Montserrat Volcano Observatory

Daily Report
Report for the period 4 pm 18 July
to 4 pm 19 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, although there has been an apparent increase in ash production.

Heavy rain overnight and throughout the day resulted in poor viewing conditions in the morning, but later in the day some views were obtained of the dome. There is a spine with three peaks above the Galway's wall, although it is not clear when this was formed, since this part of the dome has not been seen clearly for several weeks. New pyroclastic flow deposits to about 500 m away from the dome were noted over Gages, Galway's and Mosquito Ghaut, and several small pyroclastic flows with associated ash clouds have occurred throughout the day, mostly from the northern flank of the dome. Each of the ash clouds has risen to less than 10,000 ft, and they have usually been followed by strong steam emission.

Small to moderate sized rockfalls signals have been the main type of seismic events recorded on the seismographs. The number of rockfall signals was about the same number as yesterday with 27 events being recorded, and none were of particularly long duration or high amplitude. No long period, volcano-tectonic or hybrid earthquakes were recorded by the broad band system. Moderate amplitude tremor on the St. Patrick's, Windy Hill and St. George's Hill seismometers occurred for approximately 30 min at 3.54 am. It is likely that this was due to a mudflow, possibly in either Fort Ghaut or Aymers Ghaut since the tremor was most apparent on the St. George's Hill station. However, no significant deposits could be seen in any of the ghauts. Several high frequency signals with impulsive starts due to thunder also occurred early in the morning. The Chances Peak tiltmeter does not currently show any obvious cyclical behaviour but the long term deflationary trend has continued, although at a lower rate. A reconnaissance flight was taken today to review the state of the Roche's Yard, Bethel and Galway's Estate broad band seismometers.

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