The application of InSAR for crustal deformation and land subsidence in Taiwan

We apply InSAR technique between August 1995 and November 2001 for crustal deformation in central Taiwan. We utilize 2-pass and 3-pass methods to obtain interferograms from 12 ERS SAR images in central Taiwan. The result of D-InSAR (Differential-InSAR) reveals land subsidence and earthquake deformation in our study area. The critical land subsidence region was located in the mouth of Choshui River and Erhlin River. After comparing with the distribution of aquaculture in Changhua County, we find that in the aquacultural area of Tacheng Village has the greatest subsidence value of 14-17 cm/yr due to groundwater overdrafted. The subsidence region is not only located in aquacultural area but has also been detected in urban area near aquacultural area. Comparison with the result of our study is consistent with the report from Water Resources Agency during 1985-1995. In temporal analysis we find the interferograms would lose coherence on the mouth of Choshui River (approx. 408 km2) when the time interval between 2 images was longer than 3 months. The low coherence here is mainly affected by vegetation and lower backscattering buildings density. The Chi-Chi earthquake on 21 September, 1999, with magnitude M_w=7.6 and an epicenter near Chi-Chi in central Taiwan caused surface rupture along thrust Chenlungpu fault. Interferograms obtain during Chi-Chi earthquake reveal the crustal deformation on the footwall, and the result of interferograms shows that the earthquake caused more than 33.6 cm uplift in line of sight of ERS satellite relative to coastline at Chenlungpu fault east of Taichung City. There are 2 interferograms show the coseismic and 1 shows the post-seismic. The coseismic interferograms reveal an increasing gradient from the coastline and deep on Chelungpu fault. The post-seimic group shows a slightly increasing phase line relative to the global trend, which corresponds to the Changhua fault trace. We consider this maybe the reactive motion after the mainshock triggered the motion of this fault. Constructing the displacement map using the GPS data, we can simulate interferograms from GPS and reveal the same trend between the results. This technique doesn¡¦t need to do the field work, which is well suited for crustal deformation monitoring and analysis.