Montserrat Volcano Observatory

Daily Report
Report for the period 4 pm 16 August
to 4 pm 17 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. However the pattern is now slightly less regular with longer intervals and higher intensity of tremor at the end of a swarm. Over the last 24 hours two cycles have occurred followed by extended periods of rock fall and pyroclastic flow production.

354 hybrid earthquakes and 23 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 5:34 pm yesterday afternoon and lasted until about 11.22 pm yesterday evening. The swarm merged into tremor which lasted 90 minutes. Rockfall and pyroclastic flow signals then occurred culminating in a significant pyroclastic flow signal at 4.55am. The location of the flow could not be determined. Rockfall activity then reduced to very low levels until a second hybrid swarm started at 10.54am and continued until 13.08am when the swarm again merged into tremor for a further 90 minutes. Towards the end of the period of continuos tremor several pyroclastic flows moved down the Gages valley with the largest at about 2.05pm which is estimated to have reach close to Lovers Lane on the outskirts of Plymouth. A well-defined period of monochronomatic seismicity occurred for 6 minutes towards the end of the period of tremor which resembles the signals associated with explosions.

Ash clouds have been observed between 2pm and 2.30pm the day as the result of pyroclastic flow activity down the Gages valley. Periods of enhanced steam emission has also been observed during periods of high seismicity.

Surveys of the pyroclastic flow deposits produced in the last two weeks are close to completion and will be reported soon. Cloudy weather prevented any views of the dome.

Continuing evidence of small explosions demonstrate the potential for explosivity. 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