Revealing coseismic displacements and the deformation zonesof the 1999 Chi-Chi earthquake in the Tsaotung area,central Taiwan, using digital cadastral data

摘要

The 1999 Chi-Chi, Taiwan, earthquake (M_w, 7.6) was the largest earthquake to strikeTaiwan in the twentieth century. This earthquake is associated with a 100 km long surfacerupture. In order to reveal the details of displacement near the surface rupture, we use adigital cadastral system to calculate coseismic displacement around the Tsaotung area, central Taiwan. The digital cadastral system was originally conceived to survey land andbuilding boundaries. In the Tsaotung area, Taiwan authorities have taken digital cadastralmeasurements before and after the Chi-Chi earthquake. The cadastral system affordshigh-density control points that reach ~1421 points/km~2, a system denser than that of theGPS. Accuracy is to within ±11 cm, a level that is higher than spot imaging and one thatallows us to study surface deformation in detail. Coseismic displacement is ~4.3-4.6 m atdistance from the surface rupture and decreases to 3-4 m near the surface rupture. Theazimuth of horizontal displacements is ~310°-315° and rotates to 280°-305° near thesurface rupture. This produced a compression, left-lateral deformation zone with -10-3compression strain near the surface rupture. Coseismic displacement of the footwall is1-1.2 m in 110°, which is similar to that from using GPS data. In the Tsaotung thrust slice,we observed that the azimuth of horizontal displacement rotates from a NW trend to asouth trend as a result of slip partitioning and gravity slide effect.