| dc.description.abstract | The 1999 Chi-Chi earthquake (Mw=7.6) produced great impact on the Taiwan island. This earthquake induced the 90km-long and NS-trend Chelungpu fault which possesses many interesting and unusual rupture behaviors. Among them, the northern portion of the fault (i.e. the Shihkang and Cholan areas) had attracted much attention of many researches. At this northern end, the fault turned east, created huge (up to 12m) and complicated surface ruptures. In this study, we used shallow reflection seismic method to detect subsurface structures of the area, attempting to find the relationship of the structure and large ruptures. Over one hundred seismic lines (each about 500 m long) have been acquired, which were distributed over a 10 x 20 km area around the Tachiahsi and Taanhsi rivers. The irregularities of the structure are believed to be the factors which control abnormal ruptures in this area.
After combining the seismic sections, the drilling data, CPC deep seismic sections, and geologic maps, we obtained the following conclusions:
1. The Tungshih anticline develops above the Chinshui shale, and behaves as a detachment fold. This anticline plunges toward the southwest until reaching the axis of the Shihweichiang syncline. The interlimb angles of the anticline become broader in the southwest.
2. The Shihweichiang syncline (north of the Tachiahsi river) and the Toukoshan syncline (south of the Tachiahsi river) should be the same syncline. This syncline forms the basis of the Chinshui shale structure system. Several shallow folds (e.g. the Tungshih anticline) develop above it.
3. The Hsinshe terrace, south of the Tachiahsi river, is a pop-up structure basing on the Chelungpu fault as the sliding bottom. The Chelungpu fault surface under the Hsinshe terrace not only rises toward the west, but also climbs up near the surface toward the north. This up-lifting may connect with the Tungshih anticline and form a structural boundary (called the Tachiahsi-Tungshih boundary, TT-boundary).
4. The TT-boundary is obviously a structural barrier which may interfere ruptures of the Chelungpu fault during the earthquake. This west-east trend barrier blocks fault slippages, causing them to turn east and splitting into ‘en-echelon’ branches.
5. The Chinshui shale gives the major detachment basis for the Chelungpu fault. This layer comes close to surface (about 500 m depth) under the Tachiahsi river. A drilling data (Tanaka et al., 2002) has shown high water content (45%) within the fault gouge. It is not unreasonable to assume that the water from the two rivers may sink into the fault surface and cause certain degrees of lubrication. This could be one of the factors which result in huge ruptures around the ‘two river’ area. | en_US |