Montserrat Volcano Observatory

Daily Report
Report for the period 8 pm 3 August
to 10 pm 4 August 1997

The cyclical pattern of activity at the volcano with hybrid earthquakes and tiltmeter cycles has continued during the period. There were two intense periods of activity at about 6:00 am this morning and 4:45 pm this afternoon. These both had explosive components and generated pyroclastic flows which reached the sea in Plymouth.

Although the reporting period is extended, the following earthquake count is for the period from 4 pm yesterday to 4 pm today. However, following an intense thunder storm at lunch-time, the broadband system was not able to transfer events to the picking system and therefore these counts are not complete. 22 rockfalls, 1 long period events and 93 hybrids were recorded during the day up to 1:30 pm. No volcano-tectonic earthquakes were recorded. Most of the hybrid earthquakes occurred in two swarms.

The major pyroclastic flow activity of yesterday evening continued until after 7:30 pm. This was followed by small pyroclastic flows and rockfalls throughout the night. A moderate pyroclastic flow occurred at just after 2 am in the middle of the tiltmeter cycle. This emphasises that pyroclastic flows can occur at any time. An early morning helicopter flight confirmed that all the flows of yesterday evening were confined to the Gages valley. The flows which extended into Plymouth had reached the sea at Port Plymouth and ignited buildings along the entire stretch of Fort Ghaut. New flows had extended onto the northern flank of the Gages valley and had caused fires in Gages village area as far as the intersection with the St. George's Hill road where the police checkpoint used to be.

The first of the two hybrid earthquake swarms during this reporting period began at 3:58 am this morning and ended at 6:13 am. This was followed by a very intense period of pyroclastic flow and ash generation activity at 6:30 am. Loud rumbling was heard from the volcano at this time and rock fragments up to 5 mm diameter rained down on the Observatory. This was probably a result of an explosion from the dome.

The activity then consisted of rockfalls and small pyroclastic flows until the early afternoon. At about 2:30 pm a further hybrid swarm commenced and continued until about 4:30 pm. At this time ash venting was seen from the volcano, and at approximately 4:44 pm a dark grey jet was seen to project from the north flank of the dome at angle of about 60 degrees to the horizontal and to a height of about 2000 feet. Ballistic fragments could be seen showering the northern flank. A loud rumbling was also heard at this time. Shortly after this jet was initiated, pyroclastic flows started to pulse to the north down Mosquito Ghaut and to the west down Gages Valley. The flows down Mosquito Ghaut reached to Harris, and the strong pulse down Gages valley could be seen to convect strongly from behind St. George's Hill. This pyroclastic flow was the biggest seen so far into Plymouth and dark thick smoke was soon seen from the vicinity of Plymouth. A vertical ash cloud rose quickly to over 25,000 feet, and fragments of rock and pumice up to 1.5 cm diameter fell at the observatory. Views of Plymouth from the Police Launch after this episode showed that the centre of Plymouth was burning strongly. Burning buildings that could be identified included Government Headquarters, Police Headquarters and the Texaco station. A small flow down Tar River also occurred, but brief views of the dome suggested that no substantial loss of material had occurred.

The activity is following a cyclical pattern in the tiltmeter with regular inflation and deflation of the volcano corresponding to the earthquake swarms and continuous pyroclastic flows or explosions respectively. However, large pyroclastic flows have also occurred outside the most probable time period, and so at no time is it safe to enter the exclusion zone, including the Belham river valley. This type of activity is similar to that observed towards the end of June and the beginning of July, and it is possible that further major pyroclastic flows or explosions could occur.

The Gages valley is now filled with hot pyroclastic flow deposits and under no circumstances should people venture into this area. It is expected that as the current elevated level of activity continues further pyroclastic flows would occur in the Gages valley or other flanks of the volcano. This makes Plymouth extremely dangerous. The Belham River valley is also very dangerous and should not be entered at all. Access to the exclusion zone is completely restricted, and people should stay completely away from any of the flanks of the volcano. The central zone is evacuated overnight and people should not return to their homes until advised to do so. The observatory will assess the situation after overnight activity and report to the government of Montserrat tomorrow morning. Everyone should continue to stay alert, and listen to Radio Montserrat for any announcements. The new ash is still in the air in the west of Montserrat and therefore dust masks should always be worn in these areas. Drivers should also be considerate whilst driving in ashy conditions.

Montserrat Volcano Observatory