General Situation
This week has seen a decrease in the number of major explosive events compared to the previous week. Only one pyroclastic flow at 15:59 on 12 April was observed. This flow, like previous ones, was confined to the upper part of the Tar River valley away from inhabited areas. Seismic activity at the volcano continues at an elevated rate in the form of frequent hybrid events.
Seismicity
The seismicity continued at a level similar to that observed in the previous review period. The dominant feature was the continuation of the intense hybrid activity which began on 09 April. Throughout the period under review the size and frequency of these events have varied. During the early part of the period small hybrid events were occurring at rates between 1 and 5 events per minute. The number of triggering events reached a peak on 11 April and then declined to a trough on 14 April. A further increase in triggered events occurred up to 15 April followed by a slower decline to 17 April. The number of triggers in this period did not reach the number of triggers for the period 11-14 April. During the periods of reduced numbers of events the size of individual hybrid events was greater. RSAM data can be divided into two distinct periods. A steady increase in energy release took place up to the evening of 15 April. From then to the end of the review period the energy release has returned to the level seen at the beginning of April. At the end of the review period the hybrid events were occurring at a rate of 1 every 2 minutes.
Broad band tremor was recorded on the closest stations to the active area (Gage's and Chance's Peak). The longest episode of this activity occurred on 17 April and lasted for over an hour.
Small explosion and rockfall seismic signals were recorded every day during the period under review. The majority of these have only been small events creating small, non-convecting ash clouds. The largest event since 08 April occurred at 15:59 on 12 April. This produced a small pyroclastic flow in the upper Tar River area. A twenty-five minute period of explosions and rockfall began at 20:37 on 13 April. This was followed by a decrease in the number of hybrid events until the morning of 14 April after which they increased to around 3 events per minute.
Deformation Measurements And Observations
All EDM triangles were reoccupied during the week with the exception of the triangle involving the reflector on Gage's Wall, which is now obscured by ash. The present level of activity of the volcano is too high for an expedition to be made to clean the reflector. Measurement to the reflector below Castle Peak from both Long Ground and White's Yard continue to show that the slow shortening trend of mm per day is still continuing. This trend has been apparent since the end of November 1995. The reflector is slowly moving down and outwards towards the north to northeast. Occupation of the Dagenham-Amersham-Upper Amersham-Chance's Steps EDM network showed that the very small changes (of the order of 0.3 mm per day) are continuing from December 1995. New GPS (Global Positioning System) equipment has been successfully deployed numerous times during the week. The new equipment allows rapid occupation of sites. Around 15 radial lines from the flanks of the volcano have been measured at least once in order to build up a database for future comparison. A precision level of 10 mm in any direction has been consistently achieved during the occupations. No changes have been recorded which exceed this level. A reoccupation of a GPS network established by scientists from the University of Puerto Rico was made on 11 April. This is the first reoccupation this year and will be repeated in the future to re-establish baseline measurements across the volcano.
The Long Ground tiltmeter is still transmitting data to the Observatory. No major deformation events have been recorded during this week. It is hoped that a more detailed analysis of the data collected by this instrument can be made.
Visual Observations
The activity in the crater area has been dominated by rockfalls and small explosions creating small ash clouds which have drifted westwards. The spine reported in the last scientific report (number 19) lasted until 12 April when it collapsed. Material from this collapse fell towards the southwestern part of the dome. The last measured height of this feature was 899 m ASL. On 15 April a new spine was identified growing to the east of the remnant of the last spine. Break-up of this feature and further break-up of the remnant spine occurred on 17 April.
The active area of the dome (identified by areas of repetitive rockfall/explosive activity) has again changed throughout the week. The main areas of activity have been the unstable flanks of the dome in the east and northeast. Activity was also observed in the southwest and western areas on 12 April.
Steam production has been almost continuous during the week, along with SO2 emission. The area of steam production has varied during the week.
Summary
Activity at the Soufriere Hills Volcano continues at an elevated level. The style of the eruption has changed slightly from last week in that no major explosive events occurred. However, the continuation of hybrid seismic activity accompanied by rockfall and small explosions indicates that the dome is still growing. Ground deformation continues at a slow rate. New GPS occupations are on-line and a database is being constructed.