Source Process of the 1999 Chi-Chi, Taiwan Earthquake: Comparisons to Shallow Faulting in Subduction Zones

The 1999 Chi-Chi, Taiwan earthquake (Mw 7.6) occurred along a zone of rapid convergence in central Taiwan. Since there were large amounts of slip at shallow depths on the Chelungpu fault (8 to 10 meters of thrust faulting at depths of a few kilometers to the surface), the earthquake provides a first-hand opportunity to study many of the physical properties associated with large slip on a fault. There has been a large variety of geological and geophysical observations of this event, including, a dense network of strong-motion instruments that recorded the earthquake, GPS observations of the pre-, co- and post-seismic deformation rates, borehole sampling into the fault, and surface geology studies. The combination of these studies produces one of the most complete pictures of the source process of a large thrust earthquake. Especially, the detailed imaging of the area of large slip in the northern part of the fault provides information about the fault dynamics that may control the large amounts of slip on a fault. The faulting process appears very different between the large slip areas in the north and the more moderate slip regions in the south. The large slip regions in the north appeared to be associated with 'smooth' rupture, that generated relatively low levels of high-frequency radiation and caused relatively low levels of damage. This is in contrast to the more typical high-frequency radiation produced by the fault in the south, which caused considerably more damage, even though the amounts of fault slip were less. The rather unusual characteristic of the large slip in the north may be associated with low levels of dynamic friction on the fault. The thrust faulting of the Chi-Chi earthquake may be similar to the shallow faulting in subduction zones, such as the Nankai Trough. The geometry of the Chelungpu fault, which is connected to more shallowly dipping fault structures at depth, is similar to the splay faults that are mapped in the Nankai subduction zone. For the Nankai trough, it is unclear if slip on the splay faults occurs during the great subduction earthquakes or in separate earthquakes. Part of the answer to this question may come from the results of current projects, such as the borehole sampling of the Nankai splay faults and comparisons with similar borehole samplings of the Chelungpu fault.