Summary
Volcanic activity has continued at a low level, with the occurrence of minor rockfalls from the dome. No major changes in dome morphology have occurred. Seismic activity has also been very low with occasional minor swarms of volcano-tectonic earthquakes.
Visual Observations
Activity over the last 2 weeks has been limited to a few small sporadic rockfalls down the upper eastern flanks and the summit area of the Galway's dome on the south-western sector of the dome complex. A brief window of clear visibility around the dome highlighted that there were no changes to the overall morphology of the dome confirming that no fresh growth of the dome has occurred since ca. 10 March when the large spine near the summit of the Galway's dome was extruded. A small pyroclastic flow was observed on the afternoon of 19 May originating from a steep rib of old dome lava exposed on the upper flanks of the eastern side of the dome-similar events were observed in the previous reporting period. The flow travelled ca. 200 m short of the Tar River Estate House and the whole event lasted a few minutes generating very minor amounts of convecting ash and leaving a fine-grained, thin deposit in the now well-developed chute trailing down from the upper flanks. A smaller pyroclastic flow occurred the same night probably originating from the same area.
Temperatures of the block and ash flow deposits formed during the 21 September 1997 collapse were measured in the Trant's area on 13 May. The maximum temperature was found to be 350 degrees Celsius at a depth of 2 m highlighting the fact that they have not cooled since they were last measured on 28 April.
Seismicity
Over these two weeks activity has continued to be low with intervals of complete quiet alternating with intervals of sparse volcano-tectonic earthquake activity. The intervals of earthquake activity have been many hours long with events usually occurring more than an hour apart. Only occasionally have events been close enough to constitute a swarm (Table 2).
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 10 May 98 6 0 0 6 11 May 98 24 1 0 14 12 May 98 21 1 0 2 13 May 98 2 0 0 1 14 May 98 4 0 0 6 15 May 98 8 0 0 0 16 May 98 32 0 0 4 17 May 98 5 0 0 0 18 May 98 4 0 0 1 19 May 98 10 0 0 3 20 May 98 11 0 0 5 21 May 98 14 0 0 1 22 May 98 5 0 0 1 23 May 98 8 0 0 4
Table 2: Swarms
Start Duration Hybrid LP VT 11 May 98 08:10 1.52 0 0 11 11 May 98 19:31 1.83 0 0 5 16 May 98 18:06 5.72 0 0 30
Ground Deformation
GPS occupations were made of sites at Whites, Old Towne, Dagenham, Blakes, Lookout Yard and Drummonds, with the fixed stations at Harris, Whites Yard, Hermitage, MVO and South Soufriere. With respect to Harris, the sites at Dagenham, Old Towne and Lookout Yard show an increase in height of around 5 cm since June 1996. Most probably this is due to slight subsidence of the site at Harris and the other sites on the east of the island. The site at Dagenham shows a departure in its XY position of around 4cm to the NE since June 1997. We are confident that this is not due to instability of the site at Harris which has also shown a small departure to the north of around 2cm with respect to sites in the north of the island. The movement of Hermitage continues, but appears to be slightly reduced. Preparations began for the new continuous site at Spring Estate. This will provide MVO with a triangle of fixed sites around the volcano (Harris, South Soufriere Hills and Spring Estate) together with the UPR sites (Hermitage, Whites Yard and at MVO).
The peak B reflector on Farrell's Wall was shot from Windy Hill. Its movement appears to have stopped.
Volume Measurements
There have been no volume surveys during this reporting period. A comprehensive GPS/Binocular survey of the summit area will be undertaken when the viewing conditions are favourable.
Environmental Monitoring
Results from analyses of sulphur dioxide diffusion tubes were received during this period. The results are shown in Table 3. Results suggest that sulphur dioxide was still being derived from the dome at significant levels, despite the lack of growth. A distinct sulphurous odour was smelled in the north of the island on 10 May as a result of a change in wind direction to the north. A blue SO2 haze continued all fortnight and has been easily visible from MVOsouth where it has been seen extending over Gages, Chances and Galway's Wall.
Table 3. Sulphur dioxide diffusion tube results, 17 March to 19 May 1998. Levels in ppb.
17 Mar 98 - 17 Apr 98 17 Apr 98 - 3 May 98 3 - 19 May 98 Police HQ, Plymouth 21.65 18.35 6.45 St. George's Hill 9.50 7.30 19.65 Weekes 2.95 6.95 0.90 MVO south 2.00 4.10 0.00 Lawyers 1.00 2.35 0.00
Localities of Dust Trak monitoring sites:
The air quality monitoring programme during this period involved 6 permanent monitoring sites. These were located at:
Table 4 : PM10 (concentration in mg/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 6 VA Wind (*) rain 10/5 Power outage 11/5 13 - 10 18 18 11 Low 300 None 12/5 12 - 9 9 12 15 Low 085 None 13/5 13 - 9 13 14 - Low 135 None 14/5 21 - 14 25 18* 24 Low 135 None 15/5 46 - 26 41 - 24 Low 100 None 16/5 31 - 26 23 - 25 Low 090 None 17/5 28 - 25 25 - 27 Low 100 None 18/5 34 - 28 24* - 24 Low Still None 19/5 43 - 38 - - 34 Low 130 None 20/5 78 86 67 - - 42 Raised 125 Brief shower overnight 21/5 57 51 40 - - 18 Raised 130 Heavy shower overnight 22/5 29 29 29 - - 18 Low 110 6am brief downpour 23/5 12 15 17 - - 12 Low 120 None
*Collection from site stopped
Comment:
Low volcanic activity has generally kept aerosol levels low and with no rain from 11 to 20 May, conditions became increasingly dry and dusty. On 20 and 21 May, levels reach the 'Raised' air quality bracket which was the result of the dry and breezy weather conditions over the previous 9 days. This was reduced to 'Low' again by 22 May as overnight showers dampened down the surface ash. It is believed that the small PF in Tar River Valley on 19 May would not have affected the air quality as this was a very small and cold flow and little ash was generated from it. The new Dust Trak site in central Salem, where about 100 people are now living despite being in the official exclusion zone, roughly imitates conditions at MVOsouth in Old Towne.
24 Hour averages: <50 mg m-3 Low 50-100 mg m-3 Raised 100-300 mg m-3 Very High >300 mg m-3 Alert
MVO Staff Changes
Arrivals:
Steve Sparks (Bristol University)
Departures:
Tim Druitt (Universite Blaise-Pascal, Clermont-Ferrand, France)
Patrick Tuchais (Martinique Volcano Observatory)
Willy Aspinall (Aspinall and Associates)
Kate Moore (Bristol University)
Brian Baptie (British Geological Survey)
Ricky Herd (MVO)
Visitors
Mike Higgins (Universite Blaise-Pascal, Clermont-Ferrand, France)
Current MVO scientific staff list on 24 May 1998:
Chief Scientist: Steve Sparks (Bristol University)
Deputy Chief Scientist: Gill Norton (British Geological Survey)
Billy Darroux (MVO)
Richard Luckett (British Geological Survey)
Joseph Pops Morris (MVO)
Desmond Seupersad (Seismic Research Unit)
George Skerritt (MVO)
Rob Watts (Bristol University)
Colin Walker (Edinburgh)
Dave Williams (MVO)