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SAR/InSAR
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921 Chichi Earthquake :
Preliminary Interferometric Interpretations
One may notice on fig. 2B and consequently on Fig. 3, 4 and 5, the absence of interpretable interferometric data on the eastern side of the Chelungpu Active Fault due to the large amount of displacement, the large destructions of buildings, the differenciate relief and the luxuriant vegetation. In contrast, the western down-faulted block displays almost 10 fringes interpreted herein as deformations due to the Chichi earthquake as the high base line, and the low relief of the studied area avoid classical topographic artefacts. The coherence of the three independant interferogram shown here lead also to minimize atmospheric artefacts.
On the Taichung basin, the calculated interferograms shown here, present coherent 10 to 11 phases variation modulo 2P. As the radar wavelength of SAR-ERS2 is 2.8cm, the down-faulted side of the Chelungpu thrust reaches almost 30cm of subsidence linked to the 921 Chichi earthquake. In order to get both horizontal and vertical component of this deformation, we have taken GPS data into account (work in progress with Yu S.B.). Our preliminary results combining both interferogram and GPS data show the pretty good accordance and highlight the complementarity. Especially, one may notice the interest of interferograms which show spatially the maximum area of deformation of the Western block of the Chelungpu active fault in east Taichung city, within a gap of GPS stations. The maximum of subsidence area revealed by SAR interferograms is coherent to the maximum of destructions of buildings in the city. This is closely linked to the geometry of the fault which is not a uniform fault plan and present several re-etrent (Deffontaines, 2000). The further direction of studies deal with the vertical variation of water-table which also might produce centimetric ground deformation on interferograms (work in progress).
Conclusions
The preliminary interferograms linked to 921 Chichi earthquake (Taiwan) reveal spatially the western deformation field of the Chelungpu Active Fault. It is the first example of interferogram linked to active thrust in the world studied by SAR interferometry. Such studies of active deformation within high anthropic area such as the Taichung county (third city of Taiwan) shows how important is this interferometric approach to better locate, characterise, quantify and modelize natural hazards due to tectonic effects. This interferometric approach had to be combined with classical field data (trenching, and morphotectonic studies), and with GPS, levelling, geophysics and seismological approach in order to better constrain the active faults of Taiwan.
Acknowledgments
This research has been done with various supports from the Taiwan-France cooperation program supported by the French Institute in Taipei (IFT), the National Science Council (NSC), the Ministry of Education of Taiwan, the French National Center of Spatial Studies (C.N.E.S) and the European Spatial Agency (E.S.A.) and within a collaboration between University Pierre and Marie Curie and the National Central University (Chungli) and the remote sensing of NCU-Chungli.
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