Summary
Activity was at a low level throughout the month of June. The largest signals recorded on the seismic network corresponded to mudflows. A moderate size pyroclastic flow occurred on 30 June and sent an ash cloud drifting westwards over Plymouth.
Visual Observations
Activity has generally been at a low level during this reporting period. A clear view of the dome from Jack Boy Hill on 16 June showed that there had been no obvious changes to the dome morphology since it was last seen clearly on 14 May.
A moderate size pyroclastic flow occurred on 30 June and was observed to travel down the Tar River valley. Ash from this event drifted westwards at low altitude over Plymouth. A later observation flight confirmed that a new thin flow lobe had been deposited on the north side of the Tar River valley and reached as far as the Tar River estate house. There were also some new deposits in the upper reaches of the White River valley.
The main phenomena observed during this reporting period were mudflows. Substantial events were recorded by the seismic network on 3, 4, 9, 11, 15, 16, 18, and 21 June. Detailed observations were made of the mudflows on 3, 15, 16 and 21 June. The observations are summarised in Table 1. The mudflows are actually hyper-concentrated stream flows, although when the start of an event is observed, the initial pulse consists of more viscous concentrated mud with a larger amount of debris.
Table 1. Summary of observational data recorded for mudflows at Belham Bridge, June 1998.
Note:
1: time is start of observations at Belham Bridge.
2: signal height is measured on helicorder records at Lees Yard seismic station
Date Time1 at start Signal height2/mm Max flow rate m3/s Max temp/degrees C Rainfall intensity /mm/h 3-Jun-98 19:40 49 56.3 ~45 6.2 15-Jun-98 10:45 50 282.3 ~30 >4.1 16-Jun-98 14:43 24 50.3 28.4 - 21-Jun-98 06:42 38 19.2 28.3 -
The mudflow on 3 June was observed to be steaming although measurements of temperature were not taken. The peak of the event on 15 June was not observed but flow rates were estimated from the height to which water left marks on trees, the width of the channel and using the maximum velocity from 3 June. Water from the peak flood on 15 June reached over the Belham Bridge as evidenced by pumices on the roadway and small pumice fragments on the top of the central pillar. The flood also travelled over the road along the north side of the golf course. Velocity data from the events on 16 and 21 June show a rapid acceleration at the beginning of the mudflow and also suggest that there are at least three pulses of flow, as material from separate tributaries is added to the main river course.
Seismicity
The largest seismic signals in this period were those associated with mud flows on the flanks of the volcano - the most energetic of these clipped the nearest stations for tens of minutes. Qualitatively the amplitude and duration of the signals recorded corresponded to estimates of flow volume made by observers.
Seismicity was otherwise generally low for the reporting period with intervals of scattered volcano-tectonic earthquakes alternating with intervals of almost complete quiet. An exception to this was the swarm on 4th-5th June. This swarm consisted of many small signals, most of which did not trigger the networks.
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 VT Hybrid LP Dome RF 01 June 98 4 0 0 0 02 June 98 5 1 0 1 03 June 98 5 0 0 3 04 June 98 19 2 0 12 05 June 98 34 0 0 2 06 June 98 2 0 0 2 07 June 98 2 1 0 0 08 June 98 2 0 0 2 09 June 98 3 0 1 1 10 June 98 5 1 0 3 11 June 98 4 0 1 1 12 June 98 3 0 0 1 13 June 98 5 0 0 2 14 June 98 12 0 1 4 15 June 98 5 0 0 1 16 June 98 0 0 0 3 17 June 98 5 0 1 2 18 June 98 10 0 0 1 19 June 98 2 0 0 2 20 June 98 3 0 0 1 21 June 98 4 0 1 2 22 June 98 6 0 0 3 23 June 98 2 0 0 4 24 June 98 1 0 0 2 25 June 98 12 0 0 0 26 June 98 0 0 0 2 27 June 98 1 0 0 1 28 June 98 1 0 0 1 29 June 98 2 0 0 1 30 June 98 2 0 0 11
Table 3. Swarms
Start Duration Hybrid LP VT 04 June 98, 20:06 8.5 1 0 40
Ground Deformation
GPS occupations of most of the sites around the volcano and in the north of the island were made by MVO and the University of Puerto Rico during this reporting interval. The movement of sites on the volcano has reduced dramatically. The last three occupations of Whites (June 9, May 16 and April 15) reproduced site positions lying within 2mm in easting, 1mm in northing and 3mm in height. The last six occupations of Windy Hill (June 9, May 3, April 15, March 4, January 15 and December 18 1997) have shown site positions lying within 6 mm in easting, 8 mm in northing and 39 mm in height. A significant departure occurred at the Broderick's site of 4 cm to the north between mid-November 1997 and mid-January 1998. This is the period associated with rapid growth of the dome in the Galway's region close to the Galway's site. It appears to be stable now and the last occupations show site positions plotting within a few mm of one another. A site in Galway's Estate was re-established and we hope to occupy this site more often to extend coverage of the volcano.
Volume Measurements
Unfavourable weather conditions did not allow an accurate GPS/Binocular survey of the summit area and upper flanks of the whole dome complex to be undertaken. This will be attempted when an opportunity arises.
