Interferograms of Coseismic Deformation from the May 12, 2008, Sichuan, China, Earthquake obtained by ALOS/PALSAR
Abstract
A devastating earthquake hit southern China on May 12, 2008, and claimed more than 80,000 casualties. This event is the first M8 earthquake in history in and around the Sichuan basin. The epicenter is located near the Sichuan basin, and aftershocks are aligned in the NE-SW direction parallel to the Longmen Shan fault zone, which suggests that this active fault zone bounding the basin and mountain ranges is the source of this event. It is important to reveal the fault movement from the viewpoints of future earthquake hazard evaluation and tectonics of collision zone. Japan's Advanced Land Observing Satellite (hereafter ALOS) has been conducting observation of the earth's surface since its launch on Jan. 24, 2006. ALOS is equipped with an L-band synthetic aperture radar, Phased Array L-band SAR (hereafter PALSAR), which is suitable for the observation of heavily vegetated areas. We analyzed ALOS/PALSAR images along 8 paths acquired before and after the mainshock and revealed entire features of coseismic deformation using the Gamma software and hole-filled SRTM DEM by Jarvis et al.(2008). We observe fringes around the Longmen Shan fault zone. In spite of different period and timing of acquisition of SAR images, several fringes can be traced across the interferograms. These continuous fringes can be recognized in an about 100km wide region surrounding the fault zone and regarded as coseismic deformations. We can count 8-9 fringes, i.e. 90-100cm LOS displacements, between the fault zone and Chengdu located about 50km away from the Pengguan fault, southern most trace of the Longmen Shan fault zone. On the other hand, we could not obtain correlation high enough in the source region due to large displacements. This low correlation zone can be traced nearly 240km along the surface faults. It is worth noting that numbers of fringes are almost the same on the both sides of the source region. This pattern suggests right-lateral motion on a steeply dipping fault plane. On the other hand, 6-7 broad concentric fringes are recognized in the image where the epicenter is located. This pattern corresponds to about 70-80cm LOS displacements away from the satellite around the epicenter and implies a low-angled thrust. We also performed pixel matching to estimate offsets in the region of low correlation. Positive and negative range offsets larger than 2m are found on the north and south sides of the Beichuan fault, central trace of the Longmen Shan fault zone, respectively. Clear difference in offsets cannot be seen along other faults such as Pengguan fault. This observation implies that the coseismic rupture is concentrated on the Beichuan fault. We calculated a theoretical interferogram with three segment fault model with total length of 240km. In this model, southwestern part is assumed to dip toward northwest by 33° with purely thrust motion, while central and northeastern part has a much steeper dip angle and large strike-slip component of slip. Coseismic slip may be concentrated in the shallower part of faults than 15km, especially in central and northeastern parts. The geodetic moment is estimated to be 7.31x1020Nm (Mw=7.84).
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.G33C0702H
- Keywords:
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- 1240 Satellite geodesy: results (6929;
- 7215;
- 7230;
- 7240);
- 1242 Seismic cycle related deformations (6924;
- 7209;
- 7223;
- 7230);
- 7215 Earthquake source observations (1240);
- 8118 Dynamics and mechanics of faulting (8004);
- 8123 Dynamics: seismotectonics