Structural Mapping of the 1999 Chi-Chi Earthquake Fault, Taiwan by Seismic Reflection Methods

Several seismic reflection surveys were conducted to investigate the seismogenic structure of the 1999 Chi-Chi earthquake (Mw=7.6) in central Taiwan. Two 40 km-long seismic profiles that crossed the area near the epicenter were acquired using the deep reflection method with a targeting depth of 10 km, to search for the decollement boundary. One of the ob tained sections shows a clear reflecdon event that dips to the east by 40 until reaching a depth of 8 km where the earthquake's source was located. This slant event is unambiguously related to the thrusting Chelungpu fault surface. The abundant eastward dipping reflectors on the deep reflection sections faithfully describe thrusting features predicted by the earthquake faulting model. Besides these deep reflections, we also used many shallow seismic reflection lines to delineate the structures in the northern portion of the fault zone, where large ruptures (about 10 m) occurred both on the surface and underground. The 3D structure of the fault surface can be de duced using this cost-effective approach. Although the depth imaged may be limited (e.g., 3 km), shallow seismic data still provides reliable informa tion for the study of large ruptures, and to make better plans for deep wells that might be drlled in this area in the future.