Coseismic displacements of the footwall of the Chelungpu fault caused by the 1999, Taiwan, Chi-Chi earthquake from InSAR and GPS data

The differential SAR interferometry technique (interferometric synthetic aperture radar, InSAR) is applied on the Chelungpu fault surface rupture zone of the September 20, 1999, Taiwan, Chi-Chi earthquake using six ERS-2 images covering the period from February 1999 to January 2000. As compared with available geodetic data, InSAR measurements result in more extensive analysis because of high spatial sampling and centimetric accuracy. However, coseismic displacements can be evaluated only on the footwall of the fault. The analysis of interferograms shows the existence of a linear trend in phase difference mainly caused by orbital errors, which we removed from interferograms using GPS data. The corrected interferograms provide a precise map of the InSAR component of the coseismic displacement, showing a continuous decrease over the footwall from a maximum of 36.7 cm at the fault east of Taichung city to a value of about 5 cm at the coastline 30 km further west. The map analysis reveals that the Changhua fault (whose surface trace is located about 20 km west of the Chelungpu one) and the Tuntzuchio fault influence the displacement field. We interpret this in terms of minor reactivation of these faults triggered by the earthquake. A 1.7 cm uncertainty, estimated from the GPS data, is proposed to quantify the precision of the map. Beyond this single value, we highlight the interest of having several coseismic interferograms to evaluate the reliability of the map in a more comprehensive way. Comparisons with displacements inferred from models of slip distribution inverted without InSAR data highlight the advantage of carrying out a joint inversion including our results as new constraints.