Summary
Activity at the Soufriere Hills Volcano between 16 and 22 May was dominated by the continued growth of the lava dome in English's Crater. Visibility was very poor for most of the week and the dome was only visible on 20 May. It showed significant changes since the previous sighting on 11 May. Rockfalls and small pyroclastic flows occurred throughout the week, although at a lower rate than the previous week. Seismic activity was dominated by the rockfall signals. There were also two episodes of repetitive hybrid earthquakes. Ground deformation measurements indicate that there is still only slow deformation of the volcano. COSPEC measurements show that the SO2 flux remains very low, at around 160 tonnes per day.
Visual observations
Visibility was very poor from 16 to 19 May, with cloud covering the upper parts of the volcano almost all the time. Brief glimpses of the lower parts of the dome were possible for short periods on 17 and 19 May, although it was not possible to make out any details. A rockfall at 09:27 on 19 May generated an ash plume which reportedly reached an altitude of about 4000 ft above sea level, observed from Brambles Airport.
Visibility was better on 20 May, although the dome was still shrouded in cloud most of the time. After 15:00, the clouds thinned out, and the entire dome was visible for short periods from a number of locations. There were significant changes in the dome since the previous sighting on 11 May. There was a large broad spine at the top although it was not possible to say whether this was a new spine or due to growth of a pre-existing spine. The spine rose about 60 ft above the top of the dome and was slightly inclined to the northeast. Several other small spines were observed through partial cloud cover during a helicopter inspection. There was no indication of the source area of the pyroclastic flows on 12 May, possibly because the rockfalls in the intervening week had smoothed out any scarp-like features. During the short periods of observation, rockfalls were observed on the active north and northeast flanks of the dome. Occasional rockfalls were also seen on the southeast flank, an area that was steaming vigorously. The sulphur deposits around the fumaroles appeared to have grown.
Visibility was very poor on 21 and 22 May and no observations were possible
Seismicity
The seismicity at the Soufriere Hills Volcano throughout the week has again been dominated by small- to moderate-sized rockfall events. The table below lists the daily counts of the different types of event recorded - volcano-tectonic, long-period, hybrid, and rockfalls. It also includes an estimate of the amount of the broadband seismic tremor seen on stations close to the crater.
Earthquake types 16 - 22 May 1996
Date VT LP Hybrid RF Tremor amount 16/05 0 2 12 80 Low to intermediate 17/05 0 4 8 33 Low to intermediate 18/05 1 12 2 25 Low 19/05 1 9 13 34 Low to intermediate 20/05 0 7 8 43 Intermediate 21/05 0 4 0 32 Intermediate to high 22/05 0 7 0 60 Intermediate to high
The number of rockfalls was fairly constant throughout the week, although there were significantly more on 16 and 22 May. Some of the rockfalls generated small ash clouds and caused light ashfall down-wind of the volcano. A rockfall at 10:47 on 20 May was associated with a small pyroclastic flow which travelled down the Upper Tar River Valley as far as Hermitage.
There was significant hybrid seismicity during the week, although this does not show in the table above which only includes events recorded at four or more stations. Small hybrid events had been recorded on the seismic stations closest to the volcano (Gages and Chances Peak) since about 02:00 on 15 May. These peaked in number on the evening of 15 May, and were recorded in declining amounts on 16, 17 and 18 May. A different type of hybrid event started to be recorded on 19 May. These had a different waveform and were only seen on the Chances Peak seismic station. They increased noticeably in number and amplitude from about 03:14 on 19 May and reached a maximum frequency of about 2 per minute. From mid-afternoon on 19 May, these events became reduced in amplitude, although not in numbers. The number of the hybrid events again increased overnight on 20/21 May and reached a peak of about 4 per minute by 07:00. The events were still quite small. At 08:00 they started to decline in number and had stopped being recorded by 22 May.
Variable amounts of low-amplitude broadband seismic tremor were recorded throughout the week; the amount gradually increased during the week. There were sustained periods of continuous tremor from 08:07 on 17 May until 03:30 on 18 May, from 05:34 until 08:30 on 20 May from 19:00 until 22:00 on 21 May
A few long-period earthquakes were recorded. Two volcano-tectonic earthquakes were recorded, on 18 and 19 May. Both were located at depths of about 2 km, slightly north (18 May) and north-west (19 May) of English's Crater.
Ground Deformation
Few EDM measurements were made during the week due to the unfavourable weather conditions. However, both the western and eastern triangles of the network were measured, on 18 and 20 May respectively. The data from the western triangle shows no consistent trends, as it has since monitoring began. The line lengths on the eastern triangle show about 7-10mm of shortening since they were last measured on 11 May, consistent with the long-term trend of about 1 mm per day seen since mid-November.
The University of Puerto Rico GPS experiment was concluded on 18 May. The data has yet to be fully processed, but indicate that changes since October 1995 have been relatively small (e.g. about 2 cm of extension on the line lengths between Roche's Yard-Reid's Hill and Roche's Yard-Harris).
The MVO GPS programme had to be curtailed because of equipment problems this week. Measurements were only made on 17 and 21 May; the latter was a long-period occupation to determine accurate positions for two of the base stations.
Gas Measurements
SO2 flux measurements were made daily using a COSPEC mounted in a car and driven between Cork Hill and St. Patricks under the gas plume. The estimates of the SO2 flux for the week under review are listed in the table below. No measurements were possible on 16 May due to technical problems with the instrument. The results show some variability but are consistently low, with a mean value of 160 tonnes per day.
Date Number Mean SO2 flux Standard
of runs (Tonnes/day) Deviation
16/05 0 - -
17/05 5 130 29
18/05 5 129 39
19/05 5 203 54
20/05 4 164 31
21/05 5 205 56
22/05 n/a 130 n/a