Non-planar Fault Model of the 2008 Yutian Normal Faulting Earthquake (M7.2), Xinjiang, China, and its implications

On 20 March 2008, a normal-faulting earthquake (M7.2/USGS) struck Yutian county, Xinjiang, China. The epicenter is close to the southern-edge of the Tarim basin, where three prominent fault systems, the Altyn-Tagh Fault, Karakax Fault, and Longmu-Gozha Co Fault, meet together at the northwest of the Tibetan plateau. The associated crustal deformation signals, if detected and examined in detail, will provide us with important constraints on how the continental crust in Tibetan plateau has been deforming, which has been contentious over the decades. Also, it could suggest clues on why normal faulting earthquakes prevail over the central to northern areas of Tibet. Here, based on our detected co-seismic crustal deformation signals by synthetic aperture radar (SAR) data, we report a non-planar kinematic fault source model that suggests the normal faulting rupture occurred in a dilatational step-over region of two oblique left-lateral strike slip faults. We used L-band ALOS/PALSAR data for the ascending track and C-band Envisat/ASAR data for the descending track. The observed data showed that the overall strike was from NE to SW, consistent with Shao and Ji (2008)'s preliminary source model. The largest slipped area in the InSAR data, however, indicates that the strike direction is almost NS, and that the rupture reached to the surface. The pixel-offset data also illustrate that the strike direction changed around the middle of the fault trace on the surface, suggesting that the fault slip occurred on a non-planar surface. In order to realistically represent the non-planar geometry, we employed an analytical formulation for the triangular dislocation element to invert the slip distribution (e.g., Mearten et al., 2005; Mead, 2007). In inverting the fault slip, both the non-negativity constraint on the slip direction and the smoothing constraint on the slip distribution were applied. Optimum fault source and its implications for regional tectonics and mechanism for the normal-faulting event are going to be presented.