Decoding the Initiation of an Earthquake-Induced Mega-Landslide: Role of Rock Bridge Failures in the Daguangbao Landslide

Abstract

The Daguangbao (DGB) mega-landslide is the largest-scale landslide triggered by the 2008 Wenchuan Earthquake. The slope failure involved approximately 1.2 × 10⁹ m³, exposing a head scarp with a projected area of 1.85 km² and a maximum height of 800 m. However, the initiation mechanisms remain inadequately understood due to a lack of detailed structural data on the failure boundaries. This study employs 3D laser scanning to analyze rock bridge distribution and fracture patterns on the earthquake landslide scarp. Results indicate that rock bridge failures constitute 22.49% of the total rupture area. The main scarp is characterized by tensile failure involving inplane basal and downslope rock bridges linked with steep discontinuities, while the northern boundary exhibits combined tensile and shear failure through the linkage of in-plane lateral rock bridges with discontinuities of varying dips. The distribution and morphology of rock bridges indicate that the northern boundary experienced tensile failure under SN-oriented horizontal seismic loading, while the main scarp underwent tensile cracking parallel to the sliding direction due to EW-directed seismic forces. These processes led to the destabilization of a wedge-shaped block bounded by the southern and northern margins together with the main scarp, which subsequently slid along the basal bedding plane. Prior to the landslide’s detachment from the source area, seismic activity had already caused 64.01% of the total rock bridge failures. During downslope movement, landslide traction further caused tensile failure of the remaining rock bridges on the main scarp and shear failure of those along the northern boundary. This study demonstrates that rock bridge analysis provides crucial insights into boundary fracture development during the rapid initiation of large earthquake-induced landslides.

Keywords

Daguangbao Landslide / Rock bridges / slope stability / terrestrial laser scanning / initiation mechanism / disaster preventation

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