Coseismic Deformation from the 2008 Wenchuan, China, Earthquake Derived from ALOS/PALSAR Images

Abstract

We analyzed ALOS/PALSAR images along eight paths acquired before and after the mainshock of an earthquake in Wenchuan, in the Sichuan province of China, May 12, 2008. We found entire coseismic deformation features and detected many fringes in an approximately 100-km wide region surrounding the Longmen Shan fault zone. We found 6–7 broad elliptic fringes north of the epicenter, which indicated around 70–80-cm increase in range (distance between the ground and satellite). This pattern corresponded to subsidence or eastward shift of ground, which is often seen above the lower margin of a low-angled thrust. We also counted 8–9 fringes, i.e., 90–100-cm range decrease, between the fault zone and Chengdu. Chengdu is located roughly 50 km away from the Pengguan fault, which is the southernmost trace of the Longmen Shan fault zone. As this area is located on the footwall side, the pattern suggested a westward shift of the earth’s surface. However, we could not obtain high correlations due to large displacements in the about 240 km long belt along the surface faults. Several concentric fringes were found along the Beichuan fault, which is the central trace of the Longmen Shan fault zone. This implied that the rupture process of this earthquake was complicated. We also performed pixel matching to estimate offsets in the region of low correlation. Positive and negative range offsets larger than 2 m were found on the north and south sides of the Beichuan fault, respectively. We observed a belt of negative range and azimuth offsets between the Beichuan and Pengguan faults. We calculated an interferogram using a multi-segment fault model with uniform slip along the Beichuan and Pengguan faults and estimated the slip distribution. Based on previous seismological studies, we assumed that the southwestern part dipped toward the northwest by 33 degrees, whereas the central and northeastern parts had a much steeper dip angle. We found two regions of slips larger than 4 m in the southwest and central parts of the Beichuan fault. The Pengguan segments may have much smaller slips (around 1 m). The southwestern segments of the Beichuan fault and the entire Pengguan fault had dominant thrust components, whereas dextral slip prevailed in the central and northeastern segments of the Beichuan fault. The estimated geodetic moment was 5.87 × 10 20Nm (Mw = 7.78).