Summary
The activity at the volcano changed markedly on about 11 December, when dome growth resumed within the crater. The new dome growth is concentrated in the southern part of the crater, within the scar left by the September explosion. Volume estimates indicate a rate of growth comparable to the initial stages of the October 1 dome. The renewed dome growth coincided with cessation of the VT swarm activity and slowing of the deformation of the Galway's Wall.
Visual observations
Observation of the crater area were made during the week from the helicopter. The conditions were variable, with low clouds restricting views. There were excellent viewing conditions on 13 December.
In the early part of the period, only limited views of the dome were possible, and no major changes were seen to the dome complex on 9 and 10 December. On 13 December, a new dome was sighted for the first time, in the area to the south of the October 1 dome, within the scar caused by the 17/18 September explosion. This sector of the scar, which is to the west of Castle Peak, has been steaming vigorously during the past two months, and was intensely oxidised. The new dome is thought to have first appeared on the 11 December following the cessation of earthquake swarm activity, and will be called "December 11" dome.
On December 13, the new dome was a pale grey colour, and very active with minor rockfalls and some ash venting. The dome was a pancake shape, with an approximate height of 870 m. Following a dome survey on 13 December, using laser-ranging binoculars from the helicopter, the volume of December 11 dome was estimated at 500,000 cubic metres. This implies an average extrusion rate of around 200,000 cubic metres per day, which is similar to the rate measured at the start of extrusion of the October 1 dome.
The Galway's Wall continued to show signs of stressing and instability, particularly in the early part of the week. The top of the wall continued to crumble with large (5 m diameter) blocks falling from it, northwards into the crater moat. There were two main sets of cracks within the wall; transverse cracks, and highly-pervasive wall parallel cracks, that propagated deeper into the wall with time. One large fracture that runs across the face of the wall about 200 m below the top dips at a steep angle into the wall.
The talus slope at the base of the wall continued to build up, and was estimated to have covered about one-half of the wall height by 11 December. Much of the landslide activity was from a gully towards the eastern end of the wall. In this area, the lowest point of the top of the wall was only about 1 m above the dome talus on 13 December.
The cracks on the south-east side of Chances Peak continued to widen and extend. One crack showed an opening of 6.7 cm/day and a right-lateral shear movement of 1.5 cm/day between 4 and 9 December. Re-measurement on 10 December showed that the rate of deformation had slowed slightly, with extension of 0.6 cm/day although the shear increased to 2.3 cm/day. In addition , new cracks were seen near the summit of Galway's Mountain, at the eastern end of the wall.
A major slump was noted near the Galway's Soufriere on 10 December. Part of the Soufriere had slumped by about 2 m along a NW-striking plane, which is a line with a fault in the eastern section of the Galway's Wall.
Seismicity
Earthquake types: 9 to 14 December 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 Dome RF Galways Wall Landslides 09 DEC 96 116 0 0 3 2 10 DEC 96 164 1 0 1 10 11 DEC 96 37 0 0 1 9 12 DEC 96 0 0 0 4 4 13 DEC 96 1 0 0 1 0 14 DEC 96 0 2 0 1 0
The level of seismicity decreased markedly during the week. A swarm of volcano-tectonic earthquakes started at 09:27 on 9 December, only 26 hours after the abrupt end of the previous swarm. The earthquakes in this swarm were as large as those in the previous swarm. This swarm lasted until the morning of 11 December. This was the 14 VT swarm recorded since 21 October (see Scientific Report 45) The swarm details are given below.
Swarm Start End Duration No. of Earthquakes/hour (hours) Earthquakes Mean Peak 14 09:27 13:58 52.5 315 6.0 15 9/12/96 11/12/96
More Galway's Wall rockfalls were recorded, at the same time as the intense VT activity. Seismic activity was low for the rest of the week. There were virtually no rockfalls from the new dome, as growth was constrained within the scar caused by the September explosion.
Ground Deformation
EDM measurements were made of the eastern triangle on 10 and 13 December, and lines on the western side of the volcano on 9 December. The eastern triangle links Long Ground, Whites and Castle Peak. The measurements on 10 December were limited by low cloud, but indicated a continuation of the approximately 1 cm/day shortening trend for lines to Castle Peak. Better measurements were made on 13 December, and showed a line shortening of 4 cm from 4 December, consistent with the long-term trend of 6 mm/day. The lines on the western side showed no significant changes since the lines were last measured on 7 November
Static GPS surveys of the eastern and northern networks were carried out this week. The GPS program continues to detect no significant changes in any line.
Gas Measurements No gas measurements were possible, as the COSPEC instrument is awaiting repair.
Environmental Monitoring
The analysis of recent rainwater samples will be complete out soon. The program of deployment of sulphur dioxide diffusion tubes continues at sites around Plymouth.
Staff changes
Arrivals
Dr. Willy Aspinall, BGS
Dr. Sayyadul Arafin, SRU
Mr. Lutchman Pollard, SRU
Departures
Dr. Simon Young, BGS (short break)
Visitors
Anthony Langlais, Gaudeloupe Observatory