921 Chichi Earthquake :
Preliminary Interferometric Interpretations
Deffontaines B.1, Liang L.S.2, Pathier E.1,
Wang C.T. 2, Fruneau B.3, Lee C.T.4,
Raymond D.1, J.Angelier1 and Rudant J.P.3
Centre National de la Recherche Scientifique ESA 7072,
and Université Pierre et Marie Curie,Paris, France,
Remote Sensing Center Chungli, National Central University,
Chungli,TAIWAN
Université de Marne-La Vallée, France
E-mail : benoit.deffontaines@lgs.jussieu.fr
Keywords :
Chichi Earthquake, 921, Neotectonics, Active Faults, SAR-ERS, Interferogram, Foothills, Taiwan.
Abstract
Taiwan island displays active tectonics expressed by high seismicity. The 921 Chichi earthquake (M
w= 7.3 - Foothills, Western Taiwan), occured last 21
st September 1999 at 1 :47am. By SAR-ERS interferometry, we show herein the deformation field in the western side of the active Chelungpu thrust fault (Taichung area) induced by the Chichi earthquake. It shows a slight subsidence, with a maximum displacement of 30cm. This new methodology combined to precise GPS measurements, field work (trenching, fault and syndeposits analyses), geophysics, sismologist and morphotectonic works, lead to a better knowledge of active faults in Taiwan.
Introduction
Taiwan is located on plate boundaries between both Philippine and Eurasian plates to the East and West respectively. This island shows a spectacular compressional tectonics, underlined by a recent mountain range culminating almost at 4.000m. Otherwise, the present deformation is also expressed by high seismicity, including the last destructive Chichi earthquake (21.09.99), and by less known slow deformations, partly recorded by G.P.S methods. Our aim in this paper is to better know the deformation field within the Chichi area, so we have carried out SAR interferometric studies in the western part of Taiwan (Taichung cities and surrounding areas).
After a brief review of Taiwan geodynamical setting, and Western coastal plain geological features, we present herein three new and complementary SAR interferogram of the studied area, evidencing the tectonic deformation linked to the earthquake. One may notice the major contribution of these results for a better knowledge of natural hazards of tectonic origin in this densely populated area.
Taiwan geodynamical setting (fig 1)
To the NE of Taiwan, the Philippine plate subducts towards the Northwest beneath the Eurasian plate into the Ryukyu arc-trench system. Contrasting to the S of Taiwan, where the Philippine plate, bounded by the Luzon volcanic arc, overrides towards the West the South China Sea oceanic crust into the Manilla trench. This latter subduction is transformed northward and onshore into a collision between Philippine and Eurasian plates. Therefore Taiwan is an accretionary prism linked to this collision. The obliquity between the Chinese continental margin (Eurasian plate) and the Luzon volcanic arc (Philippine plate) involves a progressive migration towards the South of the active collision (e.g. Biq, 1972; Bowin et al., 1978; Wu, 1978; Suppe, 1981; Ho, 1986). A present 70 mm/yr northwestern relative motion between Philippine and Eurasian plates, predicted by Seno et al. (1977, 1993) has been recently raised to 81 mm/yr according to GPS measurements (Yu et al., 1997).
Fig. 1. Taiwan geodynamical setting
The Philippine Sea Plate subduct under the Eurasian plate to the NE
and is overriding the Eurasiatic plate to the SW (from Angelier 1986-99).
SAR interferometry data acquisition and processing (table 1)
With the aim of detecting deformations linked to Chichi earthquake, we have analysed the western Foothills of Taiwan, in the Taichung area, by the means of SAR interferometry. This method has already been used for recording and measuring ground "fast" motions linked to earthquakes (normal and strike-slip faults, volcanic eruptions, and landslides (Massonnet & Feigl, 1998). But it is the first time that a thrust to be studied with this methodology. The problem with Chichi earthquake is to get good base line (distance between the two satellites orbits) in order to get rid of topographic artefacts. Unfortunately,
with SAR-ERS2 the dates close to the September 21
st are of poor base lines which imply to take into account longer time interval before and after the 921 quake. Consequently, an interferogram with a long time interval between the two images involve a coherence loss. Nevertheless, weak ground motions have been already characterized by the means of interferograms with long intervals (until 3 years) either in desert areas like Iceland (Vadon & Sigmundson, 1997), within towns like Paris (Fruneau & al. 1998) or within a compressive environment in Tainan (Taiwan, Deffontaines et al., 2000). In Taiwan, SAR interferometry use for ground motion measurements is difficult because coherence problems induced by extensive tropical vegetation, water stretches, rice and tea crops. In this context, the Foothills is nevertheless a good target because this area is rather low relief, with several important towns connected by wide suburbs, so favourable to a coherence preservation, during a long time (Deffontaines et al., 2000).
The Taichung studied area is covered by several ERS-SAR 2 frame (descending and ascending orbits). Among the frames, we have selected, with the help of ORBISCAN
Ó (CNES) software several pairs of interferometrable images (track 232 and frames 3123, see table 1 for parameters).
Processing of the images had been done using the DIAPASON
Ó (CNES) software which lead to interferogram where the effects of satellite geometry and topography are removed using 80m ground resolution digital terrain model (hereafter DTM).
| Date(1st) |
Date(2nd) |
Difference |
V or parallel Baseline/ |
H-Perpendicular Baseline/ |
| 99/02/25 |
99/09/23 |
210 days |
39m |
36m |
| 99/05/06 |
99/01/21 |
105 days |
-96m |
-91m |
| 99/10/28 |
99/09/23 |
35 days |
219m |
91m |
| 99/05/06 |
99/09/23 |
140 days |
213m |
54m |
| 99/05/06 |
99/10/28 |
175 days |
-6m |
37m |
Table 1 : Characteristics of the different ERS-2, 3123/232 interferograms realized
