## 2 Geodetic Data Sets and Processing
### Sub-Pixel Correlation of SPOT Images  
In order to obtain observations of the hanging-wall, the COSI-Corr (Co-registration of Optically Sensed Images and Correlation) software was used to correlate pre- and post-earthquake optical images. SPOT satellite panchromatic images were processed to determine the horizontal cooseismic displacement using the subpixel correlation technique ([23; 24]). In this study, we used SPOT 1-2 images with a 10 m resolution and with lower cloud coverage (see Table 2). The locations of the SPOT images are showed by the orange dashed box in Figure 1A. The pre-earthquake image is from 29 January 1999 and 2 months following the event for the second image (23 November 1999). In order to minimize uncertainties in the correction of topographic effects, we selected images covering the area with near-vertical incidence angle (2.9\({}^{\circ}\) for the pre-earthquake image and 2.5\({}^{\circ}\) for the post one). We used a Tandem-X DEM with a spatial resolution of 12.5 m for the processing as the resolution of the images is 10 m (InSAR resolution is 80 m). Even though the images selected are covering 8 months before the event and two months after, no significant pre-seismic deformation was present on the GPS \begin{table}
\begin{tabular}{c c c c c c c} \hline
**Acquisition Date** & **Track** & **Frame** & **Orbit** & **Temporal Baseline [Days]** & **Perp. Baseline [m]** & **Height Ambiguity [m]** \\ \hline
28 October 1999 & 232 & 3123 & 23633 & - & - & - \\
21 January 1999 & 232 & 3123 & 19625 & 280.00 & \(-\)92.89 & 102.07 \\
06 May 1999 & 232 & 3123 & 21128 & 175.00 & 20.08 & \(-\)469.46 \\
15 July 1999 & 232 & 3123 & 22130 & 105.00 & \(-\)247.71 & 38.28 \\ \hline \end{tabular}
\end{table} 
Table 1: ERS-2 data used in this study. Figure 4: Interferograms of the Chi-Chi earthquake (color bar in cm) as presented in Table 1, the primary image is on 28 October 1999. **(A)** 28/10/1999-21/01/1999. **(B)** 28/10/1999-06/05/1999. **(C)** 28/10/1999-15/07/1999. observations and only a few cm of horizontal post-seismic deformation, so we considered that the offsets are essentially representing the co-seismic displacements [10]. In order to obtain the ground deformation, SPOT pairs were processed using COSI-Corr. It is a module developed in IDL (Interactive Data Language) and integrated in ENVI (Environment for Visualizing Images) by the Caltech Tectonics Observatory (USA). The methodology allows an automatic and precise orthorectification and coregistration of satellite images [25]. The main steps of the procedure are an ortho-rectification of the primary image with Tandem-X DEM due to precise selection of ground control points (GCPs), then ortho-rectification of the secondary image using GPS data values in the far field to correct for long wavelength bias. An optimization is applied to improve the ortho-rectification--reduction of the mis-registration unavoidable from the manual tie points selection by optimizing the GCPs of the secondary image (iteration until ground offset between the primary and secondary images is negligible). Finally, the subpixel correlation of the pre and post-event ortho images leads to three bands (East-West (Figure 5A) and North-South offset fields (Figure 5B)), and the Signal-to-Noise ratio from which the horizontal ground displacement is retrieved. For the joint inversion, we only kept pixels with a Signal-To-Noise ratio (SNR) above 0.1 and removed the area covered by clouds. An RMS (Root Mean Square) of about 2 m is present between GPS and SPOT offsets (Figure 6) mainly due to the different number of points and the issues in the far field. Long-wavelength artifacts are observed on the offsets (see Figure 5), mainly for the North-South image, due to satellite attitude changes.