Ejection Landslide at Northern Terminus of Beichuan Rupture Triggered by the 2008 Mw 7.9 Wenchuan Earthquake

On 12 May 2008, the M-w 7.9 Wenchuan earthquake triggered widespread damaging landslides in many parts of the Longmen Shan area. Among these landslides, the Donghekou ejection landslide is quite special. It is located at the northeast end of the Beichuan rupture, and it has caused a great loss of life at the villages of Donghekou, Qinchuan, and Sichuan Provinces. Because of its special location, this ejection landslide differs from landslides caused by gravity or rainstorms only; the sliding surface is not uniformly continuous. Instead, two sections can be distinguished: an upper section with a step sliding surface and a lower section more gently dipping. The landslide started with material ejection caused by large local seismic acceleration, throwing rocks into the air with a parabolic trajectory before they fell back to the ground. In this paper, we analyze geologic and geomorphologic conditions that favored the occurrence of this landslide, and we introduce a simple tectonic-geomorphology model to explain the mechanism that led to the ejection landslide. We find that the location of the landslide zone, along with the domino-like ground tension cracks observed on both sides of the Beichuan rupture, is controlled by the propagation of the rupture. Our result also suggests that, in addition to local seismic shaking intensity, horizontal acceleration, and geomorphologic and geologic conditions, the vertical acceleration and the style of faulting could also play an important role in the occurrence of earthquake-triggered landslides.