Revealing Surface Deformation of the 1999 Chi-Chi Earthquake Using High-Density Cadastral Control Points in the Taichung Area, Central Taiwan

Abstract

The 1999 Chi-Chi earthquake, M w 7.6, ruptured a major thrust fault along the western foothills of the Central Mountain Range of Taiwan. Here, we use cadastral control points to detect horizontal displacement in Taichung, central Taiwan. The cadastral control points were used to identify the coordinates of buildings and acreage. The coordinates of each point are measured by total station and connected to Global Positioning System (gps) control points. The density of these control points is nearly 36 points/km2 and the accuracy of each point is within less than 2–3 cm. The government of Taiwan measured cadastral control points before and after the Chi-Chi earthquake in the Taichung area (northern end of the Chelungpu fault); 1269 control points were measured over a 35-km2 area with most control points being located on the hanging wall of the Chelungpu fault. On the footwall, the displacement is about 1.1–1.2 m toward 123° to 128°, which is consistent with gps data. At the hanging wall, the displacement direction concentrates at 326° to 330°. In the eastern section, the displacement is from 7.5 to 8 m, which is consistent with gps and strong- motion data, but in the western part displacement changes from 8 m to 4–6 m and then increases again to 8 m along the 326° trending. We conclude that the changes in displacements were controlled by the geometry of the fault plane. Online material: Horizontal displacement of cadastral control points.