Montserrat Volcano Observatory

Daily Report
Report for the period 4 pm 17 August
to 4 pm 18 August 1997

Activity at the volcano has remained at a high level. As in the last few days, activity is still dominated by alternations of hybrid earthquake swarms and dome growth. The seismic swarms appear to be occurring rather regularly at intervals of about eight hours at present. Over the last 24 hours two cycles have occurred followed by extended periods of rock fall and pyroclastic flow production.

248 hybrid earthquakes and 16 rockfalls triggered on the broadband seismic network over the last 24 hours. No volcano-tectonic earthquakes or long period events were recorded. The hybrids occurred in two swarms. The first hybrid swarm began at around 11:54 pm yesterday evening and lasted until about 3:40 am this morning. The swarm merged into tremor at around 2:30 am which lasted 40 minutes. During this period near-continuous ash production was observed. 140 separate hybrid earthquakes were recorded by the broadband network before the events became too frequent to trigger the detection system. Rockfall activity then reduced to very low levels until a second hybrid swarm started at 11:31 am and continued until around 3:00 pm. 88 separate hybrid earthquakes were recorded during this swarm. Continuous tremor occurred for approximately the last 40 minutes of this period of elevated activity. Small pyroclastic flows were observed in the Gages Valley during the swarm this afternoon. Seismic signals produced by small pyroclastic flows have been detected following this swarm but low cloud has hampered the determination of region in which the flow took place.

Clear views of the dome were possible between 3 and 4 am this morning. String incandescence was observed from the dome which spread into the Gages Valley on two occasions. This is believed to be the result of hot material avalanching into the upper parts of the Gages Valley. The hybrid swarm this afternoon was accompanied by continuous ash venting. This became most intense at the peak of the seismic activity and developed a plume of ash and steam at approximately 5000 feet which was blown westward out to sea.

Further explosions are possible and these may be more intense and longer lived than those already experienced. If explosions do occur, the central zone should be evacuated immediately, and people in the northern zone should seek shelter under a strong roof as soon as possible. After an explosive event, small rocks and ash can be expected to fall anywhere on the island. Ash and falling rocks can make driving hazardous. Ash is present in the atmosphere and dust masks should be worn outdoors. The enhanced pyroclastic flow activity indicates increasingly instability of the dome and further pyroclastic flow activity is anticipated.

All ghauts on the volcano are now filled with hot pyroclastic flow deposits. It is expected that as the current elevated level of activity continues, further pyroclastic flows could occur on all flanks of the volcano. This makes all of the exclusion zone extremely dangerous. Pyroclastic flows could reach into the Belham river valley. Access to the exclusion zone is completely restricted, and people should stay away from the flanks of the volcano. The MVO is working on new hazards maps in view of the increased level of activity. People should remain vigilant and continue to listen to Radio Montserrat for any announcements.

Montserrat Volcano Observatory