Volcanic activity at the Soufriere Hills Volcano was generally low although there was a series of small explosive eruptions from early- to mid-January. Some of the explosive eruptions generated substantial ash clouds and were followed by short periods of ash venting. Brief episodes of ash venting, correlating with seismic tremor, occurred throughout the reporting period but became shorter and weaker towards the end of the month. Small volume pyroclastic flows occurred periodically, the majority were generated by dome collapse but some flows may have been generated by fountain collapse during the small explosive eruptions. The average sulphur dioxide flux was elevated throughout January. Eastward movement of the Long Ground and Tar River GPS sites continued.
There were minor ash venting episodes on 1 and 5 January.
At 03:58 on 7 January, a large long period seismic signal immediately preceded a 30 minute episode of tremor (usually associated with vigorous ash venting). Apart from a sprinkling of ash around the vent no fresh deposits were observed following this event. Later the same day, a small dome collapse event occurred generating a pyroclastic flow that travelled half way down the Tar River valley. This collapse generated a low level ash cloud that was carried westwards over Plymouth.
At 06:30 on 13 January, an explosive event generated an ash cloud to 20,000 ft and a pyroclastic flow that travelled down the Tar River valley to the sea. The onset of the seismic signal had a long period component and a pressure wave was recorded at a pressure sensor at Long Ground. A booming sound was reported by a number of people. Ash venting continued for about 30 minutes after the initial event corresponding to seismic tremor signals. The pyroclastic flow deposit in the Tar River valley was small in volume but its extent suggested that it was very mobile. Narrow, small volume pyroclastic flow deposits, as far as the former position of Galway's Soufriere, were also observed on the southern side of the dome following this event.
Two small dome-collapse pyroclastic flows occurred on 14 January. At 08:27 on 15 January a small explosive event generated an ash cloud that rose to at least 15,000 ft. The cloud moved to the north-west and light ashfall affected Salem and Old Towne. Pulse-like ash venting occurred for over 15 minutes after the event.
Another small explosion occurred on 16 January generating an ash cloud to over 10,000 ft. A number of rockfalls were triggered by this event especially on the inner walls of the July 3rd scar and on the outer south-east and north-east flanks of the dome. Weak ash venting and associated tremor continued for almost 15 minutes after this event.
A minor dome collapse pyroclastic flow occurred on 20 January and almost reached the sea at the Tar River delta. The low ash plume generated was very steam-rich and dissipated rapidly.
There were several brief (20 minutes) episodes of tremor corresponding to weak ash venting on 24 January. Each episode of venting was preceded by a rockfall. Further short episodes of ash venting occurred on 25 and 27 January.
Very clear conditions on 26 and 27 January enabled MVO staff to survey the dome. The canyon, which has been incised through the dome, was clearly visible. It bisects the dome in a north-west to south-east direction from the top of Tar River Valley to the top of Gages Valley. The inner walls of the canyon are vertical and surfaces looked fresh as a result of repeated small rockfalls.
Overall, January was quiet seismically. There were several pyroclastic flow signals (for example, at 03:58 on 7 January) which had low frequency precursors. These events were associated with booming noises heard in the south of the island and were followed by periods of vigorous ash venting; this suggests that the collapses were caused by violent degassing of the dome.
