Montserrat Volcano Observatory, Montserrat, West Indies

Scientific Report 28
12 June 1996


Poor visibility for most of the week prevented good observations of the volcano, although a switch in the focus of dome growth led to the rapid filling of the moat area behind the Gages Wall early in the week so that the first material to overtop the crater wall avalanched into the upper parts of Fort Ghaut late in the week. Seismicity was at a generally low level, and the rockfall count was somewhat lower than last week. Deformation and gas production both remain at low levels.

Visual observations

Visibility at the start and end of the week was very poor, but two good days of visibility in the middle enabled good observations to be made. Views on 9 June revealed that a switch in the focus of dome growth had occurred, as suspected from a reduction in the frequency of rockfalls on the northeast flank over the previous week or so. Material was seen to have filled the moat adjacent to the Gages Wall to within 15 or 20 ft of the top of the wall and, by the following day, the moat was filled totally and some material had begun to avalanche out of the crater into the uppermost reaches of Fort Ghaut. Rockfall activity was seen on a broad sector of the dome from the northeast all the way around to the west, and also, fresh rockfall material was seen in the southern fork of the Tar River to the south of Castle Peak.

Good nighttime visibility on 9 June enabled observations of the areas of most intense incandescence, which coincided with the source areas for rockfalls. Abundant steaming and gas emissions were noted from a number of places on the dome, although there was no obvious increase over normal levels. Only poor views of the top of the dome were possible during the week - a pair of new spines were noted in the western summit area on 9 June; one had fallen over and was resting on the talus slope below by the following day.

Ash generation during the week was at a generally low level, with few ash clouds visible out of the crater area.


Seismicity at the volcano has again been dominated by low to intermediate levels of rockfall signals; these are at a generally lower level than during last week. The hybrid swarm of the middle of last week has not been repeated during this week, and long period earthquake activity has been at a similar level to the previous few weeks. Only one volcano-tectonic earthquake was recorded during the week - this was located at a depth of c. 6 km in the Roche's Yard area. Broadband tremor was at a generally low level, with extended periods of low-amplitude tremor coinciding with heavy rainfall. A period of higher amplitude tremor on the Gages station during the late morning of 6 June was generated by a sediment-laden flood in Fort Ghaut.

The table below lists the daily counts of these different types of seismic activity.

Earthquake types 6 June - 12 June 1996

Date     VT        LP        Hybrid         RF        Tremor Amount

06/06         0         13        4         49        Low to intermediate
07/06         0         0         1         13        Low to intermediate
08/06         0         0         1         51        Low to intermediate
09/06         0         3         1         54        Low to intermediate
10/06         1         15        2         54        Low to intermediate
11/06         0         12        5         87        Low to intermediate
12/06         0         2         1         59        Intermediate

A regional earthquake, magnitude 5.2, was recorded by the local network on 11 June; it was located 95 miles SSE of the Dominican Republic.

Ground Deformation

EDM measurements were made this week on the eastern, northern and western triangles. The eastern triangle continues to show the largest movements, although these are still at relatively low rates: c. 12 mm of shortening was recorded on the Long Ground to Castle Peak line for the week, suggesting a slowing of the enhanced rate noted in the last scientific report. The western and northern triangles continue to show no changes in line length above the error of the method.

The GPS programme remained on hold for the week due to continued technical problems.

The electronic tiltmeter at Long Ground did not record any tilt events during this week, continuing the trend of the past 10 months.

Gas Measurements

SO2 flux measurements were made during the week using a COSPEC correlation spectrometer which was mounted in a car and driven between Cork Hill and St. Patrick's under the gas plume. The results of the measurements for the week are given in the table below, as the mean and standard deviation of all the runs made each day. The results show some variability but are consistently low, with a mean value of 141 tonnes per day.

COSPEC measurements of SO2 flux

Date     Number    Mean SO2 flux  Standard
         of runs   (tonnes / day)      deviation
07/06         3         84             54
08/06         5         269            103
09/06         4         126            39
10/06         5         59             21
12/06         5         168            39
Additional measurements

Gas measurements were made at Galway's Soufriere during the week. Rainfall collectors and SO2 tubes were installed at a number of locations to the west and northwest of the volcano to monitor air and water quality.

Staff changes
Christian Antenor-Habazac, IPG Volcano Observatory, Guadeloupe
Lloyd Lynch, Seismic Research Unit, Trinidad
Mark Davies, Open University, UK

Adam Maciejewski, Open University, UK
Richard Robertson, Seismic Research Unit, Trinidad

Montserrat Volcano Observatory