Montserrat Volcano Observatory

Daily Report
Report for the period 8 pm 4 August
to 10 pm 5 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 4:45 am this morning and 16:58 pm this afternoon.

Although the reporting period is extended, 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. 10 rockfalls, 78 hybrids were recorded during the day up to 4:00 pm. Two volcano-tectonic earthquakes were also recorded. Most of the hybrid earthquakes occurred in two swarms, this first started at 0250 and continued until after 0500, consisting of over 55 events. The second swarm started at 1417 and continued until after the recording period.

Both periods of activity culminated in violent explosions and intense pyroclastic flows. The explosion columns rose vertically then spread out umbrella fashion. Pumice blocks of up to 8cm in diameter fell over the observatory and elsewhere in the buffer zone. Loud audible rumblings were heard at the time of both the explosions.

Pyroclastic flows from the afternoons activity were observed by MVO field teams positioned at strategic locations around the volcano. Flows occurred down the Tar river valley extended into the sea and vigorous steaming was observed. Preliminary observations suggest as much as 80% of the delta was covered by new material. Flows also travelled down Tuitts and Mosquito Ghauts reaching as far as the junction between Paradise and Tuitts and Harris respectively. There were also reports of over-spill from Mosquito Ghaut into the Farrells Yard area.

Successive pulses of ash generation followed the pyroclastic flow activity, occurring every 20-30 seconds. This was coincident with the development of 1 Hz harmonic tremor recorded on the seismic network which lasted for about 40 minutes before falling back down to below the background levels.

An early morning helicopter flight confirmed that flows overnight had occurred in Tuitts Ghaut and Gages. The flow down Tuitts reached as far as the junction with Paradise Ghaut The new flow deposits in Gages were probably the combined result of yesterdays 1645 and todays 0445 events. Flows reached the sea at Plymouth and avalanche jumped over the North wall of the Ghaut and spread over parts of Northern Plymouth. A new lobe of deposit was also observed in the depression east of St. George's Hill. Fort Ghaut no longer exists in most places west of the lower Soufriere, with pyroclastic flow deposits spreading out over the old fan. New deposits were also observed in the Tar river valley extending on to the delta and also in the Galways area. Meter sized impact crates were observed extending to 1.5 km south of the dome, scattered over the NE side of the South Soufriere hills.

The activity continues to follow a cyclical pattern with an approximately 12 hour periodicity, however tiltmeter data is no longer available and it seems likely that this is due to damage caused by ballistics However, the periodicity also shows up well in the RSAM data.

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. The observatory will assess the situation after overnight activity and report to the government of Montserrat tomorrow morning. Everyone should continue to stay alert, and listen to Radio Montserrat for any announcements. The new ash is still in the air in the west of Montserrat and therefore dust masks should always be worn in these areas. Drivers should also be considerate whilst driving in ashy conditions.

Montserrat Volcano Observatory