Montserrat Volcano Observatory, Montserrat, West Indies

Scientific Report 02
13 December 1995

The general level of activity at the Soufriere Hills volcano has decreased significantly during the past week, although it is too early to say whether this is a temporary lull in the current phase of effusive activity or another more fundamental change in the overall style of activity. Seismicity has dropped to levels not seen for some time and, apart from low-amplitude tremor related to steam venting, is at background level for this volcano during quiescent periods. Deformation measurements continue to show only very minor changes in the edifice; some of these appear to represent slight reversals of earlier trends. Visual observations were possible at times and showed a slowing, or possible cessation, of dome growth.

The decrease in seismic activity began early on 6 December, with the amplitude of background tremor decreasing and also the number and amplitude of individual long-period earthquakes decreasing. Small, frequent, long-period earthquakes continued until 1500 on 9 December, accompanied by low and variable levels of tremor recorded at the Gages seismometer (MGAT). Tremor on all other stations disappeared by 8 December. The number of located earthquakes has dropped to one or two per day, a level not previously observed during this crisis. The located earthquakes were mostly volcano-tectonic and occurred at slightly greater depths (0-5 km) than the long-period and hybrid-type earthquakes that have dominated since 24 November. The change in seismicity is thought to be related to a slowing of active lava dome growth. High-amplitude, high-frequency tremor recorded at MGAT for several hours during 10-11 December was probably due to an increase in steam venting from several areas on Castle Peak dome.

Deformation measurements have continued throughout the last week. Some major rationalisation of the EDM network has been undertaken, with the establishment of a number of new target sites. A new target placed high on the crater wall in the Gages area is a key improvement, as it will allow monitoring of any possible movement of the crater wall on the west side. However, no measurement of this target has yet been made because of continuous low cloud cover. Other improvements have included the upgrading of a number of single or dual lines into triangles, so that the measuring error can be better understood, as triangles allow a redundancy in measurements. The absolute movement of the targets is now being measured by the recording of eastings and northings relative to one station in the triangle.

GPS data continue to be collected for a 20 hour period each day for an established triangle around the volcano, between St George's Hill in the northwest, Harris' Lookout in the northeast and Radio Antilles in the southwest. An additional station at Roche's Yard was deployed temporarily on 7 December to collect data for that location, last occupied in October.

Both GPS and EDM measurements are showing the same broad trend of a continuous but very low level of deformation of the volcanic edifice. Two electronic tiltmeters have not shown any significant tilt events during this crisis. The latest EDM data show lengthening of the lines to Castle Peak during 10-12 December, indicating some deflation of the edifice. The GPS net is sensitive to changes at a lower level in the volcano structure, and has shown an overall lengthening of over 1 cm in the last week, with some shorter term variability superimposed. This rate is equal to or greater than the average rate during the month of October.

Visual observations have been possible at times, although cloudy weather has restricted viewing of the crater area. A prominent spine on the east side of the new dome continued to grow in height until 7 December when it began to collapse. The maximum vertical growth rate of this spine was estimated to be 5 to 8 m per day. Further dome growth at a slower rate occurred until 9-10 December, after which no obvious dome growth has been observed. Small rock falls from the flanks of the new dome were witnessed by scientists on several occasions, and debris from a larger rock avalanche was seen in the moat of English's Crater at the base of the Gages wall. The rock avalanching to date has not moved material out of the crater area, suggesting that the events were not driven by gas explosion within the dome.

Since 11 December small ash-generating events have occurred on a number of occasions, depositing ash on the upper western flanks of the volcano. This ash has been of reddish brown colour and may contain a significant juvenile component; analyses are being undertaken at present to check this suggestion. Some of these ash-producing events appear to be related to rock avalanching from the new dome, whereas others appear to be the result of single or repeated small explosions in or around the new dome. On 13 December a small, radial crack on the north of the new dome emitted steam and ash; the volume of ash varied but was probably continuous for most of the day. Ash and steam columns in excess of 500 m above the crater rim have occurred at least twice during the past 2 days. The increased level of steam venting and small explosions may be related in part to the heavy rainfall which has occurred in Montserrat since 11 December.

Output of acidic aerosols from the active area appears to have decreased in line with the considerable slowing in dome growth, although no actual measurements of gas or aerosol levels have been taken. Scientists at the summit of Chance's Peak on 7 December encountered a distinct, dense aerosol and gas plume which had largely disappeared by the time of the next visit on 12 December. No data for the gas emissions at Galway's Soufriere have been received this week, although sampling did resume after a break of several weeks during the recent peak in activity.

Montserrat Volcano Observatory