Mechanism of Giant Rock Ejection at Donghekou during 2008 Wenchuan Earthquake

Abstract

This presentation gives a quantitative dynamic examination of the ejection of a giant rock mass at Donghekou during the Mw 7.9 Wenchuan earthquake of May 12, 2008. The ejected rock mass is part of the upper mountain with a slope angle of 28, about 3 km to the town of Donghekou in Qingchuan County, Sichuan Province, China. The ejected rock mass has a volume of 6×106 to 24×106 m3 and has a flying distance of 750 m to 1000 m. After the flying, the rock mass impacts the mountain slopes two times and slides further one gentle ground surfaces. The overall travel angle of the rock mass debris is about 14. The ejected rock mass rapidly travels to a maximum horizontal distance of 2.4 to 2.6 km. the rock debris covers an area of 1.1×106 m2, destroys and buries four villages and a primary school and results about 800 fatalities. Under the hypothesis of highly compressed methane gas cause of the earthquake, a dynamic model is proposed and developed to back-calculate the giant rock mass ejection. The horizontal flying distances of 750 m and 1000 m are, respectively, used as the lower and upper ejection limits. For these lower and upper limits, the giant rock mass would have to be accelerated heavily within the initial 0.1 second. Within the initial 0.5 to 0.6 seconds, the giant rock mass has to the highest upward vertical velocity of 27 to 39 m/s and the corresponding horizontal velocity of 57 to 77 m/s. The peak upward vertical acceleration has to be between 360 to 710 m/s2 (38g to 71g) within the initial 0.012 seconds. The peak forward horizontal acceleration has to be between 660 to 1250 m/s2 (65g to 125g) within the initial 0.012 second. The total flying time of the giant rock mass is about 12 to 14 second. The rock mass can have the high speed of 100 to 130 m/s before the impacting. After the impacts, the rock mass can rapidly slide over the ground surface of gentle hillside slopes and stream course with the common ground frictional resistance angle of 30.