Landslides associated with the May 12, 2008 Wenchuan earthquake: Implications for the erosion and tectonic evolution of the Longmen Shan

Satellite images and aerial-photography following the Wenchuan earthquake show that a broad area experienced severe landsliding in association with the main earthquake and aftershocks. A zone of high density landsliding (> 10% landslide surface area affected) stretches for ~ 240 km along the earthquake region, in proximity to the Yingxiu, Beichuan and Pennguan faults, which ruptured during the earthquake. The width of this severe landsliding zone varies along the strike, from ~ 25–30 km wide in the southwestern section of the earthquake region, to ~ 3–5 in the northeastern section. These variations coincide with differences in hillslope gradients, bedrock lithologies, co-seismic slip and ground shaking observed in these sections of the earthquake region. In general, there is good correlation between the magnitude and distribution of ground shaking experienced during the earthquake and mapped landslide density, though limited data and varying fault models prevent any in-depth comparison with high-resolution maps of ground shaking experienced. Landslides associated with Wenchuan earthquake highlight interesting aspects of erosion and tectonic evolution of the Longmen Shan. Short-term erosion rates in the Longmen Shan region measured from cosmogenic nuclides prior to the earthquake were ~ 0.2–0.3 mm/yr, reflecting the last 2000–3000 years — a time period shorter than the estimated recurrence interval of large earthquakes along the fault that ruptured, while long-term erosion rates measured by low-temperature thermochronology were higher, 0.5–0.7 mm/yr for the last 8–10 Ma. Integrating the total amount of erosion associated the earthquake into the short-term erosion rate estimates pushes the rate much higher. This suggests that the earthquake and associated landslide erosion was necessary for the landscape to ‘catch-up’ and balance the long-term tectonic growth of the Longmen Shan, particularly in the southwestern section of the earthquake region. Taken together, the earthquake and associated landslide erosion can be seen as the most recent expression of the rapid deformation/uplift and erosion that previous geomorphic evidence and geodynamic models had argued for along Longmen Shan front.