Slip vectors of the surface rupture of the 1999 Chi-Chi earthquake, western Taiwan

About 100 slip vectors were surveyed along the 1999 Chi-Chi earthquake rupture (100 km long), western Taiwan, to understand the coseismic reverse-faulting processes. The surveyed slip azimuths vary locally (over 80°) where the rupture is irregular or associated with flexure. Even where the rupture is straight and has consistent slip azimuths, the fault-scarp heights, horizontal slips and fault dip-angles oscillate in a distance of hundreds of meters along the rupture. Despite these local variations, the net slip (up to 11.5 m), vertical and horizontal slips are significant greater in the north, on which maximum shortening (6 m; by pure thrust) and strike slip (8 m; by left-lateral fault) occur in association with the bend of the rupture. The slip azimuths also progressively rotate from north/northwest in the north to west/southwest in the south. These patterns generally agree with those revealed by GPS surveys, except that the slips by GPS in the south move to the west/northwest and have values unexpectedly smaller than our measurements. Our obtained SW-directed slips resulted in dominantly right-lateral faulting (up to 2.4 m) where the rupture ends to the southeast. This, however, does not correspond to focal mechanisms of any main shocks or aftershocks.