SPATIAL AND TEMPORAL ANALYSIS OF NON-RECOVERABLE STRAIN GEOMETRY AS DOCUMENTED BY THE INVERSION OF EARTHQUAKE FOCAL MECHANISMS IN WEST-CENTRAL TAIWAN

The geologically young (<4Ma) Taiwan Orogen reflects ongoing, rapid (~80mm/yr) arc-continent collision. The collision of the northeast-trending Chinese passive continental margin with the north-trending Luzon volcanic arc on the Philippine Sea plate has caused extensive deformation within the orogen as the collision has propagated from north to south. Additionally, magnetic anomaly patterns in western Taiwan suggest that mountain building in this region is occurring above a relict fracture zone in the lower plate. The relative motion between the Eurasian and Philippine Sea plates here has resulted in numerous large thrust earthquakes. One such earthquake was the 1999 shallow, moment magnitude 7.6, Chi-Chi earthquake in west-central Taiwan. The main shock ruptured the Chelungpu fault horizontally over 80km. Earthquake ruptures such as these reflect non-recoverable strain in the crust related to tectonic forces. By modeling pooled earthquake focal mechanism data from the 1999 Chi-Chi earthquake and related events, using an adaptation of the micropolar continuum model, we were able to solve for best-fitting, partial strain tensors and thus examine the 3-Dimensional geometry of non-recoverable strain. Our results indicate that the dominant mode of deformation in the orogen southeast of the fracture zone is orogen subparallel stretching accommodated by both normal and strike-slip faulting.