Summary
Activity during this reporting period continued to be low. There were no changes in dome morphology and only three small pyroclastic flows occurred to the east. Seismicity was generally low, with volcano-tectonic earthquakes being the predominant signals recorded by the seismic network.
Visual Observations
Minor activity continued in a similar manner to the previous reporting period with only 3 small pyroclastic flows travelling half way down the Tar River Valley. These events occurred on 26 April and the morning of 27 April with each event lasting ca. 2 minutes and generating very small ash clouds and all originating from the steep upper flanks of old dome material on the eastern side. Rockfall activity has also been minor with small rockfalls occurring down the eastern flanks and the south-western flanks with these events gradually carving deeper chutes on the Tar River side and the southern flanks of the new Galway's dome.
A brief clearing of the dome in the early morning of 6 May highlighted that no changes of morphology had occurred from new growth since extrusion of the summit spine around 10 March. Moderate fumarolic activity was apparent from a point source in the main chute on the upper eastern flanks, and in several areas within the trench between the Boxing Day scar and the fresh growth within it i.e. the Galway's dome. A short reconnaissance flight to Chances Peak on 6 May showed the area to be blanketed in at least ca. 5 m of tephra, consisting mainly of fine ash and blocks of glassy dome rock (up to 1 m diameter).
The temperature of the pyroclastic flows deposited at Trant's during the 21 September, 1997 collapse were measured on 28 April and highlighted only very minor changes since they were last measured 2 weeks ago. A maximum temperature of 348 degrees Celsius was obtained at a depth of 2 m.
Seismicity
Over the reporting period seismicity remained at a low level. Most of the time activity was dominated by low amplitude volcano-tectonic earthquakes, which occurred in groups of a few events but were not numerous enough to constitute earthquake swarms. The exceptions to this are shown in Table 2, including a swarm of hybrids on 6 May. These were the first hybrids of high amplitude seen for many weeks but no more have followed. All of the volcanic earthquakes located for the reporting period were found to originate beneath the dome at apparent depths of about 2.5 to 3 km.
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* 26 Apr 98 17 0 1 2 0 0 27 Apr 98 7 0 0 2 0 0 28 Apr 98 8 0 0 6 0 0 29 Apr 98 2 0 0 0 0 0 30 Apr 98 3 0 0 0 0 0 1 May 98 7 0 0 0 0 0 2 May 98 10 0 0 3 0 0 3 May 98 2 2 0 7 0 0 4 May 98 4 0 0 4 0 0 5 May 98 6 2 0 3 0 0 6 May 98 5 12 0 0 0 0 7 May 98 6 0 0 0 0 0 8 May 98 25 0 0 2 0 0 9 May 98 4 1 0 2 0 0
*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.
Table 2: Swarms
Start (local) Duration Hybrid LP VT 26 Apr 98 22:47 1.45 0 0 9 6 May 98 04:45 1.40 11 0 2 7 May 98 22:57 5.75 0 0 11 8 May 98 08:41 8.32 0 0 15
Ground Deformation
GPS occupations were made of stations at Gages Mountain, Windy Hill, Dagenham, Old Towne, Blake's and Drummond's with the MVO permanent stations at Harris and South Soufriere Hills and the University of Puerto Rico permanent stations at Whites, Hermitage and at Mongo Hill. Although no dramatic differences are reported since the last occupations there are some longer term trends which are now apparent in the data. The stations at Dagenham, Old Towne and Windy Hill show height increases with respect to Harris of around 5 cm, 5.5 cm and 4 cm respectively since December 1996. These values are preliminary, as the height component is the least well constrained by GPS. The site at Lookout Yard in the north of the island shows an increase in height of around 6 cm. It is more likely then that the reference at Harris is actually sinking. Height differences between Harris and sites on the east (Whites, Long Ground, Tar River and Perches) are small and not outside the limits of error.
The northern crater wall reflector was shot from Windy Hill. Its movement has fallen dramatically since the dome stopped growing, and the line has shortened by only 1 cm since the middle of March.
Volume Measurements
No volume surveys were undertaken during the reporting period however if a window of clear visibility occurs in the near future a comprehensive GPS /Binocular survey of the summit and flanks of the dome will be attempted.
Environmental Monitoring
Localities of Dust Trak monitoring sites:
The air quality monitoring programme during this period involved 5 permanent monitoring sites and 1 temporary site. These were located at:
Table 3: 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 VA Wind (?) Rain 26 April 14 * 13 9 8 P/Low 330 9 am 27 April 11 7 15 8 7 P/Low 300 10 am 28 April 10 8 18 11 10 Low 300 Dry 29 April 9 9 15 10 10 Low 260-280 4 pm 30 April 8 6 21 9 6 Low Variable 8:15 am 1 May 9 - 13 6 6 V 310 Dry until midnight 2 May 8 7 13 8 8 Low 300 Overnight 3 May 5 5 12 6 7 Low 290 10pm 4 May 8 6 5 9 8 Low 030 Overnight 5 May 7 8 15 10 9 Low 280 Overnight 6 May 8 7 13 8 9 H 320 Dry 7 May 13 24 25 14 - Low 080 Dry 8 May 13 11 21 25 13 Low Still Continuous 9 May 6 6 14 15 10 Low 060 Dry
V = very small venting of ash observed at the top of the Tar River Valley. Also scientists working in the south-west of the island over the weekend noticed a strong smell of rotten eggs (hydrogen sulphide).
H = half hour swarm of hybrid earthquakes beginning at 05:25 on Wednesday 6th May.
P = pyroclastic flows in Tar River Valley.
* = power cut
Low volcanic activity and a number of rain showers have generally kept aerosol levels low. The small level of ash observed being generated by the volcano has not affected any of the inhabited areas in the north of the island. It appears that with each remobilization of ash by vehicles more material is removed by wind and rain. The three pyroclastic flows that occurred on 26th and 27th April have had no effect on the levels of airborne ash and dust though the wind was blowing from 330? to the south-east. Although dust levels remained low, aerosol levels roughly doubled the day following the hybrid swarm. Heavy rain two days later once more reduced levels to a fraction of that observed in UK cities.
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:
Richard Luckett (MVO)
Gill Norton (British Geological Survey)
Colin Walker (Edinburgh)
Patrick Tuchais (Martinique Volcano Observatory)
Departures:
Joe Devine (Brown University, USA)
Sue Loughlin (British Geological Survey)
Visitors:
Mike Higgins (University of Blaise-Pascal, Clermont-Ferrand, France)