Discrete element simulation of the Jiufengershan rock-and-soilavalanche triggered by the 1999 Chi-Chi earthquake, Taiwan

摘要

We present Contact Dynamics discrete element simulations of the earthquake-triggered Jiufengershan avalanche, which mobilized a 60 m thick, 1.5 km longsedimentary layer, dipping ,22°SE toward a valley. The dynamic behavior of theavalanche is simulated under different assumptions about rock behavior, water tableheight, and boundary shear strength. Additionally, seismic shaking is introduced usingstrong motion records from nearby stations. We assume that seismic shaking generatesshearing and frictional heating along the surface of rupture, which, in turn, may inducedynamic weakening and avalanche triggering; a simple "slip-weakening" criterion wasadopted to simulate shear strength drop along the rupture surface. We investigate themechanical processes occurring during triggering and propagation of an avalanchemobilizing shallowly dipping layers. Incipient deformation forms a pop-up structure atthe toe of the dip slope. As the avalanche propagates, the pop-up deforms into anoverturned fold, which overrides the surface of separation along a decollement.Simultaneously, uphill layers slide at high velocity (125 km/h) and are folded anddisrupted as they reach the toe of the dip slope. The avalanche foot forms a wedge that ispushed forward as deformed rocks accrete at its rear. We simulated five cross sectionsacross the Jiufengershan avalanche, which differ in the geometry of the surface ofseparation. Topographic and simulated surface profiles are similar. The friction coefficientat the surface of separation determined from back analysis is abnormally low (u_(ss) = 0.2),possibly due to lubrication by liquefied soils. The granular deposits of simulatedearthquake- and rain-triggered avalanches are similar.