Revealing coseismic displacement and displacement partitioning at the northern end of the 1999 Chi-Chi earthquake, Central Taiwan, using digital cadastral data

The 1999 Chi-Chi, Taiwan, earthquake (M_w 7.6) was the largest earthquake to strike Taiwan in the twentieth century. Its surface rupture extends more than 100 km in a north-south trend. However, the northern end of this surface deformation abruptly terminates at an area between the Tachia river and the Taan river, where a broad pop-up structure with east to northeast strike (quite different from the north- south-striking main thrust) is found. This pop-up structure is composed of several regions of parallel thrusts and back thrusts and total shortening reaches about 9 m. In order to reveal the displacement field in the pop-up structure and slip vectors of branch faults, we use a digital cadastral system with about 17,000 control points to calculate coseismic displacement around the Shihkang area. The digital cadastral system was originally conceived to survey land and building boundaries. The cadastral system affords high-density control points that reach about 2800 points/km², and the accuracy is to within ±11 cm. Outside of the pop-up structure, the horizontal displacements vectors are 8-9 m in about 330°-340°, changing as they pass through the surface rupture. The azimuth of horizontal displacement shows large variation with the azimuth from 360° to 315° and displacement amounts from about 4-8 m inside the pop-up structure. In addition, according to these cadastral data, we found surface ruptures that had not been observed in previous studies that could be of importance to future disaster assessment.