Summary
The level of seismic and volcanic activity remained low throughout the reporting period. Seismic activity was mainly manifest as occasional small volcano-tectonic earthquakes. The level of rockfall activity was particularly low but showed an increase after heavy rain showers. Several mudflows were generated during the reporting period, most of them travelled down Dyers River into the Belham River valley.
Visual Observations
Visual observations of the dome during this period indicated that there were no major changes in its morphology. Minor rockfall activity was sourced from near the top of the dome over Galway's wall, and travelled down a chute on the southern flank of Chances Peak. Moderate fumarolic activity continued in the gully between the December 26 scar and the new growth within the scar, and on the eastern side of the dome. A small pyroclastic flow down the Tar River occurred in the morning on 19 April and was seen on the remote video camera at MVO. It travelled as far as the remains of the Tar River Estate House. Another small pyroclastic flow occurred down the Tar River on the morning of 25 April, but could not be seen from MVO due to low cloud. Both of these flows are believed to have originated from the steep upper flanks on the eastern side of the dome. Fresh pyroclastic flow deposits were also seen along the northern side of the Tar River Valley on 14 April.
Seismicity
The observed level of seismic activity remained low. Earthquake activity continued to be dominated by volcano-tectonic earthquakes. Epicentres for locatable events lie on the eastern flanks of the volcano. Focal depths for these events were tightly clustered at depths of 3.0 to 3.5 km below the summit of the dome. Fault plane solutions were calculated using P-wave f irst motion from the 7 broadband stations along with first motions from the Lee's Yard and Jack Boy Hill stations from the short period network. The calculated fault plane solutions are consistent with a strike-slip fault mechanism.
The number of recorded rockfall signals was very low. However, in many cases, there was a correlation between the occurrence of the rockfalls and periods of heavy rainfall.
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* 12 Apr 98 15 1 - 10 - - 13 Apr 98 15 - - 6 - - 14 Apr 98 1 - - - - - 15 Apr 98 2 1 - - - - 16 Apr 98 4 - - - - - 17 Apr 98 4 - - - - - 18 Apr 98 1 - - - - - 19 Apr 98 5 - 1 4 - 1 20 Apr 98 14 - 2 5 - - 21 Apr 98 8 1 - 1 - - 22 Apr 98 - - - 1 - - 23 Apr 98 - - - 4 - - 24 Apr 98 1 - 1 4 - - 25 Apr 98 3 1 - 2 - -*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 Windy Hill, Long Ground, Tar River, Perches, Roches, Brodericks, Dagenham and Old Towne running base stations at Harris and Whites. The sites on the east (Long Ground, Tar River and Perches) all show continued slow movement to the north east, around 7 cm in the last year, Whites and Roches have moved slightly less in different directions. The line to Windy Hill from Harris is stable. Occupations of Windy Hill since December 1997 give site positions lying within a box 3 mm by 7 mm indicating stability in that area. Brodericks has shown accelerated movement. It moved around 3 cm to the north between November 1997 and January 1998 and appears to have stabilised in its new position. This would coincide with the period of rapid extrusion in the southern area of the dome during December, 1997.
A new permanent GPS site was installed in the South Soufriere Hills. Telemetry equipment used by the station was installed by the University of Puerto Rico on Antigua and in the Centre Hills. The Peak B reflector on the northern flank of the volcano was shot from Windy Hill. The measurement suggests that the site movement has slowed dramatically following the cessation of dome growth.
Volume Measurements
A new theodolite site known as Fergus Ridge was set up on the high ridge on the western flanks of South Soufriere Hills and to the north of Fergus Mountain overlooking the White River Valley. Measurements from this site and triangulated with measurements from Perches Mountain were obtained on 16 April. In conjunction with the combined photo/GPS data collected on 6 April, a revised total dome volume has been calculated of 113 million cubic metres. This figure differs with the initial estimate of 120 million cubic metres however this revised figure incorporates a greater number of theodolite and photo/GPS points which improve constraints to the summit area and the new dome on the south-west sector of the complex.
Environmental Monitoring
Localities of Dust Trak monitoring sites:
The air quality monitoring programme during this period involved 6 permanent monitoring sites (1-6 below) and two temporary sites (7, 8 below). Results are shown in Table 2.
Table2: 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 12th Apr 23 18 25 33 18 21 16 18 Low 050o Evening 13th Apr 22 20 12 45 16 22 19 9 Low 140o AM/evening 14th Apr 10 8 11 32 12 - 8 - Low 130o AM 15th Apr 7 5 9 11 9 7 - - Low 320o Dry 16th Apr 13 9 8 22 8 13 - - Low gusty 11:30 am 17th Apr 7 4 5 23 6 8 - - Low 330o Dry 18th Apr 6 4 11 12 7 4 - - Low 300o Dry 19th Apr 6 5 8 17 5 - - - PF 320o 16:00 onwards 20th Apr 8 9 8 25 8 5 - - Low 330o Dry 21st Apr 5 6 2 31 - 6 - - Low 110o Dry 22nd Apr 8 6 5 25 10 5 - - Low 150o Dry 23rd Apr 7 5 9 21 12 5 - - Low 320o Dry 24th Apr 20 7 26 21 10 11 - - Low 320o Dry 25th Apr 17 8 10 20 12 11 - - PF 300o Late am
PF = pyroclastic flows down Tar River Valley - small on the 19th April and reaching the area where the Estate House used to be on the 25th April.
Low volcanic activity and occasional rains have generally kept aerosol levels low. Extremely wet weather on the 14th - 15th April produced the lowest aerosol levels since the heavy ash fall at the beginning of February. Rain also prevented the ash produced by the small pyroclastic flow of 19th April being transported north by wind to any of the sites that were being monitored. Aerosol levels continued low later in this period despite much more dry weather conditions, except in Salem which is likely to have been affected by ash blown west from the pyroclastic flows.
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:
Tim Druitt (Universite Blaise-Pascal, Clermont-Ferrand, France)
Desmond Seupersad (Seismic Research Unit, Trinidad)
Departures: Rick Hoblitt (US Geological Survey)