The relationship between degassing and rockfall signals at Soufrière Hills Volcano, Montserrat

Abstract

AbstractOne of the most common types of seismic event recorded during the eruption of Soufrière Hills Volcano from 1995 to 1999 is known as a rockfall signal because signals recorded when rockfalls were observed on the dome are of this type. Evidence is presented that two seismic sources contributed to these events. The action of falling debris on the dome generated seismicity between 2 Hz and 8 Hz, while many rockfall signals also have a marked spectral peak between 1 Hz and 2 Hz. Deployment of a pressure sensor near the volcano has shown that the 1-2 Hz energy was associated with degassing at the surface of the dome; however, the relative timing of gas escape and seismic signal showed that the first was not the direct source of the second. Resonance of the magma conduit linked to degassing at the surface is invoked as a probable source for the 1-2 Hz seismicity. The relative importance of the two seismic sources that contributed to rockfall signals is examined in the context of the behaviour of the volcano.