Montserrat Volcano Observatory

Daily Report
Report for the period 16:00 22 July
to 16:00 23 July 1996

The phase of activity which started on the evening of 20 July continues. Rockfalls from the dome continued at a slightly increased level.

Seismic activity is still dominated by small- to moderate-sized rockfalls from the flanks of the growing lava dome. The rockfall activity, which was mainly located on the northeastern flank of the lava dome, was near-continuous at times, resulting in the recording of almost continuous low- to moderate-amplitude broadband tremor at the closest stations to the crater. More than 150 rockfalls were recorded in three distinct periods. The first was between 17:00 and 24:00 on 22 July and shows a gradual increase in activity. The other two were between 05:00 and 08:00 and between 11:30 and 13:30 on 23 July. The largest recorded rockfall was in this last period, at 11:58. Approximately 15 VT earthquakes were recorded in this reporting period, a significant decrease compared to 82 in the previous 24 hours. Complete locations for these earthquakes have not yet been calculated due to the processing of the backlog of data. Preliminary examination of the data, however, indicates that the locations are probably similar to those determined in the last two days, beneath the crater at depths of less than 3 km. Small hybrid earthquakes were recorded at an unchanged level - 101 compared to 114 yesterday. The number of long-period earthquakes declined noticeably, with 24 compared to 105 yesterday. Intermittent low- to moderate-amplitude broadband tremor was recorded at stations near the crater throughout the reporting period.

Visibility was good in the early morning and a number of observations were made from the ground and the helicopter. Near-constant rockfalls were observed at this time on the northeastern flank of the dome, resulting in almost continuous ash production. Most of the rockfalls were channelled down the gully that has been eroded on the northeast flank. None of them reached as far as the Tar River Soufriere. The ash cloud drifted southwest on the prevailing wind over Chances Peak, Gingoes and St Patricks. A few rockfalls were seen on the western flank of the lava dome. No rockfalls were seen on the south and southwest flanks of the dome and there has been no further material deposited against Farrells or Galways Walls, nor any over Gages Wall. Visibility was variable during the rest of the day, with brief intervals when the slopes of the dome were visible. Rockfall activity was noted throughout the day on the northeastern and northern flanks of the dome. Two small pyroclastic flows were observed, at 12:30 and 12:41 on 23 July. The first of these set fire to some trees in the Upper Tar River Valley.

Gravity and GPS measurements were made today at stations along a radial line on the eastern flank of the volcano, ending at Hermitage. Gravity and GPS measurements were also made along the ridge between Perche's and Roaches Mountains. COSPEC measurements of the SO2 flux were carried out, but the data is still being processed.

The current phase of activity may well continue. More rockfall activity from the eastern and northeastern parts of the dome and small pyroclastic flows in the Tar River Valley should be expected. These will be associated with ash clouds and possible ashfall downwind.

The Soufriere Hills Volcano is considered to be highly dangerous to people and property on it's eastern and upper western flanks. Visits to the evacuated zone must be kept to a minimum. The Tar River and Long Ground areas to the east and upper Fort Ghaut, Gages Village and Upper Amersham areas to the west are all extremely dangerous. All access roads to these areas remain closed and people should not enter these areas under any circumstances. If they do, they put themselves and others at direct risk of serious injury or death.

Montserrat Volcano Observatory