Government Information Service

The Volcanic Explanation
For The 24 Hour Period Ending 4 PM On Thursday July 17, 1997
A Presentation Of The Government Information Services
In Conjunction With The MVO.

The alert level system has been revised by zones.

The level of activity at the Soufriere Hills Volcano was at a lower level than yesterday (Wednesday) and was dominated by rockfall signals.

The weather cleared somewhat today and the volcano could be viewed from the helicopter and on the ground. There were several new rockfalls seen on the eastern and northern sides of the dome. Three pyroclastic flows occurred during the early morning hours in Mosquito Ghaut and extended less than 1 km. The spine over Gages Wall collapsed and is the only major change in the northern and western sides of the dome since July 13. The scar left after the June 25 event is almost completely filled in and there is still a large blocky extrusion over Gages Wall. There was considerable steaming from the top of the dome on the eastern side over Tar River. The highest point of the dome is now on the new growth within the scar and measures 948 m or 3110 feet. An estimate of the current size of the dome will be made from photographs taken from five points around the volcano.

There were 17 rockfalls today (Thursday) this is less than yesterday. Four long period earthquakes, 2 of which triggered rockfalls. There were no hybrid or volcano-tectonic earthquakes recorded. The level of tremor on St. George's Hill decreased during the day. A new repeater site on Jack Boy Hill was installed so that signals from Long Ground and Roche's Yard can now be received at the Observatory.

The EDM line between Waterworks and Lees Yard has still not shown any changes since 8 July when it was set up. The results of the GPS survey taken yesterday show that the lines from Harris Lookout, Whites and Long Ground shortened by 16 mm and 13 mm respectively and the lines from Windy Hill to Harris Lookout, Whites and Long Ground lengthened by 22 mm, 24 mm, and 33 mm respectively. The comparisons were made to a survey done on June 24. Chances Peak tiltmeter showed a very low amplitude peak during the past 24 hours and a continuation in the long term deflationary trend.

COSPEC measurements were made in static mode from the top of Garibaldi Hill, the data will processed and the results published at a later date.

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.

Mr. George Skerritt rejoined the observatory team after a training course in Hawaii and a short holiday.

Government Information Service