Montserrat Volcano Observatory, Montserrat, West Indies

Scientific Report 04
27 December 1995

Activity at the Soufriere Hills volcano has stayed constant over the last week. New dome growth has been occurring in a number of areas on the western side of the old Castle Peak Dome, and the growth and subsequent collapse of several spines has occurred. Seismic activity has continued at a low rate, with occasional large long-period earthquakes and a small number of volcano- tectonic events located beneath the volcano. EDM measurements have in general shown only small changes.

Seismic activity has continued at a similar rate to last week. Locatable earthquakes have occurred at a rate of 0 to 8 per day. These volcano-tectonic earthquakes have been scattered widely over the southern part of the island, although they have been clustering beneath the volcano in the later part of the period under review. Depths have been in the range of 0 to 7 km. The small, frequent long-period earthquakes recorded by the Gages seismometer have continued at a constant rate throughout the week, although the amplitude of these events has been variable. Occasional periods of low-amplitude, broadband tremor have also been recorded; these are thought to be associated with periods of increased steam venting in the crater area. The number of large long- period earthquakes has increased again from last week to a rate of 11 to 25 per day. Some of these earthquakes have been accompanied by small ash clouds that are visible from the observatory and with rock avalanches from areas of new dome growth witnessed from the east side of the volcano. It is thought that some of the large long-period events represent surface waves generated by small explosions at the surface within the crater area.

A total of five regional earthquakes have been recorded by the local seismic network; the largest was a magnitude 3.9 quake at 05:11 on 22 December, located at a depth of 23 km some 50 km NE of Guadeloupe.

All of the EDM lines around the volcano were measured at least once during the past week. None of the lines have shown changes in slant distance greater than the error of the data on a day to day basis. The Gages Wall reflector has been stable during this period; the 4 cm lengthening reported last week was an artefact of atmospheric corrections and not real deformation. Re-occupation of the original Tar River to Castle Peak EDM line on 24 December showed that Castle Peak has not moved in the 3 weeks since the last occupation of the Tar River site.

Occupation of the GPS network continued throughout the week, although the data have not yet been processed. The Harris' receiver was moved to Tar River on 24 December for the final few days of the current occupation; the Tar River - St George's Hill - Reid's Hill triangle now has a very long baseline of data beginning immediately after the start of phreatic activity at the volcano in mid-July. The Reid's Hill receiver was moved to Chance's Peak for a day on 26 December in order to remeasure the line lengths to St George's Hill and especially Tar River; this triangle has shown the largest line length changes during the crisis.

The electronic tiltmeter at Long Ground continues to show no significant change. The tiltmeter at Spring Hill is currently off-line.

Good visual observations were possible throughout the week from the air, from Chance's Peak and from the east side of the volcano. The large spine in the centre of the easternmost area of new dome growth collapsed on 21/22 December with no spillage of debris out of the crater area. A new spine grew rapidly adjacent to this collapsed spine on 23/24 December, reaching a maximum height of c. 15 m higher than Castle Peak before collapsing late on Christmas Eve, again with no spillage of debris out of the crater area. This whole area of the new dome continues to grow slowly and another new spine was seen on 26 December in the western part of the crater.

Small avalanches of glowing rock have been reported by nighttime observers on the east side of the volcano throughout the week, but no material has yet fallen from the old Castle Peak Dome area. Observations of small to moderate rock falls have been made by scientists during the day; the larger of these rock falls produce significant ash which occasionally drifts over the western crater wall. A series of explosions at about 11:00 on 21 December produced a mildly convecting ash cloud to c. 1500 m above the volcano and ash fall to the north, reaching the northern part of the island. This event was more vigorous than any other seen since the cessation of major phreatic eruptions in early to mid November, and appears to have been driven magmatically.

Steam production has been at a constant rate throughout the period, with a well-defined plume being seen drifting away from the summit crater in the wind. This plume occasionally contains small amounts of ash.

Analysis of SO2 tubes placed in the Plymouth area west of the volcano for a period in early December has shown that gas levels are at a very low level in inhabited areas. Analysis of rainwater collected over the same period shows that acid aerosols are being flushed out of the steam plume by rain, and the vegetation damage on the western flanks of the volcano appears to be due mainly to acid rain. The most recent observations of vegetation damage in the worst- affected area suggests that acid aerosol levels may be reduced from those early in the month.

Active dome growth continues almost aseismically across a growing area of the old Castle Peak Dome. A number of spines have grown rapidly (5 to 10 m/day) to an equilibrium height and then have stopped growing and/or collapsed. The equilibrium height is very similar to the height of Castle Peak itself, suggesting a similar magmatic driving system to that which produced the Castle Peak Dome c. 350 to 400 years ago. Dome growth has been essentially non-explosive, and the gravitational collapses from the flanks of the dome have not yet reached out of English's Crater.

Montserrat Volcano Observatory