ALOS Interferometry and coseismic slip inversions of 2008 Wenchuan earthquake, China

Xiaopeng Tong, David Sandwell and Yuri Fialko, contact Xiaopeng Tong ( for more information

Ascending strip mode interferograms
The devastating Wenchuan earthquake occurred on the Longmen Shan thrust belt, which marks the boundary between eastern Tibet and Sichuan basin. Satellite images were collected by the L-band synthetic aperture radar aboard the ALOS spacecraft that is operated by the Japanese Space Agency - JAXA. We investigated the near-field displacement field by processing 6 tracks of ascending interferograms and 1 tracks of desending interferograms. The ascending interferograms are shown on the left. The surface rupture mapped by geologists (red lines) matches the decorrelation zone of the interferograms (see the zoomed-in figure). To the south decorrelation in the interferograms is caused by intense ground shaking on the hanging wall side of the thrust fault. The complicated pattern of the fringes of the interferograms are probably related to complicated rupture process of this earthquake.

Descending ScanSAR mode interferograms
ScanSAR mode interferometry allows for mapping a wide area of deformation or topography with one single track. Because two independent looking directions from the radar are needed to resolve the ambiguity between horizontal motion and vertical motion, the descending interferograms can be combined with ascending interferograms to tightly constrain the coseismic slip inversion. Due to long baseline (~900 meters) and steep relief part of the interferograms on the hanging wall side of the fault are decorrelated.

Coseismic slip inversion with planar fault geometry
We constrain the coseismic slip distribution of the Wenchuan earthquake by combining GPS data, InSAR observations and fault scarp data from geologic field survey. The fault model consists of 4 planar fault planes. The best fit model has fault planes that rotate from shallow dip in the south (35°) to nearly vertical dip toward the north (70-80°). Our rupture model is complewith variations in both depth and rake along two major fault strands. In the southern segment of the Beichuan fault, the slip is mostly thrust (<13 m) and occurred principally in the upper 10 km of the crust; the rupture progressively transformed to right-lateral strike slip as it propagated northeast (with maximum offsets of 7 m).

Slip-depth distribution and aftershocks
One interesting characteristic of this earthquake is that the biggest slip occurred near 0 km depth, consistent with large fault scarp height measured after the earthquake. Aftershocks were primarily distributed below the section of the fault that ruptured coseismically.

Coseismic slip model with listric fault geometry
We updated the coseismic slip model of 2008 Wenchuan earthquake using listric fault geometry. As expected this model doesn't differ much from the planar fault model. In general, most of the slip is concentrated at the upper part of the fault, in the same way as the planar fault model, but slip reaches maximum at depth for part of the southern Beichuan fault. Apparently, the goodness of fit to the observation (94% variance reduction) suggests the listric fault model is applicable to the Wenchuan earthquake. Our listric model lends further support to the idea that Wenchuan earthquake indeed ruptured on an imbricate fault. However, on the other hand, the match to the observation of either planar or listric fault model reveals the weakness of geodetic inversion, or even all the coseismic slip inversion: large uncertainty in the modeled fault geometry.