360050

(was 1600-05)

Montserrat Volcano Observatory
Mongo Hill, Montserrat
 PO Box 318, Flemmings
Montserrat, West Indies

Institut de Physique du Globe de Paris
IPGP - Bureau 445-1, rue Jussieu - 75238
Paris cedex 05

Seismic Research Centre (SRC)
Department of Physics
University of the West Indies 
St. Augustine, 
Trinidad & Tobago, West Indies

Scientific Staff: A large number of scientific staff have been utilised during the ongoing volcanic eruption of the Soufriere Hills volcano (SHV), drawn from SRU, BGS, UK Universities and the USGS-VDAP. A number of other researchers from universities around the world have also contributed to the monitoring operations of MVO.

Dr Paul Cole - Volcanologist
Dr Henry Odbert - Volcanologist
Dr Thomas Christopher - Volcanologist
Dr Paddy Smith - Seismologist
Dr. Adam Stinton - Volcanologist

Local staff are drawn from the Government of Montserrat and assist with all operations at MVO.

Venus Bass - Seismic and Environmental Monitoring Technician 
Racquel 'Tappy' Syers - Volcanological Technician
Carlisle 'Pyiko' Williams - Electronics Technician
Marlon Fergus - Trainee Electronics Technician 
Dave Williams - Electronics and Computer Engineer
Andrew Simpson - Software Engineer
Sonya Melander - Outreach Officer

Cheri Ann Rankin - Operations Manager
Gunjan Jeswani - Financial Assistant 
Sharon Charles - Clerical Officer

Observatory overview

MVO was rapidly inaugurated in July 1995 with the onset of surface activity at SHV. SRU, in conjunction with USGS-VDAP installed a seismic and tiltmeter network which has since been developed into the multi-faceted monitoring operation which MVO now operates. BGS became involved at MVO several months into the crisis and draws upon internal and UK university staff to assist in operations at MVO in partnership with SRU.

During the current volcanic activity, the primary objective of MVO is to provide real-time monitoring of the eruption and advice on volcanic hazards to the local authorities. MVO also partakes in a wide-ranging programme of information dissemination to both the local population and worldwide.

Organisation

MVO reports directly to the Chief Minister of the Government of Montserrat and to the Governor of Montserrat during the current crisis. The Chief Scientist manages all scientific operations whilst an administrative officer oversees other aspects of MVO. The observatory is currently housed in temporary accommodation in the Old Towne area, although a dedicated facility is planned for the near future, to be located at Flemings, 6 km to the northwest of the volcano.

Monitoring at the MVO

Several methods are used to monitor the activity of the volcano. The main areas are seismic, deformation, volcanological, gas and environmental monitoring. In addition the MVO supports research into the various aspects of the eruption that aid in our understanding of the volcano, and plays an important role in the education of the public about the threats posed by volcanoes.

1. Seismic monitoring

The main type of monitoring comprises measurements of the earthquakes associated with the volcano. The MVO has two seismic networks running concurrently which are positioned around the volcano. Part of the first network was installed by the SRU before the volcano started to erupt, and was subsequently augmented with equipment from the USGS. The second network was installed in October 1996 by the BGS. Both networks work in parallel and complement each other. All the stations are powered by solar panels and car batteries, and the signals are telemetered back to the MVO by UHF or VHF radio. The signals are received at the MVO and recorded onto a computer network. Four of the signals are also written to paper helicorder records to provide an immediate visual record of the current seismic activity. The earthquakes are analysed on the computer network. Five main types of seismic signal have been recognized from the Soufriere Hills Volcano: volcano-tectonic earthquakes, long period earthquakes, hybrid earthquakes, rockfall or pyroclastic flow signals, and explosion signals.

Volcano-tectonic earthquakes (commonly known as VTs) have impulsive starts and then rapidly decrease in amplitude. They are predominantly high frequency signals (>2 Hz). They are interpreted as due to rock fracturing under the volcano, and often occur in swarms. They were common at the start of the eruption and now occur sporadically, often in swarms. Due to their impulsive nature, these earthquakes can often be located. Generally VTs have occurred beneath English' s Crater and at depths up to 7 km. However, early in the eruption, swarms were also located under St. George' s Hill and to the north-east under Long Ground.

Long period earthquakes (LPs) have a more emergent start and generally low frequency content (1-2 Hz). These are interpreted as gas or magma movement under the volcano.

Hybrid earthquakes are, as the name suggests, a mixture between VTs and LPs; hence they tend to have impulsive starts but contain significant amount of low frequency signal. They are thought to represent magma forcing its way to the surface. These signals are often associated with periods of rapid dome growth, and are sometimes precursors to major dome collapses. At peak times up to 3 hybrids have been recorded every minute.

Rockfall or pyroclastic flow signals are one of the most common types of seismic signals. They have an emergent start and a gradual tapering towards the end of the signal; they contain a mixture of frequencies. They are interpreted as due to material falling off the dome and traveling down the flanks. Pyroclastic flows are simply long duration and high amplitude rockfall signals.

