Abstract
The Chi-Chi earthquake in Taiwan ( ML =7 .3) occurred on 21 September, 1999 and caused widespread damage over a large area in central Taiwan. A distinct rupture appeared along the Chelungpu fault, which marks the transition from a sedimentary basin to the thrust terrain of the Western foothills. In order to investigate the subsurface geometry of the Chelungpu fault and the structural features that may have influenced the sistribution of Chi-Chi aftershocks, a three-dimensional P-wave velocity model has been developed. The velocity model was determined by simultaneously inverting 41,347 P-wave arrival times from 2,582 pre-mainshock “background” earthquakes. The most striking feature imaged by the velocity model is the high velocity wedge extending from surface to about a 15 km depth beneath the Western foothills. The Chi-Chi mainshock just occurred in the transition area from the high velocity wedge to a basin-type low velocity material where the sub-surface geometry of the Chelungpu fault is proposed. The model also images an elongated high velocity volume at midcrustal depths beneath the eastern flank of the Central Range. West of the Chelungpu fault, another high velocity volume is found at the similar depths which could be associated with the Peikang basement high. Within these two high velocity volumes, the Western foothills and the Central Range exhibit relatively low velocity in the middle-crust. A possible explanation for the nucleation of the Chi-Chi earthquake, the thrust faulting and deformation in central Taiwan is attributed to the interactions between the three high-low-high velocity volumes in response to regional compression.
Highlights
The 21September, 1999 Chi-Chi, Taiwan earthquake (ML =7.3) was the most damaging earthquake to have struck Taiwan since the Hsinchu-Taichung earthquake in 1935
The near surface high and low velocity patterns separated by strong lateral velocity gradients seen in Fig. 2b are similar to the pattern of high and low-gravity anomalies seen in the gravity map (Fig. 2c) compiled by Hsu et al (1998)
This study has demonstrated the utility of three-dimensional seismic tomography in in vestigating the velocity structure around the source area of the 1999 Chi-Chi earthquake in Taiwan in order to get the picture of the faulting and deformation prior to the mainshock
Summary
The 21September, 1999 Chi-Chi, Taiwan earthquake (ML =7.3) was the most damaging earthquake to have struck Taiwan since the Hsinchu-Taichung earthquake in 1935. The epi center of the earthquake was located near the Chi-Chi town in Nantou County, and caused distinctive surface rupture along the Chelungpu fault (Fig. 1). Their only minor to no fractures were observed in the area east of the Chelungpu fault (Lee et al.1999). Since the Western foothills and the Central Range include many active and upthrusted structures formed under a regional NW-SE compressional stress system between the Eurasian and Philippine Sea plates (Ho 1986; Yu et al 1997), the crustal structure could have certainly influenced the initiation and the spatial distribution of the mainshock and aftershock sequence
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