Summary
Activity in August was dominated by several small dome collapse events into the White River valley and the Tar River valley and a period of enhanced steam and ash venting in the middle of the month. The dome collapse events were caused by the gravitational collapse of weakened dome rock. The ash venting was intense for about 24 hours but waned over the following days back to normal levels. MiniCOSPEC results show a peak which coincided with the enhanced venting but an overall decline from about 1000tonnes per day at the beginning of the month to about 500 t/d at the end of the month.
Visual Observations
The early part of August was very quiet with little rockfall or earthquake activity. On 13th August there were two episodes of pyroclastic flow activity in the White River valley at 05:19 and at 14:55. The flows travelled 1.8km from the dome and were caused by the collapse of weakened dome rock. Active fumaroles located on the Galways side of the dome near to Chances Peak undermined part of the dome. A scar immediately above the fumarolic area is believed to be the source of the pyroclastic flows. Each episode was followed by about an hour of continuous rockfall activity.
A steep buttress overhanging the July 3rd scar collapsed on August 16th generating pyroclastic flows that reached the Tar River delta. Large fragments of the buttress are strewn about the mouth of the July 3rd scar.
On August 19th fumarolic activity in the July 3rd scar increased in intensity. One particularly vigorous fumarole on the back wall of the scar discharged jets of steam and ash. Another vigorous fumarole in the base of the scar produced copious quantities of steam and ash. This activity continued throughout the afternoon and at a lower level through the next day although by this time the fumaroles were producing mainly steam. The fumarolic activity declined in intensity over the next few days and some of the fumaroles were temporarily buried following a rockfall within the scar on August 20th. By August 22nd the level of activity had declined to the low levels observed in early August.
Seismicity
Seismicity for the whole month was generally low. Activity was dominated by small volcano-tectonic earthquakes located approximately 3 km below the top of the dome and rockfall/pyroclastic flow signals. On the 19th August a rockfall signal was followed by tremor which, from observations, corresponded to vigorous ash venting. This signal lasted for two days with varying amplitude. At times of highest amplitude the tremor was nearly monochromatic at 4 Hz.
Table 1. 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 VT Hybrid LP Dome RF 01 August 98 3 0 0 11 02 August98 10 0 0 10 03 August 98 16 2 0 8 04 August98 3 1 0 13 05 August 98 5 0 0 21 06 August 98 2 0 0 23 07 August 98 4 0 0 17 08 August 98 3 2 0 13 09 August98 8 0 0 15 10 August 98 5 0 1 5 11 August 98 8 0 0 7 12 August 98 8 0 0 8 13 August 98 6 1 0 51 14 August98 7 0 1 14 15 August98 18 3 0 16 16 August 98 9 0 0 27 17 August 98 4 1 0 7 18 August 98 7 0 0 10 19 August 98 8 0 2 13 20 August 98 0 0 2 7 21 August 98 4 1 2 4 22 August 98 8 0 1 8 23 August 98 17 0 2 9 24 August 98 1 0 0 2 25 August 98 5 0 0 2 26 August 98 6 0 0 10 27 August 98 2 0 0 9 29 August 98 8 0 0 3 30 August 98 9 0 0 7
Ground Deformation
GPS occupations were made of all the sites this month. The rates of movement of sites are now extremely small and are within the instrumental error. For instance, measurements of the line length between Harris and Whites have ranged by 2.5mm since April this year.
The EDM reflector was shot from Windy Hill. The line continues to lengthen slowly and it has now recovered to within 1cm of its original length. The line initially shortened by 9cm between January 25 - May 13. Since mid-May the line has lengthened by 8cm possibly indicating a relaxation in the confining pressure.
Volume Measurements
No volume measurements were completed this month. Several attempts were made to survey the July 3rd collapse scar, although these were all foiled due to deteriorating weather conditions during the survey and the helicopter running out of fuel.
Environmental Monitoring
Mini COSPEC results throughout early August showed a consistent SO2 flux of about 500 to 1000 tonnes per day. On August 19th levels rose to about 1400 t/d as a result of the enhanced venting. Towards the end of the month poor weather limited the number of COSPEC runs attempted but there appeared to be a slight decrease to an average of about 500 t/d. Throughout late August the wind direction was variable because of tropical storms in the area. On occasions when the wind blew to the north or north west a strong smell of sulphurous gases was detected in the inhabited part of Montserrat.
