1Montserrat Volcano Observatory, Montserrat, West Indies
2British Geological Survey, Edinburgh, UK
A state-of-the-art broad band seismic network is now installed at the Soufriere Hills Volcano, Montserrat. Initially, the network will be used in parallel with the existing short-period network, providing high-quality data for the analysis of seismic activity associated with the current eruption. It will eventually replace the existing network and be used as part of the permanent monitoring facility to be established on Montserrat. The data from the network will be made available to interested groups.
There are eight stations in the new network. These are deployed around the volcano at distances between 1.5 and 6 km from the active crater. Station locations have been largely dictated by access and telemetry considerations. Five of the stations use three-component broad-band seismometers which record ground velocity over a frequency range from 0.03 to 30 Hz and with a dynamic range of 145 dB. The other three stations use short-period seismometers which record ground velocity between 1 and 30 Hz with a dynamic range better than 100 dB. Signals from all the sensors are digitised at the stations using 24-bit digitisers. The digital data is telemetered to Montserrat Volcano Observatory (MVO) using UHF radio links and dedicated telephone lines.
At MVO, the data is interpolated to provide synchronised samples across the network, time-stamped using a GPS clock, and recorded using the SEISLOG data-acquisition system. Both triggered and continuous data are recorded. A number of different trigger set-ups can be used simultaneously, tailored for different types of seismic events. The data are transferred automatically to a Sun SPARC 5 workstation using Ethernet.
SEISAN software is used for routine analysis of events which trigger the data-acquisition system. The locations of volcano-tectonic earthquakes are stored in a database. The continuous data are stored on DAT tapes.
The data collected to date are of excellent quality. Since the network was installed in October 1996, there have been many tens of volcano-tectonic and rockfall events which the new system has detected and recorded as full waveform time histories. The first result of note is that the long-duration signals from Gages that have previously been identified as broad-band tremor are relatively narrow-band, with energy only between 4 and 30 Hz. This is consistent with the previous interpretation of a very localised source, probably steam venting on the dome or the nearby Gages Soufriere.
Like all other seismic data-acquisition systems, this system is primarily intended for locating shear-type earthquakes. It has no facilities for the analysis of signals from the other types of events recorded at volcanoes and which, at Soufriere Hills, make up the majority of the seismicity. Planned developments include: