Montserrat Volcano Observatory, Montserrat, West Indies

Scientific Report 16
20 March 1996


The seismicity over the last week has been at a low level, and of similar character to that observed over previous weeks. Efforts have continued in classifying the long-period seismicity and attempting to determine the source mechanisms. The number of true long-period earthquakes has increased slightly, and large long-period events (magnitude greater than 1) varied from 1 to 13 events per day. Some of these events appear to correlate with rockfalls from the dome, although further detailed observations are required. A large number of higher-frequency, very emergent signals were also recorded, which clearly correlate with rockfalls. Some of these rockfalls involve large volumes of material, and excite seismic signals that are recorded by most of the stations in the network. There is still no evidence of an explosive component of these rockfalls; instead, they appear to be driven gravitationally.

A total of 16 small volcano-tectonic earthquakes have been located. These occurred beneath the crater and north and west of the crater at St George's Hill, Windy Hill and Harris', at depths of 1-3 km.

Deformation measurements

Three of the four EDM triangles were occupied at least once during the week and, in addition, the line from Tar River to Castle Peak was also re-occupied for the first time in 3 weeks. All of the lines measured showed continuation of the trends which have been established over the past 4 months or so. The lines to the Castle Peak reflector continue to shorten by c. 1 mm per day, although this week's readings showed more scatter than normal. The other two reflectors, at Farrrell's and Chances Peak, are both moving at a similar or slower rate, although the scatter on these data is generally higher. Re-occupation of the Gages Wall triangle was attempted, but the high ash production had covered the reflector and conditions were not suitable for its cleaning until the end of the reporting period.

The Long Ground electronic tiltmeter continues not to show any deformation events.

Visual observations

Visual observations were possible for most of the week from the helicopter and from the Chances Peak and Farrell's lookouts. Rapid vertical growth of the northwestern part of the dome closest to the Gages Wall stopped early in the reporting period and, since that time, activity has begun to concentrate in the eastern sector of the dome. Rock falls have been occurring at variable frequency throughout the week, with 17 March being especially active. The larger rock falls are producing significant, though low-level, ash clouds which are blown on the wind towards the west. The ash is generally grey in colour and is very fine-grained.

Night viewing of the dome, especially from the eastern side of the island later in the week, shows that the rock falls are hot and that incandescent material is unroofed during each minor avalanche event. However, there is still very little evidence for explosive activity associated with dome growth or ash production. An exception during this period was an ash column seen to c. 7,000 ft on 17 March which is interpreted as being at least partially driven by explosive processes. Viewers at the Farrell's lookout were covered with a cloud of warm ash following a rock fall event on 17 March; this event had no noticeable explosive content.

Steam production as well as SO2 from the dome has continued at a constant level throughout the period, although we do not as yet have any quantitative method of measuring gas flux from the vent areas. Steam and the prevailing easterly winds are combining to pick up loose ash from the eastern flank of the dome and incorporate it into the plume even without distinct rock fall events to stir up the ash.

An inspection visit on to the Gages Wall on 20 March enabled scientists to review in detail the composition of the wall and collect samples for engineering tests. Material piling up at the internal foot of the wall is now about 40 to 50 ft beneath the top of the wall, although activity has currently stopped in this sector. Initial, qualitative views on the strength of the wall are encouraging, and no failure is expected without a great increase in pressure from within the crater.


A radical switch in the area of dome growth during this period makes estimation of growth rate difficult, although it does appear to be continuing at the relatively high rate recorded last week. Seismicity remains relatively low and deformation consistently slow. An increase in ash production by rock fall events is of some concern, although it appears to be due primarily to the switch in area of activity rather than an increase in explosions. Steam and gas venting appears to be continuing at a constant rate.

Montserrat Volcano Observatory