Seismicity at the volcano continued at a high level for most of the week with one volcano-tectonic earthquake swarm recorded which lasted for 72 hours. Measurements made using the EDM Total Station and GPS techniques have indicated low deformation, with established trends continuing. The sulphur dioxide flux has remained at a low level for most of the week. Visibility has been variable throughout the week with several clear days in which good views were obtained of the dome. The alert level has remained at Orange as the scientific team at the MVO remained concerned about the continued occurrence of volcano-tectonic earthquake swarms.
Dome growth was concentrated in the central western part of the October 1 dome at the beginning of the week. The active area of dome growth was blocky in appearance and surrounded by ashy deposits by mid-week. The top of the October 01 dome was 880 metres a.s.l. (2875 ft) on 5 November.
The eastern face of the dome was almost inactive for most of the week. Most of the rockfalls involved the movement of small amounts of fine grained material down the flank of the dome. Few of these events produced ash clouds above the crater. One moderate-sized rockfall, which produced a small ash cloud, was observed from the eastern face on 3 November.
The small spine which was first seen on 2 November increased in height during the week. This spine was shaped like a pyramid by 05 November and had partially collapsed by 9 November. A new spine was noted in the southwest of the dome on 5 November. Incandescent material was seen on the top and at the northern edge of the October 1 dome during a helicopter survey on 7 November.
A landslide was observed on the wall above Galways Soufriere on 6 November. This was a result of destruction of the vegetation in this area, combined with recent rainfall.
Large amounts of steam were observed over most of the dome complex on 9 November, particularly in the southern part of the new dome where it is in contact with the Castle Peak dome. Several small gullies from recent rockfalls were observed on the northern part of the new dome which was the only active area on the dome at the end of the week.
Earthquake types: 03 November to 09 November 1996
These earthquake counts are of events that triggered the short-period seismic network event recording system between 0000 and 2400 each day.
Date VT LP Hybrid RF 03 NOV 96 17 1 0 1 04 NOV 96 190 1 0 2 05 NOV 96 147 2 0 6 06 NOV 96 32 1 0 7 07 NOV 96 3 0 0 2 08 NOV 96 0 1 0 2 09 NOV 96 28 2 0 0
The seismicity during the week was dominated by a long-duration volcano-tectonic earthquake swarm, with events located at shallow depths beneath the crater. The earthquake swarm, which comprised 389 events, began at 19:24 on 3 November and lasted until 19:11 on 6 November. The earthquake magnitudes varied, with the largest being about the same size as the maximum recorded during the swarm of 1-2 November. Many of the VTs were large enough to be located, and most occurred at shallow depths (< 2 km).. The rate of events did not at any time reach the intensity of the 1-2 November swarm but at its maximum 11 earthquakes were recorded per hour. There were no deeper earthquakes associated with this swarm, as there was in the swarm of 1-2 November. Another swarm began at 17:50 on 9 November.
The number of other types of seismic events was very low during the period. A slight increase in the size and frequency of rockfall signals occurred towards the middle of the week with a few events on the 6-7 November which were associated with small ash clouds. A few long period earthquakes were recorded, with most being of low magnitude. The seismograph network also recorded three regional earthquakes, the largest occurred at 03:20 on 8 November and had a Richter magnitude of 5.5.
The MVO Total Station was used to make EDM measurements of the eastern and northern triangles, and on the western radial lines during the week. All the readings were consistent with the long-term trends on these lines.
Measurements were made on the eastern triangle to Castle Peak on 4, 5, 6 and 8 November. The changes in line lengths on this triangle were in most cases consistent with the well-established trend of shortening of about 6 mm per day. The measurement frequency was increased during the week on this triangle in response to the VT swarm. The onset of these swarms appear to have had no effect on the rate of shortening which has been observed for lines to Castle Peak.
The northern triangle, which includes Upper Farrells, St Georges Hill and Windy Hill, was measured on 05 November. The lines from Windy Hill and St Georges Hill to Upper Farrells shortened by 3mm and 8 mm respectively since these were last measured on 27 October. These lines are quite erratic, and show long-term trends of little or no change.
EDM measurements were completed on radial lines on the western flank of the volcano on 7 November. The line lengths between Upper and Lower Amersham shortened by 1 mm since November 1, while those between Lower Amersham and Chances Steps lengthened by 5 mm during the same period. These lines continue to show no trends.
GPS surveys of the EASTNET and WESTNET were completed on 3 and 6 November respectively. The results of these measurements indicate no major change in line length. The EASTNET consists of five stations on the north and east side of the volcano. The WESTNET comprises five stations on the western side of the volcano. A test of the reproducibility of slope length measurements was made on 4 November. This showed that repeat measurements can be made with an error of less than 1 cm.
Dome Volume Measurements
A dome survey was made on 7 November using the GPS equipment and the range finding binoculars. The volume estimate derived from this survey is, within error, the same as that obtained from the previous survey on 23 October.
Gas, Ash and Rainwater Measurements
Measurements of sulphur dioxide flux from the volcano were made on 3 November (371 tonnes/day and 5 November (155 t/d), using the correlation spectrometer. All the COSPEC measurements were made by running traverses beneath the volcanic plume along the west coast road. The values have continued to decline and are now amongst the lowest ever obtained at the volcano.
SO2 Tubes and Water
Analysis of the SO2 diffusion tubes show that the averaged concentration of SO2 at 5 sites to the west of the volcano correlate with the COSPEC measurements. Rainwater samples were collected on 4 November from 4 sites, and continue to show an improvement in the water quality. Rainwater from Upper Amersham, the site closest to the volcano still has the lowest pH (3.3) but the concentrations of chloride, fluoride and sulphate are all with WHO guidelines.
Rob Watts, BGS
The scientific staff at the MVO during this week were:
Richard Robertson, Simon Young, Paul Jackson, Paul Cole, Gill Norton,
Rod Stewart, Angus Miller, Nikki Stevens and Ricky Herd.
The scientific team works with a team of local technical staff who include:
George Skerritt, Billy Darroux, Leroy Luke, Dave Williams, Bill Tong,
Thomas Christopher, David Sillcot, Venus Bass, and Joseph Morris.