Observations of low-frequency earthquakes and volcanic tremor at Soufrière Hills Volcano, Montserrat

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Abstract
The 1995-1999 extrusive phase of the eruption of Soufrière Hills Volcano on Montserrat was dominated by low-frequency volcanic earthquakes (also called long-period and hybrid earthquakes). These earthquakes have distinctive peaked spectra and commonly occur in swarms related to the pressurization of the upper part of the magma conduit. We use data from an array of broadband seismometers to examine spatial and temporal variation in the spectral properties of these earthquakes between January and August 1997. Although spectra are generally stable over long periods of time at a given reference point, we also find evidence for changes in the spectra with time and with event magnitude, which may be attributed to changes in the source. The relative amplitude of spectral peaks varies at different stations around the volcano, leading to the conclusion that the observed wavefield is a combination of both source and propagation effects. We also analyse a number of tremor episodes related both to swarms of low-frequency earthquakes and to volcanic explosions. During certain tremor episodes we observe harmonic spectra, with shifting spectral peaks whose frequencies are the same at all stations. This behaviour can be modelled by repetitive triggering of individual earthquakes, where the trigger frequency changes with time, as well as by time-dependent changes in the acoustic properties of the magma-filled conduit caused by pressurization or depressurization.