan

The Chelungpu fault, a reverse fault with left lateral component dipping moderately to the east, was activated by the Chi-Chi earthquake (Mw = 7.6) in 21 September, 1999 with maximum vertical and lateral offsets of 5.6 m and 9.8 m. Characteristics of earthquake and related phenomena are contrasting between northern and southern regions along the Chelungpu fault. The northern region has (1) larger displacements (4 to 9 m), (2) low frequency seismic waves with higher velocity of slip surface, and (3) less disastrous except the most northern area compared to those in the southern region. Drilling into the Chelungpu fault was thus conducted at two locations, northern (Fengyuan) and southern (Nantou) sites, and successfully completed in March 2001. The project was motivated to explore the fundamental controlling factors of the mode of slip motion at northern and southern regions through analysis of intrafault materials. Meso- and microstructural examinations and measurements of static/dynamic physical properties have been conducted for each drill core. The ongoing analyses have shown interesting results: (1) fault zone architecture is totally different between the northern and southern fault zones. The rocks are mainly composed of random fabric fault breccia with extremely thin fault gouge in the northern core, whereas the foliated fault breccia is dominantly associated with ultracataclasite and pseudotachylite in the southern core, (2) possible fault zones activated by the Chi-Chi earthquake can be listed up by combining geological, geophysical logging and reflection seismic data, which are 225 m and 330 fracture zones in the core from northern well and 177 m and 180 m fracture zones in the core from southern well, (3) water contents of the core of the 225 m rupture zone in the northern well attains up to 45 vol. %, and (4) some temperature rises were detected at 330 m fracture zone in the northern well and 180 m fracture zone in the southern well by temperature logging, which could be attributed to residual heat generated during the Chi-Chi earthquake or postseismic influx of hydrothermal fluid into the fault zones.