Montserrat Volcano Observatory, Montserrat, West Indies

Scientific Report 07
17 January 1996

Continued dome growth accompanied by variable but generally low levels of seismicity has characterised events at the Soufriere Hills volcano over the past week. Understanding of the various seismic signals being recorded especially at the most proximal seismometer has improved as visual observations continue from various locations around the flanks of the volcano, at the crater rim and from the helicopter. Deformation continues but is generally below the level of detection on a day to day basis.

Seismicity early in the week was dominated by low to moderate amplitude, broadband tremor that was recorded only at the Gages seismometer. An extended period of repetitive, hybrid events up to approximate magnitude 2 began at 15:50 on 12 January and continued at variable levels of both amplitude and frequency of occurrence until about 15:00 on 15 January. These events were located at depths between 0 and 3 km beneath the Soufriere Hills volcano, although most were probably occurring at very shallow depths beneath the active dome. At the maximum frequency of occurrence, two events per minute were observed. This series of hybrid events appeared to be considerably smaller than the similar series of the previous week. Since that time, variable levels of low amplitude, broadband tremor have been recorded, along with a slowly increasing number of small long-period events that have only been recorded at the Gages seismometer, with a few of the larger ones being observed at the Long Ground seismometer also. Throughout the week, there have been very few volcano-tectonic earthquakes recorded, but there have been a number of identifiable rock fall events which have produced ash falls to the west of the volcano. Minor ash falls were reported in the Plymouth area on the 12 January at 10:26, 11:22 and 18:38 respectively. Other ash falls were reported on the 15 January at 15:20 and the 16 January at 12:59.

One regional event was recorded by the local network during the week; this was located approximately 240 km from Montserrat. It occurred at 16:59 on 11 January and had a magnitude of 3.8.

Deformation data continues to be collected on a daily basis by EDM and electronic tiltmeter. The EDM network rationalisation continued with the completion of the final of four triangles around the volcano. The final triangle, between O'Garra's, Galway's Plantation and a fixed target close to Chances Peak, completes coverage for the southwest flank of the volcano. On a day to day basis, the EDM measurements do not show any changes in slant distance above the error of the method. However, taken over a longer time period, a slow shortening is occurring on many of the lines towards the volcano, indicative of continued, very slow swelling of the edifice. The tiltmeter at Long Ground to the east of the volcano continues not to show any measurable deformation.

Visual observations have been made on most days during the week. A new spine first noted on 10 January continued to grow rapidly within the central part of the new dome until collapsing on 13 or 14 January. A new spine then appeared in a location slightly to the north, spalling material into the northern part of the moat. These and previous individual spines appear to be associated with an increase in dome growth rate within a confined area of the new dome, and the area of most rapid growth appears to switch every week or so. In addition to the increased spalling of material into the northern moat, an increase in rock falls from the southwestern part of the dome suggests that activity may also be increasing in this area. The September spine has decoupled and been pushed southwards by new dome growth so that it is now perched precariously above the moat and is slowly disintegrating. New material was also noted for the first time piling up against the crater wall beneath Chances Peak during the week. Steam emissions from the crater area were generally at a high level, with most steam coming either from the new dome or from one of the older phreatic vents close to Castle Peak which has not yet been disturbed by new dome growth.

Light ash falls have occurred to the west of the volcano on several occasions during the week. These are associated with large rock falls from the new dome. Observations of rock falls suggest that they produce ash from both stirring up ash from the surface over which they are falling as well as from self-disintegration. Very minor ash emissions have also been noted from the dome itself without any associated rock falls. Small explosions may occur prior to some of the larger rock falls, but there have still been no substantial pyroclastic flows produced during dome growth, and all new material except for very light ash falls is confined to the crater area.

Fieldwork has continued on the eastern and western flanks of the volcano, with a number of charcoal samples collected from various deposits. These will be dated in the near future and some significant increases in the understanding of the last eruptive episode at Soufriere Hills volcano should result.

In summary, this week has seen the continuation of slow dome growth within English's Crater, accompanied by varied seismicity including long-period and repetitive hybrid events as well as low to moderate levels of broadband tremor.

Montserrat Volcano Observatory