Particulate kinematic simulations of debris avalanches: interpretation of deposits and landslide seismic signals of Mount Saint Helens, 1980 May 18

Abstract

SUMMARY We construct a new class of granular landslide models in which avalanches are simulated with large numbers of independent particles moving under the influence of topographically derived gravitational and centripetal acceleration. Concurrently, the particles suffer deceleration due to basal and dynamic friction. The novel aspect of the calculation is that complex particle-toparticle interactions, fluctuating basal contacts, and unresolved topographic roughness within and below the deforming flow are mimicked by random perturbations in along-track and cross-slope acceleration. We apply the method to the 1980 May 18 Mount Saint Helens debris avalanche by constraining the initial geometry and structure of the slide mass from geological data, and the initial failure sequence from eyewitness accounts. After tuning coefficients of mechanical friction and random accelerations, the landslide simulation generates a final deposit whose extent, thickness, morphological structure and lithological variation closely replicate those observed. Moreover, the model avalanche is consistent kinematically with mapped patterns of bedrock scouring, deposit superelevation, and net force history implied from seismic records. To be successful, the slide mass must be divided into upper, high-friction and lower, low-friction members. This division corresponds to fresh, water-unsaturated and hydrothermally altered, water-saturated rock units and points to a mechanical explanation of the kinematics of the debris avalanche. Success in reproducing many features of the Mount Saint Helens avalanche indicates that debris-deposit data may be used to determine the kinematic histories of less well-observed landslides.