Summary
Activity during this reporting period was low. Seismic activity was dominated by volcano-tectonic earthquakes and small rockfalls from the dome.
Visual Observations
Observations during clear periods indicate that no growth of the dome complex has occurred throughout the reporting period. Rockfall activity has been minimal with very small rockfalls occurring down the upper flanks over Gages, the upper flanks of the new dome on the south-west sector of the dome complex and continuing degradation of the steep buttress on the upper flanks of the eastern face. Rockfalls from this buttress were seen travelling down either the gully between the eastern face and the northern flanks (Oct 22 dome) or more commonly down the incised central chute on the eastern flanks. Moderate fumarolic activity was concentrated within this same chute and around the trench between the Boxing Day scar and the fresh dome material on the south-west sector of the dome.
A field team measured the temperature of the pyroclastic flow deposits produced by the September 21 collapse in the area around Trants using a new thermocouple probe on loan from the Cascades Volcano Observatory, USA. A maximum temperature of 357 ?C was recorded at a depth of 2 m within the deposits.
Seismicity
The seismic activity over this period was at a very low level. Numbers of recorded earthquakes were low (Table 1). Earthquake activity consisted principally of volcano-tectonic earthquakes. These events occurred at irregular intervals rather than in swarms. Epicentral solutions for the volcano-tectonic earthquakes show that all of these events occurred below the dome. The number of recorded rockfall signals was also low throughout this period. These events were generally small.
Work was completed on upgrades to the broadband seismic network. The network now consists of 7 stations in total. Two new stations were installed at the South Soufriere Hills and at Mongo Hill, providing increased azimuthal coverage. Both these stations use single vertical component seismometers with corner frequencies of 1 Hz. Radio telemetry was revised for the remaining stations original network.
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 LPRF* HYRF* 29 Mar 98 - 2 2 4 - - 30 Mar 98 4 - - 8 - - 31 Mar 98 1 - - 5 - - 1 Apr 98 - 3 - 1 - - 2 Apr 98 3 2 - 18 - - 3 Apr 98 6 3 - 11 - - 4 Apr 98 7 1 - 11 - 1 5 Apr 98 17 2 - 13 - - 6 Apr 98 4 3 - 19 - - 7 Apr 98 7 1 1 1 - - 8 Apr 98 11 - - 6 - - 9 Apr 98 16 2 - 3 - - 10 Apr 98 3 1 - 11 - - 11 Apr 98 4 - - 5 - -
* LPRF: LP earthquake followed by rockfall signal. HYRF: Hybrid earthquake followed by rockfall signal. The LPs, hybrids and rockfalls in these signals are also counted in their respective columns.
Ground Deformation
GPS occupations were made at Whites, Gages, Old Towne, Dagenham, Blakes and Drummonds with the base station at Harris. The summit of Gages Mountain has moved just over 3 cm to the west-north-west since January. This direction is radially away from the dome and is almost identical to the movement direction of the pole to the tilt plane for the tiltmeter on the summit of Gages Mountain. The University of Puerto Rico station at Hermitage shows continued movement to the north-east. The site has moved 15 cm since this time last year with the highest rates of movement occurring since September, 1997.
Volume Measurements
A survey of the dome talus and the deposits in the top of the White River Valley was carried out on the 30th March. This gave a better constrained volume for the talus of 36 million cubic metres, and increases the total dome volume to 120 million cubic metres. The deposits showed an increase of 8.99 million cubic metres, since the previous survey on 17 January 1997. The total volume of the dome and deposits is 300 million cubic metres erupted since November 1995.
Theodolite measurements during a clear window on the 5 April show that the highpoint of the dome complex is the top of the large spine perched near the summit of the fresh material on the south-west sector. This has an elevation of 1031 m and the spine itself has a minimum height of 50 m. Further photos taken from the helicopter combined with GPS positional data were taken on 6 April concentrating on the fresh dome material on the south-west sector to constrain the total dome volume.
Environmental Monitoring
Localities of Dust Trak monitoring sites:
The air quality monitoring programme during this period involved 5 permanent monitoring sites (1-5 below) and three temporary sites (6-8 below). Results are shown in Table 2.
Table 2 : 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 7 8 VA wind rain 29th Mar 18 16 19 17 24 - - - Low 320o PM 30th Mar 21 23 47 53 21 - - - P 100o Periodic 31th Mar 20 27 29 91 18 - - - Low 040o Overnight 1st Apr 15 16 29 - 12 - - - Low 040o AM & overnight 2nd Apr 9 10 11 36 8 - - - Low 340o Dry 3rd Apr 47 12 43 13 11 - - - Low 340o Evening 4th Apr 25 23 45 24 10 - - - Low Still AM 5th Apr 32 15 18 18 10 - - - Low 040o Dry 6th Apr 16 16 32 52 14 - - - Low 030o Dry 7th Apr 60 58 - 100 54 - - - Low 020o Dry but hazy 8th Apr 69 48 75 90 24 - - - Low 290o Evening 9th Apr 28 24 35 37 - 32 - - S 280o Dry 10th Apr 25 19 - 32 24 12 27 - Low 300o Dry 11th Apr 15 13 19 22 15 - 8 14 S 290o Overnight
P = small plume of ash observed at 06:00 and sulphur smell in St. Peter's.
S = slight increase in volcanic seismicity.
Wind direction was variable during this period
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
Low volcanic activity and occasional rains have generally kept aerosol levels low. Slightly higher aerosol levels were measured on 7th and 8th April and may be due to Saharan dust in the atmosphere. The Davy Hill area, which was affected by traffic jams at the time, showed the highest levels.
MVO Staff Changes
Arrivals:
Willie Aspinall (Aspinall and Associates)
Joe Devine (Brown University/ Seismic Research Unit, Trinidad)
Rob Watts (Bristol University)
Departures:
Wilkie Balgobin (Seismic Research Unit, Trinidad)
Visitors:
Dave Petrie (British Geological Survey)