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[1] The geometrical structure of the responsible faults of the 20 September 1999 Chi-Chi, Taiwan, earthquake (M L = 7.3, M w = 7.6) and its aftershocks can be clearly depicted by well-located hypocenters and focal mechanisms of large aftershocks. The mainshock and two large aftershocks with M L = 6.8 were characterized by thrust faulting along a N-S striking fault plane dipping to the east. The underground structure of the Chelungpu fault, which is probably merging with the decollement beneath the Western Foothills, can be clearly associated with the seismicity pattern and the focal mechanisms of the three largest events. A group of deeper aftershocks including two moderate events (M L = 6.3 and 6.0, respectively) were located to the southeast of the mainshock along a fault plane dipping steeply to the west down to a depth of about 30 km. Our results suggest that the spatial pattern of the aftershocks in the southern part of the source area can be interpreted by a conjugate-fault system. This conjugate-fault system is comprised of the gently east-dipping Chelungpu fault and a steeply west-dipping deeper fault zone.

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[1] Based on the quarter-wavelength approximation by Boore and Joyner (1997), the frequent-dependent site amplifications at 87 free-field strong-motion stations in central Taiwan from borehole data are evaluated. Results show that the amplifications increase with frequency, and the amplification is larger at the sites in the Western Plain with Holocene alluvium than at those in the Western Foothill with Pleistocene and Miocene formations.