Probabilistic analysis of rockfall frequencies during an andesite lava dome eruption: The Soufrière Hills Volcano, Montserrat

A probabilistic analysis of rockfalls has been undertaken using seismometer-determined information on the timing and duration of rockfalls from the Soufrière Hills lava dome, Montserrat, between 1995 and 1997. Repose intervals between rockfalls fit log logistic survivor distributions. Differences in the log logistic survivor function for different sub-sets of data are accommodated by variation of the parameter k, which varies from 0.7 to 1.6, and sample median, μ. When extrusion rate is assumed to be ≈0 m³/s, k is ≤1 denoting that the hazard function has a continuously decreasing form, contrasting to the hazard maximum present during periods of extrusion. This analysis illustrates a direct relationship between k, μ, and lava extrusion rate. Extrusion rate becomes increasingly significant in determining rockfall frequencies when rates are relatively high, whereas other factors become increasingly significant when rates are low.