Montserrat Volcano Observatory

Daily Report
Report for the period 16:00 21 December
to 16:00 22 December 1996
The current alert level is ORANGE

Renewed growth of the October 1 dome continues and may have accelerated. There are real dangers of large pyroclastic flows into the Tar River Valley and of a sudden collapse of the Galway's Wall. Both could have serious consequences for much of the evacuated area. The revised risk map is in operation. No-one should be in zone A/B. The only access allowed to zone C/D is for essential purposes and by people with a means of rapid exit. Zone E is safe and the airport should be open.

The rockfall activity from the October 1 dome continued in this reporting period. Shortly before 6 pm last night there were two small pyroclastic flows from the dome. These travelled into the Upper Tar River Valley to just short of the Tar River Soufriere. More rockfall activity was seen during the late evening. The ash clouds generated by these showed little convection, suggesting that relatively cold material was involved. This was confirmed by observations during the night using an infra-red imaging system. The imaging system also showed that the entire October 1 dome was active, with no particular focus of activity. Activity continued today with numerous rockfalls and some small pyroclastic flows from the October 1 dome. These appeared to be coming from different places on the dome. The maximum runout of the pyroclastic flows was 1 km. Many small ash clouds were generated by this activity and the largest, at 1:30 pm, deposited ash in Plymouth. The ash clouds today showed noticeably more convection than those yesterday, which suggests that hotter, fresher material is involved.

A helicopter inspection this morning confirmed that the activity was restricted to the October 1 dome and that there was no sign of activity on the December 11 dome. The Galway's Wall has had some recent landslides and is still regarded as being liable to sudden failure.

Seismic activity was varied during this reporting period. The volcano-tectonic activity recorded yesterday died out in the evening, with only 7 more events being recorded. Sixteen long-period earthquakes were recorded, most of them overnight. Rockfall signals were recorded throughout. The size and numbers of these gradually increased up until about 2 pm today and then reduced noticeably. Repetitive hybrid earthquakes were also recorded. There were a few very small hybrid events yesterday but the activity started about 4 or 5 am this morning with very small events about one every minute. The size of these grew gradually until the end of the reporting period, although the event rate was relatively unchanged. This level of hybrid seismicity has not been seen since April and is probably an indicator of increased dome growth.

EDM measurements were made today on the lines to Castle Peak. These show a shortening of, on average, 6 cm since they were measured two days ago. This is a high rate of deformation, although such large movements have been seen on occasions in the past. Further measurements will be taken tomorrow if weather permits.

Initial processing of the gravity measurements on the eastern flank of the volcano has been completed. Stations on the lower slope show no significant changes since the last measurements in late July. Stations on the upper slope do show some changes, but these are felt to be consistent with the mass that has been added to the dome in that time.

Other tasks carried out today were a survey of the domes using laser-ranging binoculars from the helicopter and the collection of water and air samples down-wind of the volcano.

Dr Willy Aspinall left Montserrat today.

Montserrat Volcano Observatory