A Low Velocity Zone along the Chaochou Fault in Southern Taiwan: Seismic Image Revealed by a Linear Seismic Array

Hsin-Chieh Pu,Yih-Hsiung Yeh,Tao-Ming Chang,Kuo-Liang Wen,Cheng-Horng Lin,Wen-Yen Chang

doi:10.3319/tao.2009.11.06.01(t)

Abstract

The Chaochou fault is one of the major boundary faults in southern Taiwan where strong convergence has taken place between the Eurasian and Philippine Sea plates. The surface fault trace between the Pingtung plain and the Central Range follows a nearly N-S direction and stretches to 80 km in length. In order to examine the subsurface structures along the Chaochou fault, a linear seismic array with 14 short-period stations was deployed across the fault to record seismic data between August and December 2001. Detailed examination of seismic data generated by 10 local earthquakes and recorded by the linear array has shown that the incidence angles of the first P-waves recorded by several seismic stations at the fault zone were significantly larger than those located farther away from the fault zone. This difference might reflect the lateral variation of velocity structures across the Chaochou fault. Further examination of ray-paths of seismic wave propagation indicates that a low-velocity zone along the Chaochou fault is needed to explain the significant change in incidence angles across the fault zone. Although we do not have adequate information to calculate the exact geometry of the fault zone well, the variation in incidence angles across the fault can be explained by the existence of a low-velocity zone that is about 3 km in width on the surface and extends downward to a depth of 5 km. The low-velocity zone along the Chaochou fault might imply that the fault system consists of several splay faults on the hanging wall in the Central Range.

Highlights

Fault zone structures play an important role in understanding faulting processes in the past as well as future potential earthquake behavior and impending seismic hazards near active faults
We infer that the Chaochou fault is not a single fault near the surface, but is similar to major boundary faults that consist of several splay faults near the surface, such as the San Andreas Fault, which has been much more thoroughly studied than the Chaochou fault (Unsworth et al 1997)
To reveal the subsurface structure along the Chaochou fault, seismic data recorded at the linear seismic array have been examined in this study

Summary

Introduction

Fault zone structures play an important role in understanding faulting processes in the past as well as future potential earthquake behavior and impending seismic hazards near active faults. Aside from using traditional geological surveys on the surface or drilling at shallow depths (i.e., Boness and Zoback 2004; Chéry and Zoback 2004; Morrow et al 2007), scientists have largely found deep fault zone structures with low velocity characteristics by studying the seismic waves trapped within the LVZ (Li et al 1990, 1994; Ito and Kuwahara 1996; Kuwahara et al 2001). It was interesting to note that the particle motions of the first P-waves generated by 10 local earthquakes and recorded at the seismic array showed that the incidence angles changed significantly across the fault zone. This feature provided useful information to investigate the possibility of an LVZ beneath the Chaochou fault. Major fault zone structures can be further estimated by modeling ray-path characteristics propagated across the fault zone

Geological Background

Seismic Array and Data

Data Analyses

Findings

Discussion

Conclusion