Montserrat Volcano Observatory

Daily Report
Report for the period 4 pm 18 August
to 4 pm 19 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.

181 hybrid earthquakes, 5 long period events and 37 rockfalls triggered on the broadband seismic network over the last 24 hours. No volcano-tectonic earthquakes were recorded. The hybrids occurred in two swarms. The first hybrid swarm began at around 9:15 pm yesterday evening and lasted until about 12:35 am this morning. The swarm was followed by rockfall activity then reduced to very low levels until a second hybrid swarm started at 7:28 am and continued until 10:29 am. Small pyroclastic flows were observed in the Gages Valley and Mosquito Ghaut during this swarm. Seismic signals produced by small pyroclastic flows and rockfalls have been detected following this swarm. This second swarm was accompanied by periodic steam and ash venting from the summit area of the dome. This produced a moderate ash plume which was carried toward the north west by the prevailing wind

Views of the dome have been possible from both the ground and the helicopter during this afternoon. A helicopter inspection flight revealed that there is a large portion of dome material perched above the Gage's Wall. This area is approximately 150 meters in diameter and made up of a series of tall spines. Measurements from the Observatory indicate one of the spines is reaches an elevation of approximately 970 meters (3180 feet) above sea level. A second mass is also precariously perched above the Galway's Wall. These unstable masses may not last very long and may collapse from the dome producing pyroclastic flows in the Gages and Galway's valleys.

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