Montserrat Volcano Observatory

Daily Report
Report for the period 16:00 20 December
to 16:00 21 December 1996
The alert level has been reinstated to ORANGE

The level of alert was reduced to Orange at 6 am today. The pyroclastic flow activity which prompted the increase to Red on Thursday evening did not escalate. However the volcano remains in a dangerous state. Volcano-tectonic earthquakes and rockfalls continue and there is evidence of reactivation of the October 1 dome. The rapid switching of styles of activity is also of concern. The perceived dangers are pyroclastic flows into the Tar River Valley and a sudden collapse of the Galway's Wall. This would have disastrous consequences for much of the evacuated zone. The revised risk map is still in operation and there should be no-one in zone A/B. Access to zone C/D is restricted. Zone E is safe and the airport should be open.

Observations from the helicopter and the ground today suggest that there has been a reactivation of growth of the October 1 dome. There were numerous small rockfalls from the October dome, some of them large enough to generate small ash clouds. The largest of these was at 2:41 pm and drifted slowly towards the south. Most of the rockfall activity was on the northern side of the October 1 dome although were some smaller rockfalls from the southern side.

Measurements were made today on the two large cracks on Chances Peak. One of these continues to show movement, although the rate of deformation has slowed down. An extensometer and two tiltmeters were installed on one of the cracks. These transmit data by radio to MVO and should reduce the need to visit Chances Peak. The extensometer measures changes in the width of the crack and the tiltmeters will detect movement on the Galways side of the crack. The EDM reflector on Chances Peak was cleaned of ash at the same time.

Seismicity in this period was a mixture of VT earthquakes and rockfalls. 90 VT earthquakes were detected by the recording system. The majority of these were located at shallow depths beneath the crater area and had signals similar to those of the recent swarm activity. The swarm of VT activity which started yesterday afternoon had died down by 7:30 pm last night, although there were 3 small, slightly deeper VT earthquakes at about 10:30 pm. VT activity started again this morning at 03:45 am at a much lower level. This activity continued until the end of the reporting period. 21 rockfall signals were recorded today. One of these was from an small avalanche from the Galway's Wall. The rest were from the dome and many were associated with observed rockfalls on the October 1 dome. One long-period earthquake was recorded.

Some theodolite measurements were taken today on the October 1 dome. These will measure if there is any growing of the dome associated with the renewal of rockfall activity.

GPS measurements were made today at the gravity stations on the eastern flank of the volcano.

Richard Herd left Montserrat this afternoon at the end of his tour of duty. Angus Miller also left for a short break in the UK.

Montserrat Volcano Observatory