Montserrat Volcano Observatory, Montserrat, West Indies

Scientific Report 44
23 November 1996


The level of earthquake activity decreased significantly during the week, and the rate of dome growth appeared to increase, leading to more rockfalls and some small pyroclastic flows into the upper Tar River valley. One volcano-tectonic earthquake swarm occurred, but it was of lower intensity than other recent swarms. Increased instability of the Galway's Wall, part of the southern crater rim, was noticed. The alert level was decreased from Orange to Amber on 19 November.

Visual Observations

Good visual observations of the dome were possible at times during the week, although general cloudiness continued to hamper observations. There were excellent viewing conditions on the mornings of 22 and 23 November.

On 17 November, views of the lower slopes of the October 1 dome were possible, and two rockfalls from the northern side of the dome were observed from the helicopter. These , and subsequent, flows were channelled down the canyon on the eastern side of the dome. For the first time since the start of October 1 dome growth, these rockfalls have extended to about 500 m from the dome, beyond the break in slope at the base of Castle Peak, and small pyroclastic flows have developed in the upper Tar River valley.

Rockfall activity was low until after the volcano-tectonic earthquake swarm ended during the evening of 20 November. Thereafter, most of the rockfalls occurred during the nights. In excellent viewing conditions late on 21 November, the new growth areas on the October 1 dome were clearly visible, indicated by patterns of continuous glow, and occasional falls of glowing material. Glowing rockfalls were observed mainly from the north and north-east faces of the dome, but also from the northwest at times. Many of the rockfalls emanated from an area close to the top of the dome, just below several new, small spines.

The dome has been pale grey in colour, and blocky, since early November. This contrasts with the scoriaceous, smoother, chocolate-brown coloured appearance of the dome in October.

On 22 November, the October 1 dome was seen clearly from the northwest of the volcano for the first time. Two sources of steaming from the dome were noted, one from the south west rim of the September 17/18 explosion scar. The other steam source, to the west of the active dome, was a dark grey colour, and probably contained some ash.

There were further rock slides from the Galway's Wall, to the southwest of the crater, above the Galway's Soufriere. On 23 November, several vertical cracks were noted. These had been seen before, but now appear to be longer, extending down much of the face of the wall. The upper part of the wall now looks unstable. The lava dome and talus have piled up behind this wall to a depth of about 120 metres, with only about 30 metres of wall remaining above this.


Earthquake types: 17 to 23 November 1996

These earthquake counts are of events that triggered the short-period seismic network event recording system between 0000 and 2400 each day.

Date            VT        LP        Hybrid    RF

17 NOV 96       1         1         0         1
18 NOV 96       1         0         0         3
19 NOV 96       22        0         0         0
20 NOV 96       31        2         0         3
21 NOV 96       0         0         0         5
22 NOV 96       1         0         0         7
23 NOV 96       0         1         0         14

The character of the seismicity changed during the week. Since 21 October, the seismicity has been dominated by shallow volcano-tectonic earthquake swarms beneath the crater. In the last week, rockfalls from the October 1 dome became more common, and many of these were large enough to trigger the seismic event recording system. One VT earthquake swarm occurred from 06:27 on 19 November to 23:51 on 20 November. The swarm had 53 earthquakes, and was of lower intensity than previous swarms, although the magnitude of the largest earthquakes remained about the same. Apart from this swarm, VT activity was limited to occasional single earthquakes. The amount of other earthquake activity remained low.

The number of rockfall signals increased towards the end of the week, and the activity was more intense than any previous rockfalls from the October 1 dome. However, the level of activity was still much lower than that in the months prior to the September 17/18 explosion.

Ground Deformation

The MVO Total Station was used to make EDM measurements on 18 and 23 November of the eastern triangle which links Long Ground, White's and Castle Peak. The measurements on 18 November showed a slight lengthening of lines to Castle Peak, within measurement error. Between 18 and 23 November, the lines shortened at a rate of about 6 mm/day, continuing the trend established in July.

Static GPS surveys of the eastern, northern and BIGNET networks were completed during the week, and the results processed in near real-time in Montserrat. The GPS program continues to detect no significant changes in any line.

The University of Puerto Rico permanent GPS network was repaired during the week, with the installation of a new site a Whites to the northeast of the volcano. This replaces the Hermitage station, which was destroyed by the 17/18 September explosion. Data will be collected daily at the Whites and Reid's Hill sites, and transferred to Puerto Rico for immediate processing. Several temporary sites in the UPR GPS network were also occupied.

Gas Measurements

Measurements of sulphur dioxide flux from the volcano were made using the COSPEC instrument on 18 and 19 November. The wind direction made further measurements impossible for the rest of the week. The results show average fluxes of 363 (18th) and 250 (19th) tonnes per day. These values are similar to recent measurements, indicating that only low amounts of sulphur dioxide are emitted from the volcano during periods of dome growth.

Environmental Monitoring

Rainwater was collected and analysed for the period 14 to 17 November. The pH of these samples remained low (3.0 to 3.6), with the exception of Weekes, which was neutral. The concentration of fluoride, chloride and sulphates remained well within WHO guideline values.

Other work Following a marked decrease in the number and intensity of the volcano-tectonic earthquake swarms, and an apparent increase in the rate of dome growth, the alert level was decreased from Orange to Amber on 19 November. MVO scientists judged that the probability of an explosive episode during the following few days had significantly decreased.

MVO staff were involved in public information meetings at Salem Secondary School, the Montserrat Technical College, and in the village of St Patricks.

Staff Changes
Dr Jenni Barclay, Bristol University

Dr Gill Norton, BGS

Prof Ramsey Saunders, UWI
Mr Lloyd Lynch, UWI
Dr John Bennet, BGS

And now for the sports news....

MVO Tremors scored two decisive victories this week over local football teams. In a pre-season friendly match on Sunday, the Tremors beat a scratch team 3-1. In the first match of the Carib Montserrat National Football League on Wednesday, the Tremors had their second victory, beating the defending league champions, Police 3-2.

Montserrat Volcano Observatory