Summary
Activity during this reporting period was low with continued dome growth within the crater left by the December 26 1997 eruption. Seismicity was dominated by rockfalls and there were few other seismic signals.
Visual Observations
Visual observations were hampered by poor visibility throughout the reporting period, and few good views of the dome complex were obtained. Nevertheless, clear weather towards the end of the reporting period showed that the area of active growth was still in the crater left by the December 26 collapse, in the south-west sector of the volcano. There were occasional rockfalls, and ash venting and steaming was seen from the summit area throughout the period. A dilute steam and ash plume drifted dominantly west-north-westwards. Ash venting and rockfall activity became slightly more vigorous at the end of the reporting period and due to a switch in wind direction resulted in light ash fall over the north of the island.
Seismicity
Throughout the reporting period seismicity was dominated by rockfall signals. There continued to be an approximately 12 hour cycle in the seismic amplitude with most rockfalls coinciding with the peaks in amplitude, although peaks occurred whether or not there was any rockfall activity. This regular slight increase in seismicity without there being any sizeable events has been occurring since the collapse on 26th December and is presumably indicative of cyclical degassing as the dome grows.
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* 18 Jan 98 0 3 5 22 2 0 19 Jan 98 4 28 14 60 2 0 20 Jan 98 2 28 15 91 0 2 21 Jan 98 3 13 2 108 1 0 22 Jan 98 1 8 4 31 0 1 23 Jan 98 0 9 1 38 0 0 24 Jan 98 3 25 1 52 0 0 25 Jan 98 2 8 5 26 2 0 26 Jan 98 2 22 3 21 1 0 27 Jan 98 10 1 0 54 0 0 28 Jan 98 2 3 1 77 0 0 29 Jan 98 1 2 2 93 0 0 30 Jan 98 3 9 7 113 0 0 31 Jan 98 2 5 1 63 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 Duration Hybrid LP VT 19 Jan 98 14:54 4.17 23 1 0 21 Jan 98 3:36 2.27 12 1 0 26 Jan 98 14:54 2.92 14 1 1
Ground Deformation
GPS occupations were made at Hermitage, Tar River, Perches, Roches Yard, Dagenham and Old Towne during the reporting period, with a base station at Harris. Displacement vectors since April-May 1997 for sites around the volcano with respect to Harris are as follows:
Site Vector (mm/degrees from grid north) Whites 25 / 353 Long Ground 66 / 033 Hermitage 100 / 026 Tar River 57 / 030 (since March 1997) Perches 59 / 049 Roches Yard 66 / 342 (since October 1996) Windy Hill 15 / 283* Dagenham 16 / 077* Old Towne (M27) 19 / 084*
(* small movements give a great uncertainty in the direction of movement)
The implication from these results is that a substantial sector of the volcano incorporating the sites lying between Whites, Hermitage and Roches Yard has moved in an approximately NNE direction by around 6cm in the last nine months. The similar movements of Long Ground, Tar River and Perches suggests that these sites have been displaced in an homogenous unit with little deformation occurring within it. Hermitage shows considerably more movement than the other sites. It is much closer to the dome and may be influenced more strongly by local pressure or loading effects. Distant sites on the west and north of the volcano (Dagenham, Old Towne and Windy Hill) appear stable with respect to Harris.
The deformation programme is being strengthened with new GPS sites established and occupied on the summit of Gages Mountain and in the north of the island at Drummond's and Blakes. A triple-prism EDM reflector was installed on the remnant of 'Peak B' (a stub of the crater wall between Tuitt's and Mosquito Ghauts). The reflector is less than 100m from the northern limit of the dome and readings have been made to it from Windy Hill. The Peak B reflector and Gages GPS site will help in monitoring deformation of the northern flanks of the volcano. The new GPS sites in the north of the island will help in monitoring long-term subsidence or inflation of the volcano. They replace the previous site at Lookout Yard which was destroyed.
A 1m deep pit was dug and a concrete pad lain on the summit of Gages Mountain in preparation for an electronic tiltmeter.
Volume Measurements
Very clear weather allowed a dome survey to be carried out on 29 and 30 January. Theodolite measurements were made from Jack Boy Hill, Flemings, Garibaldi Hill and the old observatory in Old Towne. A series of photographs at different angles around the dome were taken from the helicopter and the position of the camera determined with GPS. Processing of the data is still in progress.
Environmental Monitoring
Results from sulphur dioxide diffusion tubes were received during this period (Table 3). These show slightly enhanced levels of sulphur dioxide on St. George's Hill for the period including the December 26 eruption. New tubes were put out at sites on St. George's Hill, Weekes, Plymouth Police HQ, MVO south and Lawyers on 24 January 1998.
Table 3. Sulphur dioxide diffusion tube analyses (levels in ppb). Recommended action level is 100ppb.
