At the start of the week, seismicity at the volcano was at a high level, with a volcano-tectonic earthquake swarm started on 9 November continued until 02:53 on 12 November. Thereafter, it was at a very low level apart from a small swarm of volcano-tectonic earthquakes on 14 November. Measurements made using the EDM Total Station and GPS techniques continued to indicate low deformation. The sulphur dioxide flux remained at a moderately low level for most of the week and does not appear to be controlled by the seismic swarms. Visibility was generally poor through the week, although good views were obtained of the dome late in the week. 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.
No visual observations of the dome were possible during the first half of the week due to low cloud cover shrouding the upper flanks of the volcano. Some small rock falls were seen on the lower eastern flank of the dome and others were heard by field teams on the western flank of the volcano.
Visibility improved on 14 November, when new dome growth was observed concentrated on the northern flank of the October 1 dome. This new dome growth comprised blocky, light grey material on the upper northern flank with evidence for rock falls down to the north and then east within the canyon carved by the 17 September collapse. This area of new dome growth was confirmed during the remainder of the week, with some bulging of the southern side of the dome also noted. The growth rate could not be estimated as no volume surveys were possible. However, growth appeared to quicken late in the week compared to the past couple of weeks.
Intense steaming, especially from the area around Castle Peak, was noted throughout the week. SO2 production appeared to be from a number of areas on and around the October 1 dome.
Heavy rain caused further rock slides on the southwestern face of the Galway's Wall during the week; the upper part of the wall is now very thin and looks to have become somewhat unstable.
Earthquake types: 10 November to 16 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 10 NOV 96 117 2 0 0 11 NOV 96 64 1 0 3 12 NOV 96 5 0 0 5 13 NOV 96 0 0 0 5 14 NOV 96 34 2 0 4 15 NOV 96 1 4 0 2 16 NOV 96 1 1 0 1
Seismicity during the week began with a long-duration volcano-tectonic earthquake swarm, with earthquakes located at shallow depths beneath the crater. This earthquake swarm, which comprised 212 events, began at 13:51 on 09 November (note correction from Scientific Report 42) and lasted until 02:53 on 12 November. The earthquake magnitudes varied, and were generally similar to the previous swarm. However, the intensity of the swarm was lower than during either of the previous 2 swarms, peaking at about 10 earthquakes per hour late on 10 November.
A shorter swarm of volcano-tectonic earthquakes occurred on 14 November with 40 earthquakes, mostly at shallow depths beneath the crater and of similar magnitude to those in previous swarms. However, several earthquakes early in this swarm had epicentres located to the southeast of the volcano. A single earthquake of significantly higher magnitude (c. M2.5) than any seen in recent swarms was recorded at 21:32 on 14 November.
The number of other types of seismic events was very low during the period. All of the long-period events and rockfall signals were small. However, some of the rockfalls were amongst the largest seen since growth of the October 1 dome started. The seismograph network also recorded four regional earthquakes and a teleseism from an earthquake in southern Peru on 12 November.
The MVO Total Station was used to make EDM measurements on the eastern triangle which links Long Ground, White's and Castle Peak during the week. The shortening trend on the lines to Castle Peak continued, although the rate of shortening appeared to have slowed somewhat to around 5 mm/day.
GPS surveys of all three of the networks around the volcano were undertaken during the week. The results of these measurements continued to indicate no major changes in line length. Only radial lines to the Farrell's benchmark high on the northern flank of the volcano are showing any signs of movement at present, although these movements are just within formal errors.
The dry tilt triangle at Broderick's, on the western flank of the volcano, was re-occupied during the week. This showed no changes from the previous occupation several months ago.
All the deformation data suggest that movements are confined to the upper flanks of the volcano and are thought to be due to a combination of factors, including loading of the upper part of the edifice by the new dome, localised thermal expansion and pressurisation of the magma conduit at shallow depths beneath the dome.
Measurements of sulphur dioxide flux from the volcano were made on a number of occasions during the week by ground traverses with a COSPEC instrument. All of the values were consistent with recent trends, which show that the volcano does not produce a large amount of SO2 during normal dome growth. Daily fluctuations do not suggest any strong link between SO2 production and swarms of volcano-tectonic earthquakes.
Average SO2 Flux (tonnes per day)
10 Nov 96 240 11 Nov 96 227 13 Nov 96 178 14 Nov 96 243 16 Nov 96 176
Rainwater was collected at four sites to the north and west of the volcano on 10 and 14 November. The results from 10 November show continuation of the highly acidic rainfall, particularly in sites west of the volcano. A sample from a pond in the upper Amersham area showed particularly high levels of chloride.
Prof Steve Sparks, Bristol University
Richard Robertson, SRU
Dr Paul Cole, University of Luton, UK
Nicki Stevens, BGS
Dr Paul Jackson, SRU