Montserrat Volcano Observatory

Daily Report
Report for the period 4 pm 19 August
to 4 pm 20 August 1997

Activity at the volcano has remained at a high level. Seismicity continues to be concentrated in bursts which occur at intervals of about eight hours. Over the last 24 hours two such bursts have been recorded but, in a departure from recent behaviour, only the first burst consisted of a hybrid swarm. The second period of intense seismicity consisted of high frequency signals like those normally associated with pyroclastic flows. There were several small pyroclastic flows but for the most part only ash venting was observed at the volcano in this period.

217 hybrid earthquakes, 1 volcano-tectonic earthquake and 52 rockfalls triggered the broadband seismic network over the last 24 hours. No long period events were recorded. The hybrids mainly occurred in a swarm between 7.10 pm and 11.20 pm yesterday evening. After the swarm there was very little seismicity until 10 am this morning when high amplitude, high frequency signals started to be recorded. Such seismicity is usually associated with rock falls and pyroclastic flows and over the next two hours 4 small pyroclastic flows were observed going into Gages valley. For most of the time that this signal continued, however, only vigorous ash venting was observed at the dome.

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. People should remain vigilant and continue to listen to Radio Montserrat for any announcements.

Montserrat Volcano Observatory