Implications of the rupture process from the displacement distribution of strong ground motions recorded during the 21 September 1999 Chi-Chi, Taiwanearthquake

The 21 September 1999 Chi-Chi earthquake (M-L=7.3) was the largest inland q uake occurred in Taiwan in the past 100 years. According to direct field ob servations, a significant thrust rupture was occurred along the active Shua ngtung fault and Chelungpu fault, From the integrated displacement derived from free-field strong motions around the source area, horizontal displacem ents over 9 m and vertical offsets reaching 3.4 m are obtained at the stati on TCU068 located nearby the northern section of Chelungpu fault. Based on the field observations of slip distribution over the entire 77-km surface b reak and the integrated displacement waveforms, we have arrived at the foll owing findings: An obvious small offset at about 3 sec prior to the princip al offset is observed at two sites close to the southern part of Shuangtung fault. This small offset is not shown on the other stations. We propose th at a relatively small event, probably with a magnitude of about 6.0, occurr ed about 3 sec before the major rupture, that is located at the Shuangtung fault about 8 km east of the Chelungpu fault. The induced northward propaga ting rupture along the Chelungpu fault has reached a maximum permanent offs et over 9 m at the northern section of fault. This northern section of the Chelungpu fault was identified as one of the heaviest damaged areas.