## 1 Introduction  
Taiwan is the emerged part of an orogen formed by the collision between the Eurasian and Philippine Sea plates [1] entering in convergence at a rate of 82 mm/year with an orientation of 310\({}^{\circ}\). Two subductions are consuming the island--at the south, the Eurasian plate goes beneath the Philippine sea plate in the east direction at the Manila trench, and the Philippine sea plate subducts towards the north near the Ryukyu trench. It is a high-risk seismic area (Figure 1) with the major and most destructive inland earthquake of the 20 th century being the Chi-Chi event in central western Taiwan, occurring on 21 st September 1999 with a magnitude of M\({}_{w}\) 7.6 and at a depth of 33 km (according to United States Geological Survey (USGS)). The seismic moment is estimated at 3.046 \(\times\) 10\({}^{20}\) N\(\cdot\)m by the USGS with a mean slip of 2-3 m. Thousands of people were injured or killed, and many infrastructures were destroyed. During the three months following the event [2], thousands of aftershocks occurred including six with a magnitude greater than 6.5, starting at the north and migrating downward and southward of the main event [3]. As presented in Reference [4], it is interesting to look at the seismicity (Figure 1) and its implications for the tectonics--two NW-SE patterns, one starting at the north of the fault and one at the south end between the Luliao and Tachienshan faults are clearly visible and point out the impact of this event on nearby faults such as the Luliao fault that most probably was activated during the Chi-Chi event. This earthquake was due to the reactivation of the Chelungpu fault: thrust faulting on a north-south striking fault plane dipping to the east [5]. The rupture was complex with several dislocations; it reached the surface and created a 100 km-long scrap between the hanging-wall to the east and the football to the west [6]. From GPS data (Figure 1), a clear change of surface motion is visible at the north end of the fault turning toward east. According to the focal mechanism, this event is in agreement with the SE-NW plate convergence motion [7], with an overlap toward west linked to the orogen deformation and a double plate convergence boundary, on each side of the central range [8]. Over the past 20 years, several models of the Chi-Chi earthquake were computed, each of them focusing on specific aspects, using different data sets, techniques and/or methods. For instance, Reference [2] showed by using seismological data that the total rupture duration is 28 s starting at the south of the fault and a northward propagation with static offset up to 8 s, mainly at shallow depths of less than 10 km. The GPS measurements acquired from three years before the event to a few months after [9] showed major horizontal displacements of 1.1 m to 9.1 m from NW to NNW on the hanging-wall with an uplift of 1.2 m to 4.4 m near the rupture. The deformation is less significant towards the east and increases near the end north of the fault. Regarding the footwall, 0.1 m to 1.5 m were detected toward SE to ESE for horizontal displacements and 0.02 m to 0.26 m of subsidence. Regarding studies using InSAR, only the ERS (European Remote-Sensing Satellite) constellation was previously used. Reference [6] used ERS-2 products combined with GPS to study the football and reported some issues--the existence of a linear trend in phase difference caused by orbital error and atmospheric effects, also the presence of non-coseismic displacements. Linear trend correction using GPS was applied to the interferograms to remove large scale perturbations but GPS are too sparse to constrain the error; 1.7 cm of uncertainty is estimated on the centimeter accuracy coseismic displacement map [6]. Due to limited contributions from ERS-2 interferometry to the surface coseismic deformation mapping of the hanging-wall, area presenting the major deformation but covered by a dense vegetation, a few researchers turned to optical images.