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 Hybrid LP Dome RF VT 01 January 99 2 1 5 6 02 January 99 2 1 4 0 03 January 99 0 1 9 3 04 January 99 0 0 6 3 05 January 99 2 0 8 2 06 January 99 0 1 7 8 07 January 99 1 1 13 12 08 January 99 0 0 7 14 09 January 99 0 0 10 5 10 January 99 0 0 15 8 11 January 99 0 3 22 6 12 January 99 2 3 12 6 13 January 99 1 1 28 4 14 January 99 0 1 13 3 15 January 99 0 1 16 4 16 January 99 0 0 13 2 17 January 99 1 0 5 1 18 January 99 3 0 6 7 19 January 99 0 0 6 2 20 January 99 0 1 11 2 21 January 99 2 0 10 2 22 January 99 1 0 3 4 23 January 99 0 1 5 1 24 January 99 0 0 6 0 25 January 99 1 0 10 5 26 January 99 1 0 5 6 27 January 99 0 2 16 7 28 January 99 0 1 12 6 29 January 99 0 1 4 13 30 January 99 0 0 15 21 31 January 99 0 0 12 1
All the GPS sites were occupied this month. The eastward movement of Long Ground and Tar River continues but at a somewhat reduced rate. A local EDM network of five pins was set up around the possible fault in Long Ground on 27 January. This was set up to determine whether or not the surface feature is a fault and so we can track more closely any movement that occurs on it.
A detailed photographic and theodolite survey was conducted from twelve sites around the volcano at the end of January. A photographic survey was also conducted from the helicopter with the GPS onboard. The information has been processed to produce a detailed dome map and volume measurement. The dome currently has a volume of 76.8 million cubic metres and its highest point is at the top of the White River Valley at 977 metres above sea level. The dome was split deeply by the collapse on July 3rd 1998 and by subsequent events. The northern part of the dome, which comprises three main buttresses above Gages, the northern flank and Tar River, contains two-thirds of the total dome volume. The scar in the dome cuts deeply into the ancient rock of English's Crater. The scar cuts up to 100 m deep into the pre-1995 crater floor and has removed a minimum of 5.4 million cubic metres of old rock from this area.
Ash and rainwater collection continued throughout January. Ash samples from the small explosive events tended to be very coarse with lithic and crystal fragments up to 6 mm in size in the Richmond Hill / St Georges area. Ash generated by dome collapse pyroclastic flows was very fine-grained. Water samples were sent for analysis at British Geological Survey.
Sulphur dioxide fluxes were measured using the miniCOSPEC; the results are shown in Table 2.
Table 2. Average daily sulphur dioxide fluxes measured by miniCOSPEC, January 1999
Date Flux (tonnes/day) Comments 6 January 1330 helicopter 9 January 1020 helicopter 11 January 1130 helicopter 12 January 960 helicopter 19 January Below detection limits helicopter 22 January 710 helicopter 26 January 790 helicopter 29 January 1240 helicopter
The miniCOSPEC results show a generally decreasing sulphur dioxide flux throughout most of January although levels remained elevated. The measurements made on 19 January indicated a very low flux: observations suggested that at least part of the plume was at a very low level and may have passed below the helicopter.
Sulphur dioxide concentrations were also measured at ground level by using several sets of diffusion tubes located around the island. The results are shown in Table 3. Levels continue to be elevated in Plymouth, but low at all other sites.
Table 3. Sulphur dioxide diffusion tube results, January 1999. Levels in ppb.
Location 19 Dec 1998 to 5 Jan 1999 5 to 23-Jan 1999 Police HQ, Plymouth 69.8 77.8 St. George's Hill damaged tubes 4.3 Weekes 4.8 6.3 Vue Pointe Hotel 6.3 2.2 Lawyers 2.7 1.9
On 29, 30 and 31 January FTIR (Fourier Transform Infrared Spectroscopy) was used to determine the ratios of gases in the volcanic plume. The results from this study will be available from MVO at a later date.
MVO Staff Changes
Keith Rowley, Independent
Richard Luckett, British Geological Survey
Godfrey Almorales, Seismic Research Unit, Trinidad
Lutchmann Pollard, Seismic Research Unit, Trinidad
Thomas Christopher, University of West Indies
Keith Rowley, Independent
Carlisle 'Pyiko' Williams, Soufriere Monitoring Unit, St Vincent
Lizzette Rodriguez, University of Puerto Rico
Clive Oppenheimer, University of Cambridge
Simon Hughes, Buffalo University, New York State