Rupture of deep faults in the 2008 Wenchuan earthquake and uplift of the Longmen Shan

At the Longmen Shan, the eastern flank of the Tibetan Plateau rises 6,000 m above the Sichuan basin within a distance of just 100 km. The mechanisms responsible for building this remarkable topographic contrast are debated. Before the 2008 Wenchuan earthquake, the Longmen Shan had experienced no documented large earthquakes and exhibited minimal shortening of the crust, leading to the proposal that flow of weak rock in the lower crust may instead drive inflation of the crust. Here we use high-resolution geodetic data to explore fault geometry, as well as the pattern of strain accumulation and release associated with the Wenchuan earthquake. We find that most of the earthquake slip occurred in the shallow crust, accommodated by two steeply dipping fault planes. We suggest that the maximization of slip in shallow crustal layers was caused by the accumulation of strain energy left over from past blind earthquakes that did not rupture the surface. Furthermore, we document slip of about 2-6m on a deep, sub-horizontal decollement fault that extends for 60 km beneath the Longmen Shan, implying that east Tibet has been thrust over the Sichuan basin. We conclude that infrequent, large earthquakes do accommodate crustal shortening across the eastern edge of the Tibetan Plateau, lending less support to the hypothesis that inflation of the lower crust uplifts the Longmen Shan.