Summary
There was a sudden increase in activity on 3 July with a major dome collapse to the east down the Tar River valley. The surge travelled high up the northern slopes of Perches mountain and also extended further to the north over part of the village of Long Ground. There was no precursory seismic activity and in the following weeks no fresh magma reached the surface. Nevertheless, vesicular ballistic blocks were recovered from craters on Perches Mountain suggesting that there may have been a small vulcanian explosion. The SO2 flux measured after this eruption was elevated with fluxes of 1,500-3,000 tonnes per day between July 5 and July 11. These levels declined steadily throughout the rest of July to an average of 1000 tonnes per day. Vigorous ash and steam venting continued from the scar for over two weeks.
Visual Observations
The July 3rd event occurred at 03:02 so visual observations were limited. Ashfall was reported across the west and north of the island and the ash plume was detected by satellite moving east-north-east at 30-45,000ft. Most of the ash fell as accretionary lapilli and deposits reached a maximum of 2mm thick in Salem. Lithics and crystal fragments in Salem reached 3mm in length. Heavy rain followed the event and removed or redeposited much of the ash. Pyroclastic flows were observed in the Tar River valley at 04:22 and steam was observed rising from deposits on the delta at 04:50
The elongate collapse scar was steep sided and was aligned SE - NW with the open SW side directed at Perches Mountain. The collapse removed between 15 and 20% of the dome leaving a canyon like feature extending back deep into the dome. Strong fumarolic activity was observed along a clearly defined north-east trending linear fracture 50-100m long. The deposits are channelled on the southern side of the Tar River valley above the break in slope but lower down the deposits are spread out across both sides of the valley. The ash cloud surge scoured the north side of Perches mountain and also impacted part of Long Ground village. Slightly vesicular ballistic blocks were recovered from small craters on Perches Mountain, all the blocks were similar in appearance suggesting a similar origin.
Ash and steam venting immediately after the event was vigorous. Significant pulse-like steam and ash venting continued for 2-3 weeks and fumarolic activity was evident on the south and north flanks of the dome.
Seismicity
On 3/7/98 a large pyroclastic flow signal was recorded. This signal started at 03:02 and lasted approximately two and a half hours. The maximum amplitude of this flow was several times greater than the last big pyroclastic flows down the Tar River Valley in May 1997. There was no warning of this collapse from the seismicity preceding it. Following the flow there was heightened rockfall and volcano-tectonic earthquake activity until 14:07, when there was a further pyroclastic flow. This time the signal only lasted approximately 10 minutes and the maximum amplitude was an order of magnitude less. In the days following 3/7/98 there were raised numbers of both rockfall signals and volcano-tectonic earthquakes and intervals of tremor associated with ash venting from the scar left by the large collapse.
After the 5/7/98 levels of seismicity returned to similar levels to the previous month, with the exception of a swarm of volcano-tectonic earthquakes on the 25/7/98. This swarm had no outward manifestation at the volcano and activity had returned to low levels by the end of the next day.
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 July 98 5 1 0 13 02 July 98 2 0 0 8 03 July 98 12 0 2 30 04 July 98 8 0 1 27 05 July 98 13 0 0 22 06 July 98 5 0 0 14 07 July 98 0 0 0 0 08 July 98 4 1 0 3 09 July 98 9 0 0 9 10 July 98 8 2 0 11 11 July 98 9 1 0 9 12 July 98 4 1 0 14 13 July 98 7 0 0 30 14 July 98 0 1 0 26 15 July 98 4 0 0 14 16 July 98 4 1 0 10 17 July 98 4 0 0 11 18 July 98 4 0 1 5 19 July 98 6 0 0 19 20 July 98 7 0 0 8 21 July 98 13 1 0 4 22 July 98 5 0 1 8 23 July 98 16 1 0 14 24 July 98 16 1 0 11 25 July 98 75 0 0 0 26 July 98 24 0 0 6 27 July 98 13 0 0 10 28 July 98 3 0 0 7 29 July 98 10 0 1 7 30 July 98 6 0 0 6
Table 2. Swarms
Start Duration Hybrid LP VT 25/7/98 01:32 17.27 0 0 68
Ground Deformation
All the regular GPS sites on the flanks of the volcano and in northern Montserrat were occupied this month. The movement at all sites is greatly reduced. Hermitage continues to move slowly to the north-east at rate of around 0.5cm per month. The GPS site at Perches was destroyed in the July 3rd event. Ballistics were scattered over Perches Mountain, and the GPS site is now on the edge of a 3.4m diameter impact crater.
