The cause of the apparent small friction exhibited by long rtmout landslides has long been speculated upon. In an attempt to provide some insight into the matter, this paper describes results obtained from a discrete particle computer simulation of landslides composed of up to 1,000,000 two-dimensional discs. While simplified, the results show many of the characteristics of field data (the volumetric effect on rtmout, preserved stra'ta, etc.) and with allowances made for the two-dimensional nature of the simulation, the runouts compare well with those of actual landslides. The results challenge the currein view that landslides travel as a nearly solid block riding atop a low friction basal layer. Instead, they show that the mass is completely sheafing and indicate that the apparent friction coefficient is an increasing function of shear rate. The volumetric effect can then be understood. With all other conditions being equal, differera size slides appear to travel with nearly the same average velocity; however, as the larger landslides are thicker, they experience smaller shear rates and correspondingly smaller frictional resistance.
IntroductionLong runout landslides represent a specta•ar class of geological events. They consist of large masses of rock, (of the order of a cubic kilometer in volume,) that are suddenly released by an earthquake or other catastrophic event. Starting high up the side of a mountain, the material comes crashing down toward the valley floor and may run out many times the distance of its initial fall over essentially horizontal ground. In such events, it is not uncommon to find that the runout (L) is 10 times its initial fall height (H), leading to an apparent friction coefficient (H/L) of 0.1 or lower. The rtmout distances may cover many kilometers and such events have been known to destroy populated areas [e.g. Heim, 1882; Hs//, 1978; McConnel and Brock, 1904] located at what would seem to be safe distances from potentially dangerous mountainsides. Events have been recorded on Earth (some undersea), the Moon [Howard, 1973], and Mars [Lucchitta, 1979]; in fact, the largest such events have been seen in Viking photographs of the Valle Marinaris. One such landslide, initiated by the volcanic explosion of Mount St. Helens, mn out for approximately 24 km from the mountainside [Voight et al., 1983]. Surprisingly, it has been observed that when all of these landslides are considered as a group, the effective friction decreases with increasing volume of slide material; that is, the slides with the largest volumes nm out the longest distance relative to their initial fall height. (The largest landslide, with an estimated volume of 17,880 km 3, occurred on Mars and ran out about 20 times its initial fall height [Lucchitta, 1979].)This high mobility is even more surprising in light of observations that indicate that the bulk of the landslide mass moved nearly undisturbed. The surfaces of the slides• (which are, for obvious reasons, the most carefully surveyed componen0 are covered with large bou...
