Montserrat Volcano Observatory

Daily Report
Report for the period 4 pm 14 August
to 4 pm 15 August 1997

Activity at the volcano has remained at a high level and, following the pattern of the last few days, is still dominated by hybrid earthquake swarms and dome growth.

293 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 three swarms. The first hybrid swarm began at 7:19 pm yesterday evening and lasted until about midnight. The intensity of the swarm gradually increased to a peak at just before midnight. A large amplitude, long duration signal at 11:05 pm was probably a pyroclastic flow down the north flank of the volcano. A further hybrid swarm started at 4:44 am, continued until 11:32 am and contained 83 hybrid earthquakes. Towards the end of the swarm, the hybrids merged into continuous high amplitude tremor and, as a result, many high amplitude events would not be recognisable. The swarm stopped extremely abruptly with no visible consequences. A particularly large amplitude signal was recorded at 6:08 am near the beginning of the swarm and saturated on all drums for 30 seconds. A detonation was heard from the volcano at this time, and it was followed by a pyroclastic flow down Gages valley and a gently rising ash column. A third hybrid swarm started at 2:22 pm and is still continuing at the time of reporting. Between the swarms the predominant seismic signals were due to small pyroclastic flows and rockfalls. The recent hybrid earthquake swarms are thought to be associated with extrusion of new dome material. The high levels of seismic activity suggest that extrusion rates are rapid.

EDM measurements were made today on the MVO to Lees Yard and Waterworks to Lees Yard lines. These show no significant changes since the last measurements.

The small explosions which occurred on Monday and Tuesday demonstrate the potential for explosive eruptions. 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.

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

Montserrat Volcano Observatory