Discussion on causes of ground collapse pits in the foothill belt of Longmen Mountain after 5.12 Wenchuan Earthquake

Some 20 days after Wenchuan Earthquake, more than 50 collapse pits occurred successively in Lijiayuanzhi, Sichuan Province, China. This paper considered the possible reasons for the occurrence of this cluster of collapses which occurred in alluvial deposits overlying Jurassic conglomerates with a calcareous cement. It is considered that cavities were formed in the bedrock over geological time and that the sudden vibrations associated with the earthquake may well have caused some further collapse of the conglomerates in the roofs of the cavities. During the aftershocks, which occurred over a period of some 18 days, the overlying alluvium would have suffered cyclic stresses such that it became unstable and collapses occurred. Extrapolation of a previously unmapped fault which experienced a displacement of 6 m indicated it would pass through the area where the collapses occurred.