Insights into active tectonics of eastern Taiwan from analyses of geodetic and geologic data

Abstract

About 50 mm/yr of convergence between the Philippine Sea and Eurasian plates is absorbed in eastern Taiwan, and it remains unclear how the convergence is partitioned among active faults. The Longitudinal Valley fault (LVF), the most seismically active fault in eastern Taiwan, creeps at the surface in the south and not in the north; however, it is unclear how much of the fault is locked or creeping at depth. To address these problems, we model Holocene and interseismic deformation of elastic lithospheric blocks moving over a viscoelastic asthenosphere in eastern Taiwan. Through a fully probabilistic scheme, we invert GPS, interferometric synthetic aperture radar, creepmeter, and Holocene marine terrace data for block motions, fault slip rates, and distribution of interseismic creep. The data are explained with four blocks separated by three faults, Central Range fault, LVF, and an offshore fault. The model explains the essential features of interseismic and Holocene deformation. We find that 35–55 mm/yr of slip on the offshore fault is necessary to fit marine terrace uplift rates, which is a larger fraction of the plate convergence than previously recognized. The LVF has a Holocene slip rate of 20–30 mm/yr with approximately equal magnitudes of reverse‐slip and left‐lateral strike‐slip components. Only about half of the surface area of the Longitudinal Valley fault appears to be locked. The southern segment of the LVF creeps at a rate of 5–28 mm/yr down to a depth of 15–20 km, while the northern segment is locked from the surface to a depth of 20 km.