Montserrat Volcano Observatory

Daily Report
Report for the period 4 pm 16 July
to 4 pm 17 July 1997

The alert level system has been revised by zones

Activity at the volcano has been at a lower apparent level than yesterday