Sulphur dioxide diffusion tubes (Table 2) showed a general decline in levels of SO2 in the atmosphere since the July 3rd event. An extra site in the north of the island was installed to assess the levels of SO2 the population were exposed to during the shifts in wind direction.
Table 2. Sulphur dioxide diffusion tube results. Levels in ppb.
27 July 98 - 11 August 98 Police HQ, Plymouth 131.5 St. George's Hill 9.55 Weekes 2.85 Vue Pointe Hotel 3.25 Lawyers 3.8
Air Quality Monitoring
The air quality monitoring programme during this period involved 5 permanent monitoring sites. These were located at:
Results are shown in Table 4.
Table 3. PM10 as measured by Dust Trak instruments (concentration in microgramme/m3).
These values are approximate 24 hr averages of the PM10 concentration. Aerosol values measured at each of the monitoring sites are accompanied by a note on the level of volcanic activity (VA) and prevailing weather conditions.
Date 1 2 3 4 5 VA Wind (*) Rain 1 August 98 15 25 21 20 24 Low 85 Gentle breeze None 2 August 98 ** ** ** ** ** Low 80 Gentle breeze None 3 August 98 12 16 16 30 13 Low 75 Light breeze None 4 August 98 28 25 14 18 5 Low 100 Blustery Heavy showers overnight and during the afternoon 5 August 98 13 19 7 * 10 Low 85 Light gusts Heavy showers+ 6 August 98 * * * * 10 Low 45 Light breeze Overnight shower 7 August 98 * * * * 29 Low 100 Gentle breeze Overnight shower 8 August 98 17 19 7 12 1 Low 95 Light breeze Heavy showers 9 August 98 13 16 24 10 16 Low 45 Breezy Heavy overnight showers 10 August 98 * * * * * Low 60 Strong gusty Overnight shower 11 August 98 75 75 30 25 68 Low 110 Light breeze None 12 August 98 62 57 23 17 49 Low 50 Breezy Overnight shower 13 August 98 44 45 18 13 42 Low 95 Gusty None 14 August 98 24 57 10 21 27 Low 90 Light breeze None 15 August 98 10 14 5 12 9 Low 140 Light breeze Heavy showers 16 August 98 22 6 5 - 14 Low 100 Strong breeze Showers 17 August 98 25 16 14 12 17 Low 90 Still/slight breeze Heavy overnight shower 18 August 98 16 13 7 21 16 Low 75 None 19 August 98 25 17 11 17 31 Elevated 190 Strong gusts Heavy showers 20 August 98 20 15 9 12 23 Elevated 5 Light breeze Heavy showers 21 August 98 18 * * 15 * Elevated 200 Strong gusts Overnight shower 22 August 98 38 - 27 10 12 Elevated 170 Gusty None 23 August 98 ** 24 ** 14 ** Low 190 Gusty Heavy showers 24 August 98 10 12 Low 90 Gusty Showers 25 August 98 30 17 Low 95 Strong blustery winds Overnight showers 26 August 98 18 5 Low Gusty winds Passing shower 27 August 98 ** 3 Low 85 Moderate winds Intermittent showers 28 August 98 40 26 Low 100 Gentle breeze None 29 August 98 65 35 Low 75 Light breeze None 30 August 98 26 19 Low 52 Light to moderate Overnight shower
* Power cut
** Dust traks removed
+ Mudflow
24 Hour averages: <50 microgramme/m3 Low 50-100 microgramme/m3 Raised 100-300 microgramme/m3 Very high >300 microgramme/m3 Alert
MVO Staff Changes
Arrivals:
Paul Cole (Luton University)
Lucy Ritchie (Luton University)
Desmond Seupersad (Seismic Research Unit)
Steve Sparks (University of Bristol)
Simon Young (British Geological Survey)
Departures:
Richie Robertson (Seismic Research Unit)
Gill Norton (British Geological Survey)
Joe Devine (Seismic Research Unit)
Lutchmann Pollard (Seismic Research Unit)
David 'Patch' Silcott (University of West Indies)
Levar Cabey (University of Lancaster)
Timothy Christopher (University of West Indies)
Visitors:
Peter Baxter (University of Cambridge)
Tim Druitt (University of Clermont Ferrand)