Rupture process of the Chi-Chi (Taiwan) earthquake in 1999

The rupture process of the 1999 Chi-Chi, Taiwan earthquake is investigated by using co-seismic surface displacement GPS observations and far field P-wave records. In according to the tectonic analysis and distributions of aftershocks, we introduce a three-segment bending fault plane into our model. Both elastic half-space models and layered-earth models are employed to invert the distribution of co-seismic slip along the Chi-Chi earthquake rupture. The results indicate that the shear slip model can not fit horizontal and vertical co-seismic displacements simultaneously, unless we add the fault-normal motion (tensile component) to inversion. And then, the Chi-Chi earthquake rupture process is obtained by inversion using the seismograms and GPS observations. Antidilatational fault normal motions determined by inversion, concentrating on the shallow northern bending fault where the surface earthquake ruptures, reveal more complexity and the developed flexural slip folding structures than the other portions of the rupture zone. For understanding the perturbation of surface displacements caused by near-surface complex structures, we have taken a numerical test to synthesize and inverse the surface displacements for a pop-up structure that is composed of a main thrust and a back thrust. Our result indicates that the pop-up structure, the typical shallow complex rupture that occurred in the northern bending fault zone, can be modeled better by a thrust fault added a negative tensile component than by a simple thrust fault. We interpret the negative tensile distributions, that concentrate on the shallow northern bending fault, as a synthetic effect including the complexities of property and geometry of the rupture. The earthquake rupture process also exhibits more spatial and temporal complexities at the northern bending fault. And the results clearly show an inverse correlation between the aftershock distribution and the mainshock slip.