Seismogenic structures of the 1999 Mw 7.6 Chi-Chi, Taiwan, earthquake and its aftershocks

High-resolution 3-D VP and VS velocity models around the source region of the Mw7.6 Chi-Chi, Taiwan, earthquake show significant lateral and vertical variations. The mainshock occurred within a narrow low velocity zone along the Chelungpu fault. A sudden increase of velocity and seismicity took place across the Shuilikeng fault to the east. Most aftershocks were located in areas of high VP and VS. A sharp east-dipping zone extending from the Shuilikeng fault on the surface to a depth of about 15 km separates the low velocity region in the west from the high velocity region in the east. An apparent high-angle west-dipping seismic zone lies at depths of 15-30 km beneath the western Central Range. Numerous aftershocks with normal faulting were located west of the middle segment of the Chelungpu fault.