特別セミナー
日時: 2011年6月22日(水) 10:30-12:00
場所: 地震火山・防災研究センターセミナー室(理学部E館4階 E411)
発表者: Yu-Ting Kuo (Department of Geosciences, National Taiwan University)
講演タイトル: Co-seismic Surface Displacement by COSI-Corr on optical images: Case Studies of the 1999 Mw7.6 Chi-Chi Earthquake and the 2008 Mw7.9 Wenchuan Earthquake



Co-seismic Surface Displacement by COSI-Corr on optical images: Case Studies of the 1999 Mw7.6 Chi-Chi Earthquake and the 2008 Mw7.9 Wenchuan Earthquake

Yu-Ting Kuo1, Yue-Gau Chen1, Jean-Philippe Avouac2, John Suppe1, Francois Ayoub2,
Sébastien Leprince2, J. Bruce H. Shyu1, Kuang-Yin Lai1, Yu-Ju Kuo3,
1. Dept. of Geosciences, National Taiwan Univ., Taiwan, R.O.C.
2. Division of Geological and Planetary Sciences, Caltech, California, U.S.A.
3. Mathematics Dept., Indiana Univ. of Pennsylvania, U.S.A.

Motivation
When an earthquake occurs close to an urban area, it is easy to cause the collapse of buildings and other damages following by the surface rupture. It also results in unpredictable disasters. In order to estimate the amount of damages following by an earthquake, it is important to understand how an earthquake causes the surface rupture. Furthermore, the surface rupture is closely related to the underground fault geometry and subsurface structure. Thus it is necessary to obtain detail and continuous inter-seismic or co-seismic ground slips to study surface rupture fault and fault geometry.

Introduction
1. Chi-Chi Earthquake
On Sep. 21, 1999, Mw 7.6 Chi-Chi earthquake occurred in Taiwan. It causes many damages at the area of surface rupture. At the Tsaotun area, the surface rupture always occurred along the Chelungpu fault (CLPF). In addition, about 2km from the east of CLPF, there was an Ailiao fault (ALF) which also created some surface deformation, thus there should be changes in co-seismic ground displacement. In previous study, a more complete measurement of the co-seismic ground displacement was retrieved from sub-pixel correlation of SPOT satellite images. The displacement distribution was used to study the surface deformation and underground geometry of the CLPF structure. However, it cannot accurately locate the position of surface rupture of CLPF and there were no obvious changes in displacement on ALF due to the low resolution of SPOT images.
2. Wenchuan Earthquake
On 12th May 2008 in the mountains to the west of Sichuan province a Mw 7.9 earthquake occurred on the Longmenshan fault, which not only shook many Asian cities but also killed at least 69,000 people. The co-seismic surface ruptures appeared with a strike of roughly NE and distributed up to 270 km along the boundary between the eastern Tibetan Plateau and the Sichuan basin. It was difficult to complete the detailed mapping of surface ruptures and to measure fault slip in the field because well-defined features to correlate across the fault were rare in such a mountainous area. Two major and sub-parallel surface ruptures in the study area have been mapped by post-earthquake investigation. With an attempt to obtain continuous co-seismic horizontal ground displacements we apply the COSI-Corr software on SPOT images.

Method
COSI-Corr on optical images
In addition to seismological records, the co-seismic ground displacement is another key to investigate the mechanism of seismic rupture. Geodetic measurements, such as GPS, can provide accurate displacement measurements, but they generally have limited number of points. A much more complete displacement field can be obtained from COSI-Corr of optical images, such as SPOT, ASTER, IKONOS satellite images. This research uses optical images to measure co-seismic ground deformation. It has been shown that COSI-Corr of satellite images can provide an alternative measurement of horizontal co-seismic displacements with widely spatial coverage. The use of optical images offers an attractive alternative due to the operational status of a number of optical imaging programs and the availability of archived data. However, this technique still has some limitations, mostly due to the uncertainties on the satellite attitude, or the inaccurate account for stereoscopic effects, which result from the uncertainties on the topography and the looking angles. As a result, the approach has been successfully applied to images with nearly the same incidence angle (typically less than 0.5° in all the published examples so far). Satellite images from SPOT or other systems with similar spatial resolution (such as ASTER) are particularly appropriate to measure the ground deformation over an area after a larger earthquake with magnitude 7 or greater.

Summary
1. Chi-Chi Earthquake
Our measurements are consistent with lower resolution measurements of horizontal displacements obtained from SPOT images, as well as with vertical displacements obtained from repeated leveling measurements. Horizontal strain is strongly localized along the Chelungpu fault (CLPF) and along a secondary scarp that runs parallel to the CLPF about 2km to the East, the Ailiao Fold scarp (ALF). This pattern closely matches the surface ruptures mapped in the field. Horizontal strain across CLPF correlates remarkably well with the topographic expression of long-term deformation. The cumulative horizontal shortening across the CLPF and ALF amounts to 4.89±0.36 and 6.12±0.57 m, respectively, and fault-parallel displacement is 3.44±0.35 m. The pattern of surface strain is consistent with the interpretation of the ALF as a fold scarp formed over an active axial hinge zone. This study shows that, even in this compressional setting, most surface deformation is localized within narrow fault zones or active axial hinges.
2. Wenchuan Earthquake
By this method we are able to not only identify where the sub-parallel surface ruptures are but also map continuous change along the rupture as well as across the rupture on the hanging wall up to a distance of a few kilometers. The resulted along-rupture variation is generally consistent with the observation from the field observation. However, the width of hanging wall deformation zone varies along the strike of the entire rupture system. For the western rupture, the hanging-wall deformation zone reaches the maximum in the central segment from Qingping to Beichaun, but gradually becomes as narrow as less than 2 km to the north, indicative of steepening the fault dip to the north. In the south, two parallel rupture traces are clear but characterized by relatively narrower deformation zone in the hanging wall of the western rupture than of the eastern. It also implies that the western fault dips with a higher angle and than of the eastern fault. By such a continuously and high spatial-resolution ground displacement derived by this study, a 3-D fault geometry can be further constructed.


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