Controls on rates of slope movement before catatsrophic collapse

    Rapid, giant landslides, or sturzstroms, are among the most powerful natural hazards on Earth. They are produced by catastrophic, deep-seated slope collapses with minimum volumes on the order of 10⁶–10⁷ m³. Such collapses are often the final stage of accelerating slope movement that may have continued for years. Measurements made over 60 years ago before the failure of Mt. Toc into the Vajont reservoir in the Italian Alps remain one of the best records of pre-collapse slope movement. Numerous studies have recognized that the rate of movement increased hyperbolically during at least two months of heavy rainfall before the mountainside collapsed on 9 October 1963. Two hundred million m³ of rock sent a wave of water over the Vajont dam, killing approximately 2,500 people in the downstream communities of Longarone, Pirago, Villanova, Rivalta, and Fae. Analysis of the extended record shows that the hyperbolic trend was preceded by an exponential acceleration during 1962. The earlier trend was interrupted in December 1962 when the reservoir was temporarily drained to install engineering safety measures. The acceleration resumed in July–August 1963 after the reservoir was refilled to its pre-drainage level. This combined exponential-hyperbolic acceleration trend is consistent with the activation and eventual linkage of cracks along the future failure plane.This suggests that the surface movements were a consequence of fracturing as deep as 200 m underground, rather than cracking being a result of slope movement. This interpretation points to the weakening of deep rock as the primary driver of failure, caused by factors such as increases in pore water pressure and water-induced corrosion, rather than the destabilization from the weight of a water-saturated slope.Since rock cracking occurs within a restricted range of physical conditions, this case study demonstrates that medium-term forecasts of catastrophic slope failure are a feasible goal. By identifying and quantifying these conditions, we can advance predictive capabilities and mitigate the devastating impacts of rapid landslides, such as tsunamis, seismic shocks, and downstream flooding.