Montserrat Volcano Observatory

Daily Report
Report for the period 4 pm 11 August
to 4 pm 12 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.

107 hybrid earthquakes, 19 rockfalls and 1 long period earthquake triggered the seismic network today. No volcano-tectonic earthquakes were recorded. Most of the hybrids occurred in two swarms between 06:52 pm and 11:50 pm and between 05:39 am and 10:10 am. The actual number of hybrids can be assumed to be much higher than the number of triggered events as both swarms were very intense with earthquakes occurring very close together. When earthquakes get as close together as this many of them do not trigger the system. The second swarm was followed by a small explosion at about 10:12 am. A prolonged rumble was heard at the observatory and pyroclastic flows were generated into the tops of Tuitt's Ghaut, the Un-named 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 dome has been obscured by cloud for most of the day but a brief glimpse was obtained at about 12:55 am. This revealed that a very large spine has grown in the new crater.

A GPS survey of the volcano's Eastnet was conducted this morning using full helicopter support. This involves sites at Harris, Whites, Long Ground and Windy Hill. Recent data indicate a deformation event has occurred in the last 3 months shifting the sites at Whites and Long Ground about 3 cm to The North and North East. Today's results suggest the event is not continuing as the movement of the sites has slowed and they may be returning to their original position.

Activity at the volcano is dominated by hybrid earthquake swarms and dome growth. The small explosions which occurred yesterday and 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.

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.

Professor Steve Sparks arrived at the MVO today.

Montserrat Volcano Observatory