ESTIMATING GEOMETRY AND SLIP PARTITIONING OF ACTIVE FAULTS IN TAIWAN

The Taiwan arc-continent collision represents one of the best-studied and most active orogenic belts in the world. Despite a strong effort in the last 20 years to determine the mechanism of orogeny in Taiwan, the subsurface fault geometry in Taiwan remains a debated topic. Many researchers have subscribed to the accretionary wedge theory in which most of the deformation occurs on and above a shallow, low dipping detachment. Others claim wedge theory is inappropriate for Taiwan based on various geophysical observations that suggest the entire crust is actively deforming down to ~50 km depth. To date, none of these studies have made use of an abundant set of GPS data that have recorded the spatial distribution of horizontal and vertical displacements in Taiwan for the last 10 years. We use the GPS data and 2-D viscoelastic models of interseismic deformation to image the active fault geometry under Taiwan and estimate slip partitioning among faults. Two methods are used. The geometry and slip rates on faults are estimated using simple viscoelastic dislocation models of crustal deformation that incorporate cyclic motion on seismogenic faults and steady motion on creeping faults. We also have developed more physically realistic models with stress boundary conditions on the faults. Slip on faults in an elastic layer over viscoelastic half-space is dictated by the stress state in the elastic layer. Both models are constrained by independent data including relocated seismicity, geologic cross sections, and paleoseismic estimates of slip rates and recurrence intervals. Two-dimensional profiles are taken from two ~50 km wide NW/SE swaths of data extending the width of the island. The northern profile includes the section of the Chelungpu fault that ruptured during the 1999 Chi-Chi earthquake, the presumed active Changhua and Shuangtung Faults, and the northern part of the Longitudinal Valley Fault (LVF). The southern profile includes part of the active Chukou Fault and the creeping mid-section of the LVF. We show that the GPS data can be explained with a wedge-like fault geometry consisting of a ~10° dipping detachment under the foothills and central range that becomes steep (~30°) to the east under the coastal ranges.