Recording and analysing volcanic earthquakes has been an important monitoring tool throughout the eruption of the Soufriere Hills Volcano. This Seismic Monitoring relies on a network of seismic stations, which has been continuously improved during the progress of the eruption. A network of three stations had been put in place by the Seismic Research Unit (currently, Seismic Research Centre) prior to the eruption. This was expanded during the first few weeks of the eruption with equipment and assistance from the USGS/VDAP program.
The network was upgraded by the British Geological Survey (BGS) in October 1996 and again in early 2005, by which time it included ten broadband, high dynamic range instruments with digital telemetry, located all over the southern half of Montserrat. As the stations are not easily accessible, they use solar power and the signals are transmitted to MVO using spread-spectrum radios. The signals are recorded and analysed using a computer-based data accquisition system at MVO. Like the seismic network, this has been upgraded several times during the course of the eruption and currently uses a combination of Scream! and Earthworm software for data acquisition and Seisan software for data analysis. MVO records both continuous and triggered event data which are then analysed by specialists. MVO also uses real-time automatic processing of the seismic data to provide immediate information to staff on dutyand, if seismic activity increases significantly, alarm calls to staff cell-phones. All data are archived.
There are a number of distinct types of seismic signals recorded on the seismic network at the Soufriere Hills Volcano. The most important are volcano-tectonic earthquakes, long period earthquakes, hybrid earthquakes and rockfall or pyroclastic flow signals.
The signals from volcano-tectonic (VT) earthquakes have a sudden start and last for a short time. They are predominantly high frequency signals (2 Hz) and often occur in swarms lasting from an hour to several days in duration. VT earthquakes are caused by the fracturing of the rock under the volcano as the magma forces its way to the surface. They were common at the start of the eruption and now occur much more sporadically. VT earthquakes can be located using MVO's seismic monitoring network and generally occur beneath the summit of the SHV, at depths between 0 and 7 km. Early in the eruption, some VT swarms were located under St. George's Hill and to the northeast under Long Ground.
- Long period (LP) earthquakes have a more emergent start and a lower, narrow-band frequency content (1-2 Hz). These are probably caused by resonance in the gas or magma inside the volcanic conduit. It is normally not possible to locate LP earthquakes.
- A hybrid earthquakes is, as the name suggests, a mixture between a VT and an LP. They tend to have impulsive starts but contain a significant amount of low frequency signal. They are thought to represent magma forcing its way to the surface and are are often associated with periods of rapid dome growth, and are sometimes precursors to major dome collapses, or switches in the direction of lava extrusion. These signals often merge into continuous tremor, which sometimes occurs in bands 4-24 hours apart.
- Rockfall or pyroclastic flow signals have often been the dominant type of seismic signal. They have an emergent start and a wide frequency range. They are interpreted as due to material falling off the dome and traveling down its flanks. Signals from pyroclastic flows are similar but are of longer duration and higher amplitude.
- Mudflows can create seismic signals that can be confused with pyroclastic flow signals, although they are generally of lower amplitude and longer duration. These signals usually build-up gradually and are stronger at stations close to ghauts (valleys).