DInSAR method

In complement to the STAMPS analysis developed in the paper, we also applied standard Differential SAR Interferometry (DInSAR) technique. This technique is based on the phase difference between two SAR images acquired at different times over the same area. The resulting interferometric phase contains several components, amongst them a measure of the ground movement, in the Line Of Sight (LOS) of the radar direction, that have occurred between the acquisition dates of images. After removing the orbital and topographic components (thanks to the precise knowledge of the orbits supplied by the JAXA and the use of a 40 meters Digital Elevation Model), we obtain a differential interferogram mapping the surface deformation between the two dates. It is however polluted by some potential DEM errors, and cumbersome atmospheric artifacts, due to the propagation delays of the radar wave through the atmosphere, those last ones being difficult to eliminate totally. In this study, all interferograms have been generated with the Repeat Orbit Interferometry Package (ROI_PAC). Most of interferograms present a good quality, thus confirming the advantage of the L-band data and their interest in area with luxuriant vegetation.

Among all interferograms, we focused especially on two specific independent ones (having no common images), with quite small perpendicular baselines (lower than 70 m) and an interval of time between acquisitions of about 920 days for both interferograms, so large enough to detect surface deformation. These two interferograms show a clear discontinuity located in the Longitudinal Valley that we interpret as the location of the active Longitudinal Valley Fault. This important discontinuity, characterized by a fast change of colors (from blue to red) underlying two blocks, can be observed from south Rueisuei until Luyeh (over more than 65 km). The eastern block, corresponding to the Coastal Range, is uplifting with regard to the western block, composed by the Longitudinal Valley and a part of the Central Range. This significant gap of radar LOS displacements, visible on both interferograms, corresponds to the interseismic creeping activity of the LVF.

The main trace of LVF can thus be quite easily localized by identifying the fast change of color generated by the deformation related to the thrust fault activity. In an interferogram, one cycle of color from blue to red corresponds to a LOS displacement of lambda/2, that is to say 11.8 cm, toward the satellite. First interferogram, covering January 2007 to August 2009 period, not only contains signal of deformation but also significant atmospheric artifacts over the Central Range, contrary to the second interferogram, between August 2007 and February 2010, which is less affected. Globally, the coherence for both interferograms is very high held account of frequent changes of land surface (rice growing and prominent vegetation) in this part of Taiwan. Loss of coherence is however observed over area located on eastern mountainside which corresponds to shadow areas for ascending SAR data or/and areas of high elevation.
