Volcanic activity at the Soufriere Hills volcano during this period continued at the same level as yesterday. Seismicity continues to be dominated by small- to moderate-sized rockfall signals. The largest of these occurred at 10:47 and 11:15 on 20 May. The 10:47 rockfall was associated with a small pyroclastic flow which travelled as far as Hermitage and generated a small ash cloud.
There were 5 long-period events and no VT earthquakes recorded by the seismic network today. The small hybrid events which started to appear on the Chances Peak seismograph on 18 May and peaked in numbers yesterday continued to occur at an average rate of about 2 per minute, but with very low amplitude. The low-amplitude broadband tremor which has been recorded on the Gages seismograph since yesterday morning increased slightly in amplitude at about 05:34 this morning and was near-continuous up until 08:30. The tremor was more intermittent up until 13:30, and then continuous from that time onwards.
Visibility was better today, although the dome was still shrouded in cloud most of the time. After 15:00, the clouds thinned out, and the entire dome was visible for short periods from a number of locations. There were significant changes since the last sighting on 11 May. The dome now has a large broad spine at its top. It is not possible to say whether this is a new spine or due to growth of a pre-existing spine. The spine rises about 60 ft above the dome and is slightly inclined to the northeast. Several other small spines were observed through partial cloud cover during a helicopter inspection. There was no indication of the source area of the pyroclastic flows on 12 May. This is possibly because the rockfalls in the last week have smoothed out any scarp-like features. During the short periods of observations, rockfalls were observed on the active north and northeast flanks of the dome. Occasional rockfalls were also seen on the southeast flank, an area that was steaming vigorously. The sulphur deposits around the fumaroles appear to have grown.
The increase in visibility today meant that it was possible to make EDM measurements on the eastern triangle. The line lengths on this triangle have show about 7-10mm of shortening since they were last measured on 11 May. GPS measurements were made on the eastern traverse of the MVO GPS network. The data from this survey has not yet been processed. The height of the top of the spine was estimated at 3081 ft, surveyed from the ground. Data obtained from the University of Puerto Rico GPS network on the 16-17 May has been processed. These indicate that there were about 2 cm of extension on the line lengths between Roche's Yard-Reid's Hill and Roche's Yard-Harris since October 1995.
Further gas measurements were made using COSPEC. This data has not yet been processed. The data collected yesterday shows an SO2 flux of about 200 tonnes per day, which is consistent with recent results.
The present level of activity at the Soufriere Hills Volcano continues to cause concern to the scientists. The MVO urges that visits to the evacuated zone are kept to a minimum. The Tar River, Long Ground and Whites areas are extremely dangerous and should not be entered under any circumstances.