The Spatial Pattern of Active Uplift in Eastern Taiwan

Leslie Hsu and Roland Bürgmann

Introduction

The eastern coast of Taiwan is one of the most actively deforming regions in the world, with present-day convergence at 8 cm/year where the Luzon Arc is colliding with the Eurasian Plate (Yu et al., 1997). The Eastern Coastal Range is thrust upwards along the east-dipping Longitudinal Valley Fault (LVF), accommodating about 3 cm/yr of the convergence (Yu et al., 1997). Present-day active uplift along the fault is constrained by a GPS network and leveling measurements, showing along-strike variation in rates of slip (Yu and Kuo, 2001). In addition, the marine terrace record indicates that some segments of the LVF have been uplifting more rapidly than others over the timescale of the Holocene (Hsieh et al., 2004). We are using InSAR to improve our knowledge of present-day vertical movement and strain accumulation along the Longitudinal Valley Fault. The new InSAR data, yielding an improved velocity field, will be used to both reevaluate previous dislocation models and formulate new models. These improvements may help to explore currently unresolved issues such as the location of strain accommodation near in the northern half of the LVF – within the Coastal Range or on proposed offshore faults (Bos et al., 2003; Yamaguchi and Ota, 2004). We will also investigate the uplift patterns perpendicular and parallel to the fault’s strike. The former will tell us to what extent the Coastal Range is uplifting like a rigid block and the latter will illustrate transitions between locked and creeping sections of the fault.

Results

From 9 SAR scenes, we created 13 unwrapped interferograms from pairs spanning one month to 2.5 years. We made three stacks of 3-4 interferograms in an attempt to average out atmospheric effects. In some stacks, the recent mapped fault trace correlates well with an offset in the phase signal. Near Chishang, we measured a maximum offset across the fault that gave 14-26 mm/yr movements in the slip direction (Figure 21.1), agreeing well with the nearby creepmeter data (Lee et al., 2003). However, this signal is not present everywhere along the fault. We are developing an interseismic model that will be used as the basis for estimating and removing an orbital ramp from each interferogram. The interseismic model is produced from a simple fault geometry in an elastic half-space. We have also compiled horizontal and vertical GPS measurements, creepmeter data, leveling data, and current fault traces for comparison with the interferometry. Combining these different measures of deformation, we hope to achieve a better understanding of the nature and origin of spatial variation of movement along the Longitudinal Valley Fault.

Acknowledgements

This research is made possible by the Francis Turner Fellowship and a University Fellowship granted by the Taiwanese-American Foundation of Boston.

References

Bos, A.G., W. Spakman, and M.C.J. Nyst, Surface deformation and tectonic setting of Taiwan inferred from a GPS velocity field, J. Geophys. Res., 108(B10), 2458, doi:10.1029/2002JB002336, 2003. Hsieh, M.L., P.M. Liew, and M.Y. Hsu, Holocene tectonic uplift on the Hua-tung coast, eastern Taiwan, Quatern. Int., 115, 47-70, 2004. Lee, J.C., J. Angelier, H.T. Chu, J.C. Hu, F.S. Jeng, and R.J. Rau, Active fault creep variations at Chihshang, Taiwan, revealed by creep meter monitoring, 1998-2001, J. Geophys. Res., 108(B11), 2528, doi:10.1029/2003JB002394, 2004. Yamaguchi, M., and Y. Ota, Tectonic implications of Holocene marine terraces, east coast of Coastal Range, Taiwan, Quatern. Int., 115, 71-81, 2004. Yu, S.B., H.T. Chen, and L.C. Kuo, L.C., Velocity field of GPS stations in the Taiwan area, Tectonophysics, 274, 41-59, 1997. Yu, S.B., and L.C. Kuo, Present-day crustal motion along the Longitudinal Valley Fault, eastern Taiwan, Tectonophysics, 333, 199-217, 2001.

Figure 21.1: Location map, unwrapped interferometry, and offset on the Longitudinal Valley Fault, near Chishang. The interferogram is produced from a stack of three pairs spanning 1997 to 2000 (970517-980502, 970726-980815, 980815-000122). We calculated 14-26 mm/yr offset in the slip direction across the fault from A to A'.
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