Montserrat Volcano Observatory

Daily Report
Report for the period 16:00 24 April
to 16:00 25 April 1996

The seismic activity has been at a low level during the last 24 hours. The small, repetitive hybrid earthquakes have continued, although they are smaller than before and occurring only about once every two or three minutes. A few small rockfall signals were also recorded, but none of them had large explosive components. One volcano-tectonic earthquake was located at a depth of 2 km southeast of the South Soufriere Hills. Some episodes of low-amplitude broadband tremor have been recorded at the Gages seismic station.

Several very small earthquakes have been recorded by the Gages seismic station over the last few days. Another seismometer was installed at Gages last night to confirm that the earthquakes are not the result of instrument malfunction. The cause of these small earthquakes is not known, but appears to be localised in the Gages area. Swarms of similar events have been identified in previous records from the Gages seismic station, especially during July and August 1995.

The eastern EDM triangle was measured today. The line length changes are small and consistent with the trend of 1 mm shortening per day that has been measured since November 1995. Six GPS lines were measured across the volcano today, between the points in the University of Puerto Rico network. The results will be processed overnight. The results from the GPS lines measured during the last two weeks show changes that are within the error of the technique. Further measurements will be required before any significant large-scale deformation of the volcano can be detected.

Observations of the volcano have been possible at times during the day, from the ground and during two helicopter inspections. Fewer rockfalls were observed today than on previous days, and were concentrated in the northeast of the dome. The two spines in the centre of the dome have not changed since yesterday, and several large blocks still remain in unstable positions in the eastern dome. Vigorous steaming was seen from various parts of the dome, especially the south dome, which was also emitting significant amounts of light brown gas this afternoon.

The volume of the dome has been estimated at 10 million cubic metres, following a careful analysis of photographs taken on 18 April. Further measurements from the ground and from recent photographs will now be used to estimate the current rate of dome growth.

Despite the lower level of activity during the past few days, the volcano still remains in a highly dangerous state, and further explosive activity and pyroclastic flows could happen with little notice. Therefore, the scientists continue 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.

Montserrat Volcano Observatory