Seismic activity at the Soufriere Hills Volcano during this period has decreased. It is still dominated by the occurrence of small, repetitive hybrid earthquakes. The rate has decreased from about one every two minutes to about one every three or four minutes during the period under review. The size of these hybrid earthquakes has remained fairly constant.
The number of seismic signals interpreted as representing rockfalls from the growing dome continued to increase, although these still represent a small proportion of the total seismic activity. Some of these rockfalls generated small ash clouds and most seemed to occur on the north-eastern flank of the dome, near the bottom of the newest spine. A moderate-sized explosion at 15:11 on 21 April generated an ash column which rose to a height of 5,000 to 6,000 ft above sea level. The explosion generated a small pyroclastic flow down the Tar River valley. A helicopter inspection shortly after the explosion showed that the pyroclastic flow travelled about 1,000 ft from the base of the dome. The pyroclastic flow generated an ash cloud which rose to about 4,000 ft above sea level.
EDM measurements were made on the southern triangle of the EDM network. There were no significant changes since the previous measurements on 11th April. Eight GPS lines were measured today but the results are not yet available.
We apologise for being unable to provide the revised estimates of dome volume and extrusion rate as promised in yesterday's report. The methods used in calculating these quantities are being re-examined and we expect to provide results within a week. However, it can be stated that the values quoted in the report 16:00 on 18 April 1996 were considerably overestimated.
Although there has been a slight reduction in the level of activity at the volcano during the past few days, scientists at the MVO still view the situation with grave concern. The MVO continues to urge 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.