Friction weakening in granular flows deduced from seismic records at the Soufrière Hills Volcano, Montserrat

Accurate modeling of rockfalls and pyroclastic flows is still an open issue, partly due to a lack of measurements related to their dynamics. Using seismic data from the Soufrière Hills Volcano, Montserrat, and granular flow modeling, we show that the power laws relating the seismic energy E_s to the seismic duration t_s and relating the loss of potential energy ΔE_p to the flow duration t_f are very similar, like the power laws observed at Piton de la Fournaise, Reunion Island. Observations showing that t_f≃t_s suggest a constant ratio E_s/ΔE_p≃10^-5. This similarity in these two power laws can be obtained only when the granular flow model uses a friction coefficient that decreases with the volume transported. Furthermore, with this volume-dependent friction coefficient, the simulated force applied by the flow to the ground correlates well with the seismic energy, highlighting the signature of this friction weakening effect in seismic data.