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 Es to the seismic duration ts and relating the loss of potential energy ΔEp to the flow duration tf are very similar, like the power laws observed at Piton de la Fournaise, Reunion Island. Observations showing that tf≃ts suggest a constant ratio Es/ΔEp≃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.

Activity since 1995 at Soufrière Hills Volcano (SHV), Montserrat has alternated between andesite lava extrusion and quiescence, which are well correlated with seismicity and ground deformation cycles. Large variations in SO 2 flux do not correlate with these alternations, but high and low HCl/SO 2 characterize lava dome extrusion and quiescent periods respectively. Since lava extrusion ceased (February 2010) steady SO 2 emissions have continued at an average rate of 374 tonnes/day (6 140 t/d), and incandescent fumaroles (temperatures up to 610 o C) on the dome have not changed position or cooled. Occasional short bursts (over several hours) of higher ( 10x) SO 2 flux have been accompanied by swarms of volcano-tectonic earthquakes. Strain data from these bursts indicate activation of the magma system to depths up to 10 km. SO 2 emissions since 1995 greatly exceed the amounts that could be derived from 1.1 km 3 of erupted andesite, and indicating extensive partitioning of sulfur into a vapour phase, as well as efficient decoupling and outgassing of sulfur-rich gases from the magma. These observations are consistent with a vertically extensive, crustal magmatic mush beneath SHV. Three states of the magmatic system are postulated to control degassing. During dormant periods (10 3 to 10 4 years) magmatic vapour and melts separate as layers from the mush and decouple from each other. In periods of unrest (years) without eruption, melt and fluid layers become unstable, ascend and can amalgamate. Major destabilization of the mush system leads to eruption, characterized by magma mixing and release of volatiles with different ages, compositions and sources.

International audienceThe seismic signals generated by two large volcanic debris avalanches (Montserrat, Lesser Antilles, 1997 and Mount St. Helens, USA, 1980) and a large rock-ice avalanche (Mount Steller, USA, 2005) have been analyzed. For the two debris avalanches, given the times and locations of such landslides, their signals were recorded by only a few seismic stations. Moreover, these signals cover only a very narrow frequency band and include considerable noise. The Mount Steller, on the contrary, was precisely recorded. For each event, the source mechanism (i.e., point force) has been determined by waveform inversion using at most two broadband seismic stations. The resulting force is very difficult to interpret in terms of landslide characteristics. A Monte-Carlo inversion was therefore performed by imposing a simple force model associated with the landslide, based on the schematic view of an accelerating/decelerating mass traveling down the slope. The best parameter set of the force model was then found by minimizing misfits and maximizing correlations between data and synthetic signals. This model appears to contain the minimum degree of complexity required to well reproduce the seismic data. We detail here the method for the Montserrat debris avalanche and then present it’s validation on the well studied Mount St. Helens debris avalanche and the well recorded Mount Steller rock-ice avalanche. The horizontal and vertical components of the resulting force have different source time functions. The best force model compares well with the force obtained by waveform inversion. Finally, this simple force model was interpreted using analytical and empirical relations derived from the sliding block model, granular flow model and landslide studies. This made it possible to estimate the order of magnitude of the mass, flow duration and direction, initial topography slope, mean velocity and travel distance of the avalanches. For these three avalanches, the calculated characteristics are consistent with former studies