Montserrat Volcano Observatory, Montserrat, West Indies

Scientific Report 18
03 April 1996

General situation

At 06:52 on 03 April 1996, an explosive signal was recorded by the local seismic network, which signified a major change in the eruptive style of the Soufriere Hills volcano. The level of activity increased during the following hours, and peaked with two pyroclastic flows in the Tar River area at 15:18 and 18:12 on 03 April. There were few precursors to this sudden change in activity; the number of explosive signals increased slightly, but the general level of seismic activity was low in the preceding days, and no significant deformation was detected.

Seismicity

The seismicity during this period was dominated by rockfall signals and a small number of explosive events. The number of rockfall signals that triggered the event recording on the seismic network increased during the week, to between 30-50 events per day. Many of these rockfalls were recorded at most of the seismic stations. Large rockfalls occurred on 31 March for about one hour from 20:43. The signal was composed of a series of pulses, including two probable explosive events, and was associated with a series of block and ash flows.

Explosive events are a new development during the current crisis, and the build-up of events prior to the first major pyroclastic flow is summarised in table 1. The explosive events are characterised by large-amplitude impulsive signals that follow a 5-7 s emergent, low-amplitude initial signal. They are recorded at all the stations on the network, and often are powerful enough to saturate the Windy Hill seismometer, which is about 3 km from the crater. The event at 06:52 on 03 April was followed by a period of nearly continuous seismic activity, including many small rockfall signals and hybrid earthquakes.

The number of long-period earthquakes was lower than during the previous week, and did not show any significant build-up prior to 03 April. Hybrid earthquakes were also recorded, with the rate of occurrence decreasing during the period, from 40 to less than 10 events per day. Eight volcano-tectonic earthquakes were located, mostly scattered to the east and north of the volcano at shallow depths (<3 km).

Deformation measurements and observations

The eastern, southern and western EDM triangles were measured at least once during the week. The line from Tar River to Castle Peak was also remeasured on 31 March. The lines to the Gages Wall could not be measured due to continued low cloud and haze. None of the EDM lines showed major changes. The lines continue to show slow deformation, and there is some indications that the rate of deformation has slowed in the last ?? days.

The Long Ground electronic tiltmeter did not show any significant deformation events during the period.

Table 1: Summary of explosive events prior to 15:18 03 April pyroclastic flow 

Date    Time    Coda length     Comments
		on MWH2 (s)
27Mar96 1725    95 31Mar96 0129    90      Followed by 140 s low amplitude
signal
	2043            Extended signal (1 hour) with two "impulsive"
	events:
			2110 for 90 s and 2127 for 60 s.
01Apr96 0116    85
	1030    60      Small event- didn't clip on MWH2 02Apr96 0606
85      Clipped MWH2 for 30 s.  Ash to 10,000 ft
	0849    70      Smaller, more emergent 2230    60 03Apr96 0017
60      Smaller, more emergent
	0652    80      Clipped MWH2 for 30 s.  0808    60      Smaller,
	more emergent 0916    45 1229    180     Followed by second
	similar pulse at 1236 1244    60      Smaller, more emergent
	1308    60      Smaller, more emergent 1428    120 1516    60
	1518    180     Clipped MWH2 for 40 s.  Pyroclastic flow in Tar
	River

Visual observations

Visual observations have been made throughout the week from the helicopter, from viewpoints in the east and from Chances Peak. The activity has been concentrated in the northeastern part of the dome, and the upper Tar River / Hot River valley. Following the initial rockfalls out of the crater described in the last scientific report, there were two quiet days (28-29 March), before further significant rockfalls into the upper Tar River. The largest of these was a series of events in the evening of 30 March, which lasted for about one hour. Observers in the Long Ground area saw extensive glowing avalanches from the eastern dome, and the resulting block and ash flows extended about 1 km from the dome and caused the burning of trees down to the Tar River Soufriere. Some large blocks, probably resulting from spine collapses, were deposited at the base of the slope. An ash plume caused by this event drifted west and caused heavy ashfalls in Plymouth, probably reaching a maximum thickness of about 2 mm. Further rockfalls occurred during 01-02 April and resulted in further block and ash flows that did not extend as far as the 30 March event, and ash plumes up to 10,000 ft.

At 06:52 on 03 April, an explosive event started a period of intense activity. During an observation flight in the morning on 03 April an open fissure was seen in the eastern dome for the first time, and appeared to be the source of continuous ash emissions. This fissure was at the top of the slope where recent rockfalls had occurred. During the day, further explosive events and rockfalls resulted in almost continuous ash production. The ash was carried to the north and northwest.

A major explosion and pyroclastic flow occurred at 15:18. Observers in the helicopter and at Whites had an excellent view of the development of this flow. Material bounced off the ridge that forms the northern boundary of the Tar River valley, causing extensive scorching of vegetation there and setting fire to the sulphur in the Tar River Soufriere. Some scorching of vegetation also occurred over the lip of this ridge, although none of the buildings nearby were affected. The flow extended 1.9 km down the Hot River Valley as far as the road, about 400 metres from the Tar River Estate house. A strongly convecting ash cloud rose to about 25,000 ft. The flow scoured out a channel in the upper part of the slope. Further rockfalls and pyroclastic flows occurred from the same area during the rest of the day, including another major flow at 18:12.

The rest of the dome was fairly quiet during this period, and no further growth was observed in the western dome near Gages Wall. Early in the week, a spine grew then collapsed in the southwest dome.

Summary

This period has been the most active of the current volcanic crisis, and a major change in the eruptive style of the Soufriere Hills Volcano has occurred. There was a build-up in the number of explosive seismic events prior to major explosions on 03 March, which resulted in several pyroclastic flows to the east of the volcano extending 1.9 km into Hot River ghaut. The number of earthquakes remains low, and slow deformation of the volcano continues.

Montserrat Volcano Observatory