Abstract
In the southernmost Longitudinal Valley (LV), Taiwan, we analyzed a dense GPS array composed of 10 continuous stations and 86 campaign-mode stations. By removing the effects of the four major earthquakes (one regional and three local) occurred during the 1992 - 2010 observation period, we derived a new horizontal velocity field in this area, which then allows better locating the surface traces of the major active faults, including the Longitudinal Valley Fault (LVF) system and the Central Range Fault, and characterizing the slip behaviors along the faults. Note that LVF reveals two sub-parallel strands in the study area: the Luyeh Fault to the west and the Lichi Fault to the east. Based on the results of strain analyses, including dilatation and shear strain, and projected vectors of station velocities across the major faults, we came to the following geological interpretations. During the inter-seismic periods, the surface deformation of the southernmost LV is mainly accommodated by the faulting on the two branches of the LVF; there is very little surface deformation on the Central Range Fault. The Luyeh River appears to act as a boundary to divide the LVF to behave differently to its northern and southern sides. The Lichi Fault reveals a change of slip kinematics from an oblique shearing/thrusting in the north to a nearly pure shearing with minor extension to the south. Regarding the slip behavior of the Luyeh Fault, it exhibits a creeping behavior in the north and a partially near-surface-locked faulting behavior in the south. We interpret that the two strands of the LVF merge together in the northern Taitung alluvial plain and turns to E-W trend toward the offshore area.
Highlights
The island of Taiwan is situated in the convergent boundary between Eurasia and the Philippine Sea plate
From the GPS velocities in map view (Fig. 12a), we found that the yielded surface trace of the Luyeh Fault is consistent with the previous mapped locations and that the Lichi Fault cuts into eastern edge of the Longtien terraces and the Peinanshan tableland
A dense GPS network was deployed around the vicinity of the southernmost Longitudinal Valley in southeastern Taiwan
Summary
The island of Taiwan is situated in the convergent boundary between Eurasia and the Philippine Sea plate. Our recent analysis of vertical velocity field combining leveling and GPS data (Chen et al 2012) showed that the faulting of the two branches of the LVF dominated the surface inter-seismic vertical deformation in the southernmost Longitudinal Valley: (a) the hanging wall of the Lichi Fault (i.e., the Coastal Range) is uplifting at a rapid rate, (b) the hanging wall of the Luyeh Fault (i.e., the Kaotai and Peinanshan tablelands) reveals a variation of moderate uplift or subsidence, (c) the footwall of the LVF (i.e., the flat area of the valley) shows significant subsidence, and (d) the Central Range exhibits a slight to moderate subsidence This vertical velocity cannot provide the information on the horizontal especially the strike-slip components of the aforementioned three faults. We incorporated recently published leveling results to discuss the detailed location of the surface traces of the three aforementioned major active faults, their surface slip behaviors, and possible tectonic implications
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