General Situation
The eruptive activity which began at 06:52 on 03 April 1996 with a small explosion, started a period of near-continuous seismic activity in the form of small explosions, continuous tremor on the closest seismic station on Chances Peak, and near-continuous ash emission from an area on the eastern dome. After a reassessment of the situation by the scientists at the Montserrat Volcano Observatory at 13:00, the civil authorities were advised to start an immediate evacuation of the southern part of Montserrat, with the demarcation line being that for the 01-02 December 1995 relocation. At 15:18, an eruption occurred which generated a significant pyroclastic flow in the Tar River valley area and an ash plume which rose to about 20,000 ft (~6 km) asl. Further pyroclastic flows in the same area were generated at 18:08 and 18:18 on the same day. Significant eruptions occurred at 14:45 on 06 April and 13:54 on 08 April, generating slightly taller ash columns and smaller pyroclastic flows in the Tar River area than that of 03 April. No inhabited areas have been affected so far by these pyroclastic flows.
Seismicity
Seismicity during the period was dominated by both rockfall signals and hybrid earthquakes. Numerous rockfall signals of various sizes were recorded during the early part of the week but the numbers decreased considerably from 07 April to the end of the period under review. Small hybrid-type earthquakes were recorded consistently from the morning of 04 April 1996 and became the dominant form of seismicity from 07 April to the end of the period under review. The rate of occurrence of these events varied from a minimum of 1 or 2 per 5 minutes to a maximum of about 5 per minute. The amplitudes of events within periods of the order of several hours also varied. These hybrid earthquakes were located beneath English's Crater at shallow depths (<2 km).
Several short duration episodes of low amplitude broadband tremor were recorded at most of the nearest stations of the seismic network, especially the Gages station, during the period.
The number of seismic signals interpreted as small- to moderate-sized explosions also varied daily but began to show a significant decrease towards the end of the review period. Excluding those associated with the major eruption sequences on 06 April and 08 April, the event of 06:59 on 07 April was probably the most outstanding. It was a moderately-sized explosion which was heard at the W.H. Bramble Airport Control Tower about 6 km northeast of English's Crater and caused an ash column that drifted towards the northwest, depositing ash over the St. George's Hill, Cork Hill and Fox's Bay areas.
Several regional earthquakes were recorded by the MVO seismic network. The largest was a moderate event which occurred at 12:55 on 08 April. This earthquake had a duration magnitude of 4.9 and was located at a depth of 180 km close to Dominica.
Less than 10, generally small, long-period earthquakes were recorded during the week, a number considerably less than that of last week. Seven volcano-tectonic earthquakes were located, mostly scattered beneath the volcano at shallow depths (<3 km).
Deformation Measurements And Observations
The eastern, southern, northern and western EDM triangles were measured at least once during the week. However, the lines to Gages Wall could not be measured because of ash on the reflector. None of the EDM lines showed major changes but most continue to show slow deformation. For example, the Long Ground - Castle Peak slant distance continues to shorten at an average rate of about 1 mm per day.
The Long Ground electronic tiltmeter did not show any significant deformation events during the period.
New Global Positioning System (GPS) equipment recently brought to the island was tested on 08 April and is being used to measure and remeasure the positions of stations in an established network of lines around the volcano. This GPS network will be integrated with the EDM network.
Visual Observations
Visual observations have been made throughout the week from the helicopter and from various viewpoints on the ground around the volcano under a variety of conditions. Activity in the form of rockfalls was mainly concentrated in the northeastern and southwestern parts of the dome, with material from the northeast falling into the upper Tar River / Hot River valley area.
The activity of 03 April resulted in pyroclastic flows which slightly overtopped the northern embankment of the Tar River valley but caused no destruction to property in the nearest village, Long Ground (see Report No. 18). Fires started by these pyroclastic flows continued for several days in the Tar River area. Several smaller explosions and rockfalls occurred during the period 04-05 April, and some of these generated ash clouds which were carried by the wind and deposited in Plymouth and environs. The most significant of these was a moderately strong explosive ash eruption which took place at about 12:53 on 05 April and produced an eruption column which rose to about 5000 ft and a small pyroclastic flow into the Tar River valley.
A series of eruptions starting at 08:39 on 06 April generated ash plumes with a maximum height of about 10,000 ft (~3 km) and also caused at least six separate small pyroclastic flows which carried material from the eastern dome into the Tar River area. From 13:37, the activity level increased again, with continuous ash emission occurring from the eastern dome coupled with several ash plumes. At 14:45, a significant explosive eruption began and continued for about one hour. It consisted of two main pulses which sent ash to about 30,000 ft (~ 9 km) asl and generated a relatively large pyroclastic flow. This flow, however, did not travel as far as the one at 15:18 on 03 April. The ash cloud drifted to the northwest. Several small to moderate eruptions which produced ash columns and possibly small pyroclastic flows in the Tar river Valley also occurred at 16:17, 16:45 and 17:32 on 06 April.
On 08 April, another series of eruptions, including two large explosions at 13:54 and 13:57, occurred. During this period, near-continuous pyroclastic flows occurred into the Tar River valley, and several large ash clouds which drifted out to sea, depositing very little on land, were generated. These eruptions were of a similar size as those of Saturday afternoon (06 April). The pyroclastic flow did not reach as far as the flows on Wednesday 03 April, but some trees in the Tar River valley were set alight. A new spine was observed close to the centre of the dome in the morning of 04 April. At mid-morning on 05 April, its height was measured at about 2716 ft (828 m) asl. On 06 April, it had grown to about 2975 ft (~ 906 m) asl and was visible from many points around the island. By 07 April, the spine had grown taller than Chance's Peak (the highest topographic feature on Montserrat - 3002 ft). However, during the night of 07 April, the top half of the spine broke off but the remnant continued to grow from the base throughout 08 April so that once again it became higher than Chance's Peak in the later part of 08 April and much broader than previously. This spine is the largest seen so far during the present period of dome growth at the Soufriere Hills volcano.
In general, the dome also seems to have continued growing vertically around the central area. However, there was no sign of change in the northwestern and northern parts of the dome, including that abutting the crater under Gages wall.
Continuous emission of variable amounts of steam and sulphur dioxide occurred throughout the period.
Summary
This period has been the most active of the current volcanic crisis, following in the same style as last week's activity. Two significant eruptive episodes occurred on 06 and 08 April, generating ash columns to c. 30,000 ft and several pyroclastic flows into the Tar River valley. The seismicity during the early part of the week was dominated by rockfalls but from 07 April, hybrid earthquakes increased in frequency. Deformation is still continuing at a slow rate.