Coseismic Deformations and Tectonic Implications of the two Mw>6 2013 Nantou Earthquakes in Taiwan

In 2013, two Mw>6 thrust-faulting earthquakes occurred in Nantou County of the central Taiwan with close epicentral locations within a distance of ~8.5 km: the Mw 6.1 March 27, or 0327, earthquake has a focal depth of 19.4 km and the Mw 6.2 June 2, or 0602, earthquake has a focal depth of 15 km. To evaluate the coseismic surface displacements and fault-slip distributions of the two events, we applied the GIPSY/OASIS II software to process the 1-Hz and 15-second GPS data from 18 stations around the epicentral area in a precise point-positioning mode. We adopted JPL's final orbits and clock products, the VMF1 tropospheric mapping function, and the FES2004 tide loading effect for our processing. We also applied modified sidereal filtering (MSF) to deduce common errors due to multipath effects in the high-rate (1-Hz) solutions. For the 0327 event, a horizontal coseismic displacement of 19.2×7.3 mm was obtained based on the change of daily coordinates recorded at the SUN1 station, the closest station to the earthquake epicenter. This displacement is in consistency with the value of 18.8×23.5 mm revealed by the high-rate (1-Hz) solutions (Fig. 1a). For the 0602 event, the GPS daily coordinates indicate a coseismic displacement of 43.9×8.6 mm at SUN1 that is apparently different from the high-rate solution of 29.2×20.2 mm (Fig. 1b). Preliminary fault modeling of daily-based GPS data showed a maximum coseismic fault slip of 9 cm at 20-km depth for the 0327 event, and 19 cm at 12-km depth for the 0602 event. These results suggest that the two Nantou earthquakes are located in a shallow (< 20km) seismogenic zone beneath the central Taiwan, where a conjugate-fault system was revealed by the background seismicity and the aftershocks of the 2000 Mw=6.7 earthquake. Our studies on the relation among the two Nantou earthquakes and the fault system are in progress with further fault modeling constrained by relocated seismic data and other geologic observations.