Activity during February was generally very low with only a few rockfalls and volcano-tectonic events recorded. Several ash venting episodes occurred during the first few days of February producing small ash clouds and pyroclastic flows.
The main type of activity during February consisted of short periods of ash venting or small pyroclastic flows (Table 1). The event on 5 February was the largest in the series, and the ash cloud rose rapidly to over 15,000 feet. Pulses of ash originating from pyroclastic flows could be seen on the eastern side of the volcano for several minutes after the start of the event.
Table 1. Dates and times of ash venting episodes
Date Time Comments Est cloud height/feet 5 February 16:12 Pyroclastic flow down Tar River >15,000 7 February 00:26 Venting 8 February 22:59 Small pyroclastic flow/venting 9 February 18:13 Small pyroclastic flow/venting 10 February 03:25 Small pyroclastic flow/venting 10 February 21:49 Small pyroclastic flow/venting 12 February 05:22 Small pyroclastic flow/venting 5,000 12 February 17:44 Small pyroclastic flow
Small mudflows were also recorded on 14 and 27 February.
The seismic signals for the ash venting episodes towards the beginning of the month lasted up to 20 minutes with a gradual decline in amplitude from start to finish. They had energy at frequencies between 1 Hz and 8 Hz.
Table 2. Earthquake types
These earthquake counts are of events that triggered the broadband network's event recording system between 00:00 and 00:00 each day (local time).
Date Hybrid LP Dome RF VT 01 February 99 0 0 15 7 02 February 99 0 0 3 2 03 February 99 0 0 6 2 04 February 99 0 0 5 3 05 February 99 0 0 20 4 06 February 99 0 0 13 17 07 February 99 0 0 19 6 08 February 99 0 0 17 11 09 February 99 0 1 20 2 10 February 99 0 0 16 2 11 February 99 0 1 7 3 12 February 99 0 0 12 6 13 February 99 0 0 6 2 14 February 99 0 0 6 8 15 February 99 0 0 18 13 16 February 99 0 0 13 7 17 February 99 0 0 9 4 18 February 99 0 0 11 5 19 February 99 0 0 16 3 20 February 99 0 0 14 10 21 February 99 0 0 12 8 22 February 99 0 0 13 9 23 February 99 0 0 14 1 24 February 99 0 1 10 1 25 February 99 0 0 11 5 26 February 99 0 0 0 5 27 February 99 0 0 8 10 28 February 99 0 0 7 1
A new EDM network in Long Ground was first occupied on 9 February, and then repeated on 24 February. It is too early to discern any consistent movement in this area.
All the GPS stations on the volcano and in northern Montserrat were occupied this month. All the site positions determined were within the error of the last measurement indicating greatly reduced levels of deformation. The Long Ground and Tar River sites, which moved quite quickly during the last three months of 1998, have slowed dramatically. The GPS receiver at Harris was replaced with a superior model.
Microgravity measurements on lines radial to the volcano were started during February. These lines were last measured in February 1997.
Measurements of daily sulphur dioxide flux were made during February. Results are shown in Table 3. These results show a peak immediately after the pyroclastic flow on 5 February followed by a gradual decrease through the remainder of the month.
Table 3. Average daily sulphur dioxide fluxes measured by miniCOSPEC, February 1999
Date Flux (tonnes/day) Comments 1 February 480 Helicopter 2 February 680 Helicopter 6 February 1120 Helicopter 8 February 960 Helicopter 12 February 670 Helicopter 23 February 690 Helicopter
Sulphur dioxide concentrations at ground level were measured using diffusion tubes; results are shown in Table 4. The high result from the sample point at Weekes is probably due to ash contamination on the tube.
Table 4. Sulphur dioxide diffusion tube results, February 1999. Levels in ppb.
Location 23 Jan to 13 Feb 13 to 25-Feb Police HQ, Plymouth 13.1 22.6 St. George's Hill 5.5 7.4 Weekes 1.7 36.8 Vue Pointe Hotel 1.1 4.3 Lawyers 3.7 4.3
Hazards Assessment Meeting
A meeting of chief scientists took place in Trinidad on 24 and 25 February to assess the current and future hazards posed by the volcano. A summary of the main conclusions of the meeting is shown below.
SCIENTIFIC AND HAZARDS ASSESSMENT OF THE SOUFRIERE HILLS VOLCANO, MONTSERRAT
Preliminary Statement - 24 February 1999
A. Growth of the lava dome ceased in early March 1998 when the magmatic eruption stopped. There has been residual activity and instability over the last year, but no sign of the magmatic eruption starting again. The post-eruptive residual activity has included occasional collapse of the lava dome to generate pyroclastic flows, vigorous venting of ash and gas sometimes accompanied by small explosions and generation of small but mobile pyroclastic flows, occasional swarms of volcano-tectonic earthquakes, and continued ground deformation on the eastern flanks of the volcano. There is also a large amount of loose volcanic debris and ash on the slopes of the volcano, which has been remobilised to form volcanic mudflows during periods of heavy rain.
B. The residual activity over the last year is thought to have been caused by the continued release and ascent of volcanic gases from the source of the magma deeper in the earth. The rise of pressurised gas makes the dome unstable, is associated with the continued low level earthquake activity and is responsible for minor explosions, some pyroclastic flows and the ash venting. Such activity is known at other volcanoes as an eruption is ending and may continue for many years. However, the scale and magnitude of this activity and the associated hazards are expected to diminish with time.
C. The residual activity together with the occurrence of volcanic mudflows during intense rainfall and the substantial amounts of volcanic ash deposits will continue to pose a number of hazards in southern Montserrat.
D. Further collapses of the lava dome and pyroclastic flows generated during periods of ash venting make all the major valleys down the flanks of the volcano vulnerable. The Tar River valley, Plymouth and the north east flanks of the volcano are particularly dangerous. However, it is thought unlikely that pyroclastic flows will extend as far as the largest pyroclastic flows of the eruption. Thus the areas already inundated by flows define a logical zone of high hazard. Pyroclastic flows can be generated with no warning and within the areas identified with high hazard could be lethal.
E. Volcanic mudflows are generated only during periods of intense rainfall and dangerous situations can be easily identified. The situation at the Belham River crossing is no different to many parts of the world where periods of intense rainfall can cause mudflows and floods and populations routinely cope with the situation.
F. It is now a year since magmatic activity stopped and so some of the areas in the current exclusion zone are no longer considered under threat, provided that a magmatic eruption does not restart. Thus careful consideration could be given to cautious reoccupation of certain areas south of the Belham Valley subject to levels of risk that will be identified in the main report. Many of these areas have substantial amounts of fine volcanic ash, which will require prior clean up and further minor ash falls are likely.
G. The volcano will need to be monitored carefully to help minimise exposure to the current residual hazards and to ensure identification of any signs of resumption of magmatic eruption.
MVO Staff Changes
Dr. Gill Norton, British Geological Survey
Dr. Joe Devine, Brown University
Desmond Seupersad, Seismic Research Unit
Mark Davies, Open University
Rob Watts, Bristol University
Chan Ramsingh, Seismic Research Unit
Dr. Sue Loughlin, British Geological Survey
Dr. Brian Baptie, British Geological Survey
Godfrey Almorales, Seismic Research Unit
Chloe Harford, Bristol University
Prof. Barry Voight, Penn State University
Amanda Clarke, Penn State University
Dr. Alexander Belousov, Moscow State University
Dr. Clive Oppenheimer, Cambridge University
Prof. Steve Sparks, Bristol University
Prof. Bill Rose, Michigan Technological University
Uwe Grunewald, Bristol University