Volcanic seismicity at Montserrat, a comparison between the 2005 dome growth episode and earlier dome growth

In 2005 Soufrière Hills Volcano on Montserrat started its third major episode of dome growth since the current eruption started in 1995. The style of seismicity associated with dome growth has changed, in particular the events known as ‘hybrid’ earthquakes have reduced in numbers by an order of magnitude compared to previous dome growth episodes. In the past, hybrid earthquakes have been associated with magma ascent and so it is surprising to observe prolonged periods of rapid dome growth during which very few hybrid earthquakes are recorded. In addition, the frequency of the codas of hybrid earthquakes, as well as of some of the so called ‘long-period’ events, has changed. The changes in recorded seismicity have had a marked effect on the techniques used to monitor the state of the volcano and those events that continue to be recorded in large numbers (‘rockfall events’) have been used to assess the state of activity at the volcano.