Slip recalculation for LP seismicity at Soufrière Hills

Abstract:

A 25 years eruption is still ongoing at Montserrat, West Indies, which has been characterised by explosive behaviour with episodes of pyroclastic flows and dome collapse events. The silicic nature of the magma promotes brittle failure in the magma which triggers slow waves in vertical directions within the volcanic conduit eventually recorded at seismometers as low-frequency events. The occurrence, cyclicity and repetition of these events have been studied extensively, however, the relationship between the amplitude signals and physical mechanisms of magma ascent have never been addressed. We study the seismicity between June 22nd and June 25th, 1997, identifying around 1500 events grouped in approximately 10 hours cycles, which abruptly ended with a dome collapse. Around 260 events were classified as part of the same family of similar waveforms, with correlation coefficients > 0.7. We assume that the trigger of these events is a full-ring rupture of magma at the conduit wall, therefore, the seismic moment of each event needs to be adjusted for this geometry. We use synthetic seismograms to determine the seismic moment of all the events of the family finding slip values that match geological observations of magma failure in exhumed dykes. These displacements are also consistent with tilt observations near the summit that have been linked to local shear stress through magma flow models. The magma ascent velocity due to seismic slip is 9.26 x 10^-5 m/s, while the average ascent velocity observed at the surface is 2 x 10^-2 m/s, this supports the idea that magma ascent is an aseismic process. However, the ratio between these velocities gives us an estimation of how to estimate magma ascent and extrusion from seismic moments as a first approximation. Once calibrated, this method can be applied to different volcanoes and stages of eruption, taking into account the individual dynamic conditions for magma failure which need to be constantly monitored.