Nantou earthquakes

Coseismic Deformations and Tectonic Implications of the two Mw>6 2013 Nantou Earthquake

Author: Jiun-Ting Lin

Adviser: Wu-Lung Chang, Benjamin F. Chao

Abstract

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 compare two different models to analyze the rupture properties of the events. Our daily-based GPS data resolved a maximum coseismic fault slip of 10 cm at 14-km depth for the 0327 event, and 25 cm at 12-km depth for the 0602 event, which are consistent with the centroid moment tensor (CMT) inversion results.

Introduction

Nantou County, one of the most active seismic regions in Taiwan, with some devastating earthquakes occurred over the centuries. The most recent 1999 Mw 7.3 Chi-Chi earthquake, which ruptured the Chelungpu thrust fault, caused thousands of houses collapsed and more than 2000 people died. A critical-taper wedge model (Suppe et al., 1981) had been used to explain the structure below the central Taiwan. However, whether large earthquakes could occur below the shallow detachment (Carena et al., 2002) is unclear.

The two 2013 Nantou earthquakes show an unexpectable rupture and produced deep aftershocks, which may provide us more clues for studying the structure below central range and the mechanism of the Taiwan orogeny.

Both coseismic displacements of Nantou earthquakes were recorded in the nearby GPS network in the daily solution (left) and high-rate/1 Hz solution (right). We then invert the fault of these events by following dislocation model.

This equation assumes that surface displacements are produced by combination of strike-slip and dip-slip components on each subfault, nearby subfault has the similar slip behavior, which control by smoothing & fitting trade-off.

CONCLUSION

1.Based on our model results, both the 0327 and 0602 earthquake ruptured to the deep seismogenic zone below the central Taiwan.

2.A critical taper model had tried to explain the tectonic of Taiwan, but it is seems that the deep events cannot fully explained by a critical wedge model.

3.Both of the Nantou earthquakes shed light on the complex tectonic structure of the Taiwan collision, it might be a key for understanding the evolution of Taiwan active mountain belt and tectonic below central range.