Environmental Monitoring
Results from analyses of sulphur dioxide diffusion tubes were received during this period. The results are shown in Table 4. Results suggest that sulphur dioxide was still being derived from the dome at significant levels, despite the lack of growth. In general there has been a slight increase in sulphur dioxide levels measured by the diffusion tubes since the beginning of the eruption. A blue SO2 haze continued all month and has been easily visible from MVO (south) where it has been seen to extend over Gages, Chances and Galway's Wall.
Table 4. Sulphur dioxide diffusion tube results, 19 May to 29 June 1998.
Levels in ppb.
19 May - 1 June 98 1 - 13 June 98 13 - 29 June 98 Police HQ, Plymouth 21.50 22.75 29.10 St. George's Hill 6.85 12.65 7.80 Weekes 7.05 5.70 6.75 MVO south 4.40 5.00 3.20 Lawyers 2.60 3.80 8.90
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 5.
Table 5. 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. *: power cut, -: incorrect calibration
Date 1 2 3 4 5 VA Wind (*) Rain 1 June 98 17 - 16 14 10 Low 100/light breeze Heavy 08:00 downpour 2 June 98 23 - 18 24 22 Low 100/breezy Heavy midday downpour 3 June 98 16 - 15 19 9 Low 170/still/gentle breeze Torrential downpour from 16:00 to 19:00: medium-sized mudflow down Belham Valley with light associated ash fallout. 4 June 98 21 - 19 18 9 Low 210/very gentle breeze None 5 June 98 13 - 24 19 11 Low 135/gentle breeze Torrential downpour from 19:00 to 20:00: small mudflow down Belham Valley. 6 June 98 31 - 30 22 19 Low 110/gentle breeze None 7 June 98 * - 51 * 16 Low 110/strong breeze None 8 June 98 16 - 21 16 9 Low 090/strong breeze None 9 June 98 10 - 15 13 8 Low 110/light breeze,gusty Brief showers during day 10 June 98 15 - 21 13 11 Low 130/strong breeze,gusty Brief showers overnight 11 June 98 13 - 18 14 - Low 100/mod, gusting strong Heavy voernight showers 12 June 98 23 - 38 30 - Low 135/moderate, gusting strong Few overnight showers 13 June 98 29 - 46 38 - Low 120/gentle breeze None 14 June 98 17 - 24 22 30 Low 120/still/light wind None 15 June 98 26 - - - - Low 120/moderate breeze Torrential downpour all morning, mudflows down Belham Valley. 16 June 98 6 - 21 16 17 Low 120/gentle breeze Torrential downpour from 14:00 to 15:30. 17 June 98 31 - 49 38 56 Low/raised 115 - light breeze, strengthening all day None 18 June 98 30 - 41 32 23 Low 115/light breeze Brief overnight shower 19 June 98 21 - 34 26 10 Low 125/moderate breeze, gusting strong Brief shower 06:00 20 June 98 31 - 65 58 50 Raised 110/moderate breeze, gusting strong Brief shower 06:00 21 June 98 19 - 43 - 8 Low 120/gentle breeze, strengthening all day 05:00 to 06:00 downpour, medium-sized mudflow down Belham Valley. 22 June 98 15 - 38 21 19 Low 100/moderate breeze Short overnight showers 23 June 98 13 - 33 - 40 Low 100/gentle breeze None 24 June 98 8 12 15 22 8 Low 105/gentle breeze None 25 June 98 8 19 18 15 16 Low 110/moderate breeze None 26 June 98 18 40 41 34 41 Low 110/moderate breeze None 27 June 98 15 31 30 25 - Low 110/moderate breeze Brief 07:00 shower 28 June 98 46 102 133 76 79 V. high 120/moderate breeze Showers overnight 29 June 98 63 147 185 108 118 V. high 105/moderate breeze Showers overnight 30 June 98 51 114 144 80 85 V. high 120/moderate breeze None
Low volcanic activity has kept aerosol levels low, with heavy downpours and showers maintaining the low dust levels. Aerosol levels generally increase quickly again after rain as a result of the rapid drying out of the ash and the moderate winds which re-suspend particulate matter into the air. The ashfall associated with the mudflow down the Belham Valley on 3rd June was a result of water contacting hot pyroclastic flow deposits producing collapse of deposits into stream flow and possibly generating small phreatic explosions. The ash was then carried downwind and recorded by scientists at the Belham Bridge; there was no affect on aerosol levels on 4th June. Dust levels were 'very high' on 28th, 29th and 30th June as a result of dust from the Sahara being blown over the island.
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:
Dr. Ricky Herd (MVO) [from a period of fieldwork in St. Vincent]
Dr. Keith Rowley
Christian Antenor-Habazac (Guadeloupe Volcano Observatory)
Departures:
Prof. Steve Sparks (Bristol University)
Dr. Ricky Herd (MVO) [on leave]
Dr. J-C Komorowski (Guadeloupe Volcano Observatory)
Desmond Seupersad (Seismic Research Unit, Trinidad)
Visitors:
John Bennett (British Geological Survey)
Alison Searl (Institute of Occupational Medicine)
Andy Nicholl (Institute of Occupational Medicine)