Government Information Service

The Volcanic Explanation
For The 24 Hour Period Ending 4 PM On Sunday July 20, 1997
A Presentation Of The Government Information Services
In Conjunction With The MVO.

The alert level system has been revised by zones.

There was no change in the level of activity at the Soufriere Hills Volcano today (Sunday) and rockfalls were the main type of seismic signals recorded. The weather has been overcast most of the day and prevented viewing of the dome.

27 rockfalls, none of which were of high amplitude or long duration and 3 long-period earthquakes were also recorded. There were no volcano-tectonic, or hybrid earthquakes. Shortly after midnight 2 periods of moderate low amplitude tremor were recorded on the St. George's Hill and St. Patricks seismometer, probably caused by mudflows in Fort Ghaut or Aymers Ghaut. The Chances Peak tiltmeter is not showing any cyclical behaviour.

Readings from the GPS stations located at Lees Yard, Garibaldi Hill, and Waterworks were carried out today. The results will be posted later in the week. The EDM measurements from Garibaldi Hill and Lees Yard and measurements from Waterworks will be used to see if deformation is occurring on the northern flank of the volcano. The EDM line from Waterworks to Lees Yard was measured and shows no major change daily, but the line has shortened since it was set up on 12 July 1997.

The recent pyroclastic flows and ash eruptions have not been associated with an increase in seismicity or tiltmeter readings. Anyone entering the exclusion zone is at great risk. Further pyroclastic flows are most likely to occur in Mosquito Ghaut and Gages Valley but pyroclastic flows could also occur in Tuitts, Tar River Valley and White River. There has been no warning of the pyroclastic flows in Gages Valley and this makes Plymouth very dangerous. Belham River Valley could be the sight of pyroclastic surges or hot mudflows. These mudflows are at or near boiling point and travel very fast and may go further than pyroclastic surges. A reminder to wear your ash mask if there is ash in the air.

Government Information Service