Volcanic activity at the Soufriere Hills volcano was at a relatively high level today, with rock falls occurring throughout the day and two pyroclastic flows in the Tar River valley, the larger of which almost reached the coast.
Seismicity has been dominated by moderate rockfall signals, although the closest stations have also been recording abundant, small long-period events. Rock fall signals totaled 60 during the day. The largest rock-fall signals occurred at 10:50, 13:51, 14:04 and 15:35 on 31 May, the later two were associated with the two pyroclastic flows down the Tar River valley. A total of 13 large long-period events and one volcano-tectonic earthquake were recorded by the seismic network today, the v-t being located at a depth of 1.5 km (1 mile) beneath the crater area. A renewed period of small, repetitive long-period events began at 17:15 on 30 May at a rate of about 1 per minute, this increasing to about 2 per minute around 13:40 today (31 May). In the past, such seismic activity has been associated with increases in dome growth rate.
>From late morning onwards, viewing conditions were excellent and a number of observations were made from the ground and from the helicopter. Activity in the late morning was concentrated on the eastern and northern flanks of the dome, although new material was seen to be avalanching down from the southern moat to the south of Castle Peak into the uppermost part of the southern fork of the Tar River. Large volumes of unstable, hot material were noted high on the dome especially on the eastern flank, and new growth and increased steaming was noted from the southeastern flank. The two pyroclastic flows were both generated from the eastern flank of the dome, following a well-developed channel into the Tar River valley. The larger flow reached to within about 1,000 feet of the sea, whilst the later flow reached well past the Tar River Soufriere. Both flows produced ash clouds to a height of 7 to 10,000 feet, which were blown northwestward depositing ash on the Cork Hill and Richmond Hill areas especially. Later in the afternoon, the first large rockfall from the southern moat was witnessed from the air, with material moving a few hundred feet out of the crater.
EDM measurements were made today on the eastern triangle. Line lengths to Castle Peak have shortened by 6 to 12 mm since yesterday. These results continue to indicate very small changes, although at a slightly increased rate than over the past few months. This may be the result of increased dome growth activity in the area behind Castle Peak. It was not possible to make measurements with the new GPS equipment due to continuing technical problems; it is hoped that the source of the problems has now been identified and replacement parts would soon be available.
COSPEC measurements of gas concentrations in the volcanic plume were attempted today, although poor conditions prevented good data from being collected. Results from yesterday suggest continued low levels of volcanic gases in the plume over Plymouth; sulphur dioxide levels were measured at about 200 tonnes per day, similar to levels over the past few weeks.
Today's pyroclastic flows demonstrate the continued very dangerous nature of the current volcanic activity at the Soufriere Hills volcano, which is causing grave concern to the scientists. The MVO urges that visits to the evacuated zone are kept to a minimum. The Tar River, Long Ground and Whites areas are extremely dangerous and should not be entered under any circumstances.