Rapid Shortening at the Eastern Margin of the Tibetan Plateau Prior to the 2008 Mw=7.9 Wenchuan Earthquake

Abstract:

The Longmen Shan is the steepest topographic front of the India-Asia collision and was the site of the Mw=7.9 Wenchuan earthquake. Shortening estimates across the Longmen Shan provide strain accumulation rates and clarify the eastward extrusion of the Tibetan plateau. Here, to explain the interseismic GPS velocities across the greater Longmen Shan region, we develop a boundary element model including earthquake cycle effects, topography, the westward dipping Beichaun fault, and a ~20 km deep, shallowly dipping, detachment. The detachment is inferred from observations of postseismic afterslip following the Wenchuan earthquake and from structural considerations. In contrast to analyses which neglect the detachment and earthquake cycle effects, we find that interseismic GPS data are consistent with a shortening rate of 7±1 mm/yr. These results suggest that the Longmen Shan is an active fold-and-thrust belt with Wenchuan style earthquake recurrence intervals of <500 years.