Montserrat Volcano Observatory

Daily Report
Report for the period 10 pm 5 August
to 4 pm 6 August 1997

The cyclical pattern of activity at the volcano with hybrid earthquakes and tiltmeter cycles has continued during the period. There were two intense periods of activity at about 4:00 am this morning and 2:30 pm this afternoon.

Although the reporting period is compressed, the following earthquake count is for the period from 4 pm yesterday to 4 pm today. The count was obtained using the short period seismic network. 8 rockfalls, 132 hybrids and four long period events were recorded during the day up to 4:00 pm. No volcano-tectonic earthquakes were recorded. Most of the hybrid earthquakes occurred in two swarms, the first started at just after 1:30 am and continued until after 4 am. The second swarm started at 10:34 am and continued until 2:34 pm.

Both periods of activity culminated in violent explosions and intense pyroclastic flows at 4:02 am and 2:34 pm. The explosion columns rose vertically to 15000 to 20000 feet then spread out umbrella fashion. Loud audible rumblings were heard at the time of both the explosions, and lightning occurred in the ash clouds. The explosions were followed by pulsing of ash clouds from the summit of the dome approximately every 10 seconds, and corresponded to long period tremor.

An early morning helicopter flight confirmed that the pyroclastic flows from yesterday and this morning's eruption were mostly pumice flows in the Tar River valley and Mosquito Ghaut. Samples of the pumice were collected from the Tar river delta. Pumice flows extended further to the north-east of St. George's Hill, and a new pyroclastic flow had extended into the upper reaches of the Dyers River. This was probably fed from the un-named ghaut to the wet of Mosquito Ghaut and is an indication that column collapse must have occurred during yesterday evening's eruption. Impact craters were also observed on Hermitage, Gages and the north side of Galways. There seemed to be no further activity down Fort Ghaut into Plymouth.

Pyroclastic flows from this afternoon's activity were observed by MVO field teams positioned at strategic locations around the volcano and in the helicopter. Flows occurring down the Tar river valley just entered the sea, and were not as extensive as the flows in this direction yesterday. Flows also travelled down Tuitts and Mosquito Ghauts, the White River and the Gages valley. Down Tuitts Ghaut the flows reached at least as far as the junction with Paradise River. Down White River the flows reached as far as the previous position of the Great Alps Falls. It was difficult to assess the extent of flows down Gages valley because of heavy ashfall over Plymouth. Observations of the explosion seemed to indicate that pyroclastic flows were generated very soon after the vertical blast, and initially travelled very quickly into the tops of the ghauts in all directions. Several minutes after the explosion, the flows were seen to be moving relatively slowly near to their ends. No fallout was deposited in Old Towne as a result of either of the explosions today.

The activity continues to follow a cyclical pattern with an approximately 10 to 12 hour periodicity. The periodicity shows up well in the earthquake data, and once a hybrid earthquake swarm starts, people are advised to move out of the central zone.

The Gages valley is now filled with hot pyroclastic flow deposits and under no circumstances should people venture into this area. It is expected that as the current elevated level of activity continues further pyroclastic flows would occur in the Gages valley or other flanks of the volcano. This makes Plymouth extremely dangerous. The Belham River valley is also very dangerous and should not be entered at all. Access to the exclusion zone is completely restricted, and people should stay completely away from any of the flanks of the volcano. The central zone is evacuated overnight and people should not return to their homes until advised to do so. Everyone should continue to stay alert, and listen to Radio Montserrat for any announcements.

Montserrat Volcano Observatory