Explosions have quite characteristic signals. They often have a long period component (1-2 Hz) at the start of the signal due to the initial gas burst and resonance of the conduit; this signal is then usually subsumed into high amplitude, mixed frequency signal related to pyroclastic flows formed by column collapse. After the pyroclastic flow signal has died away, the long period signal remains, but at a lower amplitude than the initial signal. This is often related to low level ash venting, and can continue for many minutes.

Mudflows also create seismic signals that can be confused with pyroclastic flow signals. These signals are usually highest amplitude at stations close to ghauts, and have a very gradual build-up of amplitude.

2. Deformation monitoring

The MVO uses several different methods to measure the deformation of the flanks of the volcano: Global Positioning System (GPS), Electronic Distance Measurement (EDM), tilt, and crack measurements.

GPS uses an array of satellites to accurately determine the position of the GPS receiver. Hand-held GPS instruments are now common: the MVO has 2 such receivers for fieldwork, and these are usually accurate within a few 10's of metres. However, much better accuracy is required to monitor the deformation of the volcano where movements are of the order of a few centimetres.

The MVO collects data from 6 permanent Leica and Trimble GPS receivers stationed around the volcano. Three of these receivers are the property of the University of Puerto Rico (UPR), and three are the property of the MVO. Collaboration with the UPR has been ongoing since 1995. The data are telemetered to the MVO where it is processed and analysed to provide the positions of these stations with accuracies to about 1 cm. In addition, the MVO has over 20 sites that are occupied by temporary GPS survey stations occasionally to provide a wider coverage of the deformation field of the volcano.

EDM was used frequently in the early stages of the eruption to monitor the deformation of the flanks of the volcano. An EDM instrument shoots an infrared laser beam to a reflective target situated at a few kilometers distance away. Usually, the target will be high on the flanks of the volcano, and is shot from two different sites placed away from the volcano. This means that the surveyor does not need to be in a hazardous site once the target has been positioned. The MVO used to have four main EDM target sites on the flanks of the volcano, but three of these sites are now inaccessible or destroyed due to the volcanic activity.

Electronic and dry tilt has been used periodically during the current eruption. Electronic tilt was particularly successful during periods of rapid dome growth when cyclical activity with time periods of 8 to 24 hours were measured on the tiltmeters, showing repetitive inflation (with associated hybrid earthquake activity and culminating in pyroclastic flows or explosions) and deflation. Measurements of the width of cracks on Chances Peak were made either manually or electronically with an extensometer during 1996 and early 1997.

In general, the majority of the deformation of the Soufriere Hills Volcano is confined to areas close to the crater. Between December 1995 and December 1996 EDM measurements on the eastern flank of the volcano showed that Castle Peak had moved by over 1 m. Other sites close to the dome have moved significantly, for example, Hermitage on the north-eastern flank has moved by over 15 cm to the north-east since August 1996.

3. Volcanological monitoring

Observations of the volcanic activity have comprised a large proportion of the monitoring effort. Many different techniques have been used, but the main parameters that have been measured include the measurement of dome and deposit volumes (and consequently rates of extrusion), the geology and petrology of the new deposits, temperatures of deposits, and measurements of ash cloud heights and speed of pyroclastic flows.

All of these measurements have aided our cataloging and understanding of the volcanic activity, and enhanced our ability to make assessments of the likely hazards from the volcano.

4. Gas and environmental monitoring

Measurements of the gas and ash emissions from the volcano have added to the volcanic risk assessment, and have also been important in mitigating against potential health problems. Two methods of monitoring gas emissions have been used regularly throughout the eruption. The first method uses a Correlation Spectrometer (also called a Cospec) to measure the daily output of sulphur dioxide from the volcano. Using ultraviolet light from the sun, passing through the volcanic plume, the concentration of this gas can be measured. High levels of sulphur dioxide were measured during periods of high extrusion rate of the lava dome. Sulphur dioxide has also been measured using diffusion tubes that are left at sites around the volcano for about 2 weeks. These give a measure of the level of gas at ground level, and can be directly related to the gas levels for people living or working around the volcano. One other method of measuring gas concentrations has been occasionally on Montserrat: Fourier Transform Infrared spectroscopy or FTIR. Using this method the ratios of different gases can be measured, for example, the ratio chlorine to sulphur dioxide.

In addition to gas measurements, the chemistry of rain water has been monitored through the eruption. At sites directly west of the volcano the pH of the rainwater reached as low as 2, i.e., very acidic. This is a result of the rain passing through the gaseous volcanic plume. Ash has also been collected from several sites around the volcano, leading to estimates of the mass of ash erupted for individual volcanic events. For example, on 17 September 1996, it is estimated that 600,000 tonnes of ash fell to the west of the volcano. Periodically, the amount of fine dust (less than 10 micron diameter particles) in the air has been monitored on behalf of the health authorities.