Montserrat Volcano Observatory

Daily Report
Report for the period 4 pm 10 August
to 4 pm 11 August 1997

Activity at the Soufriere Hills Volcano has continued at an elevated level during the reporting period. Hybrid earthquakes swarms remain the dominant seismic signal.

287 hybrid earthquakes, 28 rockfalls and 4 long periods earthquakes were recorded by the seismic network today. No volcano-tectonic earthquakes were recorded. Most of the hybrids occurred in three swarms. The first, between 4:18 pm and 9:11 pm yesterday contained 154 hybrids and was the most intense of the period. The second which included 58 events occurred between 1:32 am and 4:17 am today. The third swarm occurred between 8:31 am and 11:13 am and included 72 events. The third swarm was followed by a small explosion at about 11:38 am. No audible sound was heard from this explosion but pyroclastic flows were generated in Mosquito Ghaut and Gages valley. The flows appeared diffuse with weakly convecting ash clouds. The ash plume produced by the event convected slowly to about 8000 ft.

The summit of the volcano has been obscured for the entire day due to low cloud and vigorous steaming from the area of the new crater. A midday observation flight allowed examination of the distribution of pumice flows from last week's explosions. Pumice flows have extended along the whole of Tuitts Ghaut down to Farms village at the juncture of Farms and Pea Ghauts. In Mosquito Ghaut, the flows have extended to just south of Harris village. The upper Tar River valley is now covered with a thin veneer of pumice flows and about 80 % of the new delta is now covered. The maximum run-out of pumice flows in the Galways area is about 600 m from the sea. In the Gages valley and Fort Ghaut the pumice flows have reached about 500 m from the sea. In Tyre's Ghaut (previously called the unnamed ghaut), the flows appear to have extended just past the junction with the Belham stream.

Theodolite measurements of the new crater carried out during clear conditions two days ago gave a volume of between 5 and 7 million cubic metres for this feature. The highest point measured on the dome at that time was 981 metres or 3266 feet above sea level.

The activity of the volcano has changed over the last three days. The 12 hour periodicity seen previously is now broken and appears to have been replaced by hybrid earthquake swarms and dome growth. The small explosion which occurred today demonstrate the potential for explosions to occur with little or no warning during this period. Further explosions can occur in the future 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. 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