Montserrat Volcano Observatory

Daily Report
Report for the period 4 pm 9 August
to 4 pm 10 August 1997

The activity of the volcano has continued at a high level during this reporting period. Hybrid earthquakes have been the main seismic signals recorded by the network. New dome growth was observed in the scar left by the explosive activity of last week.

233 hybrid earthquakes, 23 long periods and 8 rockfalls were recorded by the seismic network today. No volcano-tectonic earthquakes were recorded. Most of the hybrids occurred in two swarms. The first, between about 11 pm and 3 am, was less intense than the swarms experienced yesterday. The second started at 6 am and continued until 11:12 am. This was at the same intensity as the large swarm yesterday morning, and progressed into near continuous high amplitude tremor at around 7 am. The number of hybrid events is probably far higher than that recorded since many of the closely spaced events will not trigger the seismic software.

A brief clear view of the summit of the volcano was obtained this morning. A growth of new dome material was observed in the deep oval-shaped scar formed by the explosive activity of the past few days. Thus it appears that the hybrid earthquakes are associated with rapid extrusion of new lava.

The activity of the volcano has changed over the last few days. The 12 hour periodicity seen previously seems to be broken and, although no explosions have occurred for the past 36 hours, there is no guarantee that further explosions will not take place. 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 also make driving hazardous. Ash is present in the atmosphere and dust masks should be worn outdoors. MVO are currently assessing the levels of ash in the occupied zone using a hand held dust monitor.

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 central zone is evacuated overnight and people should not return to their homes until advised to do so.

Such changes in the pattern of activity further emphasize the inherent unpredictability in the volcanic system. The risk of further explosions remains high. People should remain vigilant and continue to listen to Radio Montserrat for any announcements.

Montserrat Volcano Observatory