The GPS kit was used for one week by colleagues from the University of Rhode Island who were conducting a bathymetric survey around the fans at the mouths of the Tar River and White Rivers valleys.
The EDM reflector on Peak B was shot from Windy Hill. The line has now lengthened by 5cm since the middle of May. This may in part have been caused by releasing part of the load in the July 3rd collapse.
Volume Measurements
A kinematic GPS survey of the Tar River fan was completed. Its total volume (not DRE) is presently 22.1 million m3. The previous survey in August 1997 gave a volume of 15.7 million m3. The increase is largely from the July 3rd collapse which extended the fan 350m to the north, although a small part of the increase was due to the accumulation of pyroclastic flow deposits during the Sept-Oct 1997explosion sequence. The eastern limit of the fan was unchanged as it is defined by a steep shelf extending into the sea. A small amount of deposition occurred on the southern side of the fan, although the step above the established shoreline here was only covered by a thin layer of pyroclastic flow deposits.
Environmental Monitoring
Observations with the miniCOSPEC recommenced on 5 July. In the early part of July, SO2 flux was generally between 1000 and 2500 tonnes per day. On 13 July the highest SO2 flux yet was measured at 4150 tonnes per day. Throughout the rest of July there was a gradual decline in SO2 flux to an average of 1000 tonnes per day at the end of the month. The cause of the relatively high gas flux in the apparent absence of magmatic activity is being investigated. It probably relates to perturbations in the hydrothermal system perhaps caused in this instance by the dome collapse on 3 July 1998. Sulphur dioxide diffusion tubes exposed between 29 June and 13 July clearly reflect the high emissions of SO2 in early July. The Plymouth area in particular was subjected to very high concentrations of gas. In the second half of July SO2 concentrations in Plymouth were reduced by half. Populated areas north of the Belham River valley were, as usual, subjected to very low levels of SO2 throughout the month of July.
Table 3. Sulphur dioxide diffusion tube results, 19 May to 29 June 1998.
Levels in ppb.
29 June 98 - 13 July 98 13 - 27 July 98 Police HQ, Plymouth 207.9 116.5 St. George's Hill 22.05 8.55 Weekes 5.75 4.1 MVO south 4.3 3.85 Lawyers 2.2 0
Mudflows continued to be a problem in July. Mudflow deposits built up beneath the Belham Bridge until there was a clearance of only about 30cm.
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 4. 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 6 VA Wind (*) Rain 1 July 98 50 41 47 33 31 Low 1200 moderate breeze None 2 July 98 33 52 54 40 68 Low/raised 1100 moderate breeze, strong gusts None 3 July 98 * 76 218 63 31 Raised/Very high 1100 moderate breeze Early morning torrential rain ** 4 July 98 * 111 314 135 26 Very High/Alert Level 1050 moderate breeze Torrential downpour overnight 5 July 98 - 31 63 36 10 Low 1050 gentle breeze Over night Shower 6 July 98 - 23 45 - 8 Low 750 gentle breeze Brief over night shower 7 July 98 - 32 48 52 - Low/raised 900 still/gentle breeze Brief over night showers 8 July 98 - 48 47 41 8 Low Gentle/moderate breeze Heavy rain in Woodlands area 9 July 98 - 21 24 18 9 Low 950Moderate None 10 July 98 - 21 24 18 12 Low 1000 gentle breeze with gusts None 11 July 98 - 10 26 12 8 Low 1000 gentle breeze None 12 July 98 - * * * * Low 1800 moderate Overnight shower 13 July 98 - 20 27 44 4 19 Low/raised 900 Moderate Overnight shower 14 July 98 - 8 13 0 3 - Low 1000 Still gentle breeze Heavy Showers 15 July 98 - 14 15 45 3 Low 16 July 98 - 50 53 72 23 Low 