Kinematic and dynamic fault slip analyses: implications from the surface rupture of the 1999 Chi-Chi, Taiwan, earthquake

The 1999 Chi-Chi earthquake created a 100-km-long surface rupture on the Chelungpu thrust fault in Taiwan due to convergence between the Phillipine Sea and Eurasian plates. Fault slip measurements were made by several researchers from the entire length of the rupture directly following the earthquake and can thus be attributed to a single tectonic event. Conventional fault slip analyses are applied to these data and the results are compared with independent seismological and kinematic observations. Unlike many fault slip analyses, complications due to multiple deformations can be categorically excluded and the results can be evaluated from the seismological and plate movement data. Kinematic analyses of fault slip data that are weighted by displacement show sub-horizontal NW–SE shortening that is parallel to the plate convergence vector. A single fault plane solution satisfies almost all the data. Right dihedra and trihedra solutions also satisfy almost all the surface rupture measurements and give σ1 in a NW–SE orientation that is similar to the results of stress inversion and to inversion from earthquakes in the Chi-Chi earthquake sequence. Despite criticisms of fault slip analysis methods, these results show that fault slip analyses from data collected along major faults, which have not witnessed multiple deformation events, can be valid. Homogeneous strain and stress states exist in the sense that kinematic and dynamic solutions can be found that fit essentially all the data, and these solutions have tectonic significance.