Sample period runs from 22 October 1997 to 02 January 1998. Start Date 22-Oct 22-Oct 25-Oct 25-Nov 25-Nov 14-Dec 21-Dec End Date 25-Nov 21-Dec 25-Nov 14-Dec 21-Dec 28-Dec 2-Jan-98 Location St. George's Hill n/a n/a 1.00 2.55 n/a 11.50 n/a Weekes n/a n/a 0.00 1.05 n/a 2.60 n/a Belham Checkpoint n/a n/a 0.00 5.10 n/a 0.60 n/a MVO (S) 1.00 n/a n/a 1.95 n/a 1.60 n/a Lawyers 0.25 n/a n/a n/a 1.45 n/a 2.40 Fogarthy n/a 2.45 n/a n/a n/a n/a n/a MDF Base n/a 1.00 n/a n/a n/a n/a n/a Control 0.00 0.00 0.00 0.00 0.00 0.00 n/a
Rain water samples have been collected from sites at MVO south, Lawyers and Fogarthy's. The samples have yet to be analysed.
Dust Trak monitoring has been carried out at four fixed sites, and the results are shown in Table 4. Raised levels at the CPS site are thought to be due the high level of human activity and its proximity to a main road. Table 4 : PM10 (concentration in mg/m3)
Site 18-Jan 19-Jan 20-Jan 21-Jan 22-Jan 23-Jan 24-Jan Mango 0.013 0.008 0.008 0.026 0.039 0.033 0.024 CPS 0.015 0.023 0.054 0.055 0.067 0.057 0.053 St. Peter's 0.013 0.008 0.007 0.023 0.033 0.031 0.020 MVO north 0.000 0.001 0.005 0.000 0.027 0.022 0.016 Site 25-Jan 26-Jan 27-Jan 28-Jan 29-Jan 30-Jan 31-Jan Mango 0.023 0.023 0.016 0.016 0.016 0.014 0.035 CPS 0.026 0.043 0.053 0.040 0.044 0.050 0.042 St. Peter's 0.021 0.017 0.011 0.018 0.009 0.011 0.022 MVO north 0.017 0.015 - 0.010 0.007 0.009 0.019* These values are approximate 24 hr averages of the PM10 concentration. *43hr average. 24 Hour averages: <0.05 mg m-3 Low 0.05-0.1 mg m-3 Raised 0.1-0.3 mg m-3 Very High >0.3 mg m-3 Alert Locations: St. Peter's: The Dust Trak is outside on the balcony of a villa in St Peter's, called St. Peter's Place. CPS: Catholic Primary School The Dust Trak is outside in the area under the roof where some of the children have classes. The school is in Palm Loop near Woodlands. MVO north: The new Volcano observatory on Mongo Hill near St.John's, the Dust trak is on the third story of the building with the sampling tube stuck out of the window. Mango: The Dust Trak is outside on the verandah of a villa in Mango Drive in Woodlands
In addition to the daily averages at the permanent sites several other Dust Trak samples have been taken.
Note: Not all averages are 24 hour. Date Sample Description Duration Average (mg/m3) 20-Jan-98 Olveston 1 hour 0.057 21-Jan-98 Olveston 30 mins 0.065 23 to 24-Jan-98 Catholic Primary School. 24 hours 0.059 26 to 30-Jan-98 Salem Shelter. 24 hours 0.023 48 hours 0.018 24 hours 0.017 26 to 30-Jan-98 Brades Primary School 24 hours 0.023 48 hours 0.018 24 hours 0.017 28-Jan-98 MVO Scientist 30 mins 0.212 Olveston 30 mins 0.088 30-Jan-98 Olveston 1 hour 0.067 Salem Checkpoint 1 hour 0.086 31-Jan-98 Davy Hill 30 mins 0.047 McChesneys 30 mins 0.042
Also, several Cyclone sampling results were available during this period. All samples have <0.01mg of cristobalite present.
Date/Sample Volume (l) Dust weight (mg) Dust concentration (mg/m3) Cristobalite Weight (mg) 27-Nov-97/Woodlands 747 0.13 0.2 <0.01 27-Nov-97/St. Peter's 818 none detected (ND) ------ <0.01 28-Nov-97/CPS 992 ND ------ <0.01 1-Dec-97/Gardener 996 0.07 0.1 <0.01 1-Dec-97/Scientist 948 0.04 <0.01 <0.01 23-Jan-98/CPS 1056 0.46 0.4 <0.01
MVO Staff Changes
Arrivals:
Uche Osuji (Seismic Research Unit)
Dave Pyle (Cambridge University)
Departures:
Lutchman Crax Pollard (Seismic Research Unit)
Visitors:
Dai Stewart (British Geological Survey)
Dave Petrie (British Geological Survey)
Eliza Calder (Bristol University)