NEW VIEW OF LATERAL BLAST DYNAMICS AT MOUNT ST. HELENS

The 18 May 1980 blast at Mt. St. Helens has been simulated numerically using the 3D multiphase flow model PDAC (Neri et al., JGR 2003; Esposti Ongaro et al., Parallel Computing 2007), leading to a plausible new view of blast dynamics. Initial source conditions, e.g. gas content, mass of juvenile and entrained rocks, temperature, grain size distribution and pre-eruption pressure distribution in the cryptodome have been parameterized accordingly to geological constraints. Model results suggest that the main blast can be schematized by an expansion phase (burst), lasting on the order of ten seconds, followed by collapse and pyroclastic density current phases. In the burst phase the pressure forces dominate, the flow can reach supersonic velocities and generate pressure waves that can be tracked by the numerical model. In the PDC phase the flow is gravity-driven and its dynamics are strongly controlled by its vertical stratification and the 3D topography. The simulations suggest that much of the severe damage observed at MSH can be explained by high dynamic pressures in gravity currents, and the rapid decrease of damage and dynamic pressure from proximal to distal areas (and related parameters of PDC velocity and density) were largely related to rugged topography beyond the North Fork Toutle River valley (rather than, say, a supersonic Mach disc at ~11 km). Although the source models investigated thus far represent a simplification of the actual geometry and complex sequence of initial events, we show that the explosion mechanisms are significantly robust over a wide range of initial conditions. Multiple simulations have been run, some with >20 x 10⁶ computational cells. Simulation results for MSH are also consistent with those obtained in a previous application of a similar model to the 1997 lateral blast at Soufrière Hills volcano, Montserrat (Esposti Ongaro et al., J. Geophys. Res. 113 (B03211), 2008), which was at least 10X smaller, thus suggesting that the simulated mechanisms are largely independent of eruption scale.