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This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/3.0/).

The

On 21 September 1999, a

Differential Interferometic Synthetic Aperture Radar (DInSAR) has been used to measure the co-seismic displacements of the earthquake. Pathier

We will first present results of co-seismic ground deformation measurements from DInSAR method, followed by those from the method of SAR amplitude images matching. By combining the Azimuth Offset (AZO) and the Range Offset (RO) of the SAR amplitude images, a two-dimensional (2D) surface displacement fields associated with the earthquake will be generated. The results will be compared with GPS observations at some GPS stations in the study area.

Two C band (

The SAR images are first processed with the two-pass DInSAR method and the GAMMA software [

It can be seen from

A SAR image contains the phase as well as the amplitude information. We will in this study estimate the co-seismic displacements of the Chi-Chi earthquake with the method of SAR amplitude image matching. The principles of the method are based on the considerations that the post-seismic SAR amplitude image will have pixel-by-pixel shifts with respect to pixels of the pre-seismic SAR amplitude image and that the shift values can be determined through a correlation analysis. The shift values are directly related to the ground displacements caused by the earthquake. The shifts are in general estimated in two orthogonal directions, i.e., the azimuth and range directions, and are correspondingly called AZO and RO, respectively [

To determine AZO and RO, the amplitude images first need to be co-registered. The co-registration will be implemented in two steps, i.e., coarse and fine co-registration. Preliminary offsets in the azimuth and range directions can be determined in the coarse co-registration. Finer scale offsets can then be calculated in the step of fine co-registration. The AZO and RO between the two amplitude images will be measured in their original sampling space, i.e., about 5 m in azimuth and 8 m in slant range directions. The window size used for estimating the correlation is 64 × 64 pixels. The accuracy of the measurements is about 15 cm in the azimuth direction and 25 cm in the slant range direction. For each pixel, the offset is [_{offset}_{orbit}_{defo}

The non-coseismic components due to the difference in the imaging geometries in _{0} , _{1} , _{2} and _{3} are coefficients accounting for the difference in the imaging geometries.

Ten of the 25 GPS stations in the study area given by Yu _{u} r_{n} r_{e}^{T}_{defo}_{AZO} R_{RO}^{T}_{u}_{n}_{e}_{AZO}_{RO}

The non-coseismic component _{orbit}_{orbit}_{0}, _{1} , _{2} and _{3} in

Once the azimuth and range offsets and their corresponding non-coseismic components are determined, the azimuth and range displacements caused by the Chi-Chi earthquake can be easily computed by using

The ground displacements in the range direction shown in

Fifteen additional GPS stations in the area are used to validate the results (See

A complete two-dimensional co-seismic ground deformation filed associated with the 1999 Chi-Chi earthquake in Taiwan has been generated from two descending ERS-2 SAR images with the SAR amplitude image matching method. DInSAR approach has not worked well in the area due to the severe decorrelation effect in the heavily vegetated mountainous regions. In addition, the DInSAR method has not been able to provide information on the displacements that perpendicular to the radar LOS direction, although the direction is important in this study as the Chelungpu fault trace is in the direction. The SAR amplitude image matching method on the other hand has worked very well in the study area. With the assistance of 10 GPS stations in the area, the co-seismic surface displacements in both the azimuth and range directions have been extracted successfully. Comparisons with the displacements observed at other 15 GPS stations have shown that the RMS values of the differences between the two types of results are 6.9 cm and 5.7 cm in the azimuth and range directions respectively. It is unfortunate that there are no suitable ascending ERS SAR images. Otherwise a 3D surface displacement field may be determined with the method and the least squares approaches [

We thank Dr. Charles L. Werner and Dr. Urs Wegmuller for their advices in the study and comments about SAR data processing using GAMMA software. This work was supported by the National Natural Science Foundation of China (Nos. 40774003 and 40404001), National High-Tech. “863” Program of China (No. 2006AA12Z156), Research Grants Council of the Hong Kong Special Administrative Region (Project No.: PolyU 5157/05E and PolyU5161/06E), and the Project of Western China 1:50000 Topography Mapping. The ERS data used in this study were provided by the European Space Agency under category 1 user projects (AO-4458 and 4914). We thank one of the anonymous reviewers for their wonderful work that help to significantly improve the quality of the paper, both in terms of scientific content and clarity of presentation

_{w}

Location of the Chi-Chi earthquake in the WGS 84 system (all figures presented in this paper are in the same reference system except otherwise stated). The red star represents the position of the epicenter. The yellow line represents the Chelungpu fault [

Co-seismic interferogram of Chi-Chi earthquake. Each interferometric fringe represents 2.8 cm of relative displacement in the radar LOS direction. The black line represents the Chelungpu fault. The inset diagram shows the corresponding coherence map. The value zero means that the signals are completely decorrelated and value unity fully coherent.

The estimated of AZO (left) and RO (right). Unit: m.

GPS stations used in the study. The yellow triangles represent the stations used as GCPs. The blue dots represent the stations used for validating the results. The inset diagram shows the geometry of radar image acquisition.

The non-coseismic components of AZO (left) and RO (right) estimated from the GPS observations. Units: m.

The ground surface displacements in (a) the azimuth and (b) the range directions. The black solid line shows the location of the Chelungpu fault.

Comparisons between the displacements observed with GPS and from the method of amplitude image matching in azimuth (up) and range directions (low). The GPS stations used for the comparison are listed in

ERS-2 SAR data used.

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1 | 15 July 1999 | 22130 | 3123 | 232 |

2 | 28 October 1999 | 23633 | 3123 | 232 |