Montserrat Volcano Observatory

Daily Report
Report for the period 4 pm 19 July
to 4 pm 20 July 1997

The alert level system has been revised by zones

Activity at the volcano has been at the same level as yesterday with rockfalls being the main type of seismic signals recorded. Overcast conditions for most of the day prevented any clear views of the dome.

Seismicity was dominated by small to moderate sized rockfalls signals, none of which were of particularly long duration or high amplitude. The number of rockfall signals was the same as yesterday with 27 events being recorded. The only other seismic events recorded were 3 long period earthquakes. There were no volcano-tectonic or hybrid earthquakes. Two periods of low to moderate amplitude tremor occurred on both the St George's Hill and St Patrick's seismometer between 12:13 to 12:26 am and 1:08 to 1:25 am. These were most likely due to mudflows, possibly in Fort Ghaut or Aymers Ghaut, caused by remobilisation of recent deposits by the heavy rain experienced yesterday. The Chances Peak tiltmeter does not currently show any obvious cyclical behaviour.

A reoccupation of GPS stations located at Lees Yard, Garibaldi Hill, Waterworks and the MVO was carried out today. The results of this survey is currently being processed and would be reported later in the week. EDM measurements from Garibaldi Hill were made to the Lees Yard reflector for the first time today. These measurements, along with continued measurements from Waterworks would be used to help determine whether deformation is occurring on the northern flank of the volcano. The EDM line from Waterworks to Lees Yard was also remeasured today. This line continue to show no major changes on a daily basis, although the line length has consistently shortened since it was first established on 12th July, 1997.

Although there appears to be very little seismic activity from the volcano, there have been periods in the past when the dome has continued to grow with few earthquakes apart from rockfalls. Thus material is currently building up above Gages wall and Mosquito Ghaut, and the potential for large pyroclastic flows onto the northern and western flanks into the Central corridor and Plymouth is significant.

Residents should note that pyroclastic flow and ash eruptions have occurred during the past week with no direct association with seismicity or tiltmeter readings. This means that people entering the designated exclusion zone put themselves at extreme risk. People in the northern and central zones should stay alert and listen to Radio Montserrat. The current area of activity in the crater makes Mosquito Ghaut and Gages valley the most likely pathways for pyroclastic flows, but further flows in Tuitt's, Tar River or White River are possible as well.

The Belham River valley is also an area for anxiety over safety. It may provide a pathway for further pyroclastic surges and there is also a risk of hot mudflows which may form rapidly if there is a further period of heavy rain. Mudflows travel extremely fast and may be near boiling point, they may extend much further along the Belham River valley than the pyroclastic surges. The pyroclastic flow and surge deposits will remain extremely hot for several days. Residents must not approach, attempt to handle or walk on the deposits because of the risk of severe burning. Residents and workers are also reminded to wear a dust mask when there is ash in the air.

Montserrat Volcano Observatory