1200 Moderate Early morning shower 17 July 98 - Low/raised 750 Light breeze Brief overnight shower 18 July 98 - Low 850 Gentle breeze Heavy rain over night and in the morning 19 July 98 - 46 42 35 13 - Low 80 Moderate/ Gusty None 20 July 98 - 34 30 46 57 19 Low 95 Strong Gusty Heavy overnight shower 21 July 98 - 33 29 133 41 21 Low 80 gentle breeze Heavy showers in the morning 22 July 98 - 27 24 24 18 13 Low 100 Strong/Gusty None 23 July 98 - 16 16 17 24 14 Low 110 Gentle with strong gusts Brief morning shower 24 July 98 - 24 * 14 * 35 Low Moderate with strong winds in the afternoon Heavy rain storm in the afternoon 25 July 98 - 24 * 14 * 35 Low Strong gusty winds None 26 July 98 - Low 27 July 98 - 14 22 20 23 21 Low 160 Light breeze with gusts None 28 July 98 - 26 71 15 11 17 Low 85 Light breeze None 29 July 98 - 10 20 19 15 21 Low 95 Light gusty breeze None 30 July 98 - 12 24 25 20 - Low 75 Light breeze None
- not recorded
* Power cut
** Mud flow
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:
Richie Robertson (Seismic Research Unit)
Dr. Ricky Herd (MVO) [after a period of leave]
Dr. Peter Francis (Open University)
Lutchman Pollard (Seismic Research Unit)
Prof. Joe Devine (Brown University)
Costanza Bonadonna (Bristol University)
Sarah Dornan (Institute of Occupational Medicine, Edinburgh)
Dr. Sue Loughlin (British Geological Survey)
Departures:
Christian Antenor-Habazac (Guadeloupe Volcano Observatory)
Dr. Keith Rowley
Dr. Peter Francis (Open University)
Rob Watts (Bristol University)
Colin Walker (Institute of Occupational Medicine, Edinburgh)
Visitors
Marie Edmonds (Cambridge University)
Clive Oppenheimer (Cambridge University)
Tim Druitt (University of Clermont-Ferrand)
Yfan Formenti (University of Clermont-Ferrand)
Dr. Haraldur Sigurddson (University of Rhode Island)
Dr. Steve Carey (University of Rhode Island)
Between 14 and 16 July 1998, a meeting was held at McChesney's Estate, Montserrat to assess the current hazards and risks associated with the Soufriere Hills Volcano. The meeting brought together many of the senior scientists who have worked at Montserrat Volcano Observatory during the three year volcanic crisis.
Those who took part were Richie Robertson (current Chief Scientist at MVO), Lloyd Lynch and Dr. John Shepherd from the Seismic Research Unit in Trinidad; Dr. Simon Young, Dr. Sue Loughlin, Dr. Tony Reedman and Dr. Gill Norton from the British Geological Survey; and many other senior scientists from around the world including Prof. Steve Sparks from Bristol University, Dr. Peter Baxter from Cambridge University, Prof. Barry Voight from Penn State University, Dr. Joe Devine from Brown University, Rhode Island, Dr. Peter Francis from the Open University, U.K., Dr. Keith Rowley and Dr. Willy Aspinall. Dr. Richard Luckett and Dr. Richard Herd from MVO provided up-to-date information about the current status of the Soufriere Hills volcano.
This was the first time that the six monthly assessment meeting had taken place on Montserrat, previous meetings being held on Antigua in December and in Nottingham, U.K. in April. The July meeting was brought forward from the scheduled date in November because of the decrease in the level of activity at the volcano since March.
Lengthy discussion was held on the various aspects of the volcanic activity over the previous 6 months, including the event on July 3. Various specific issues were also addressed including the safety of Bramble airport. An assessment of the level of risks associated with the volcano was undertaken.
A report on the meeting was presented to the Government of Montserrat and the Government of the U.K. on 29 July after which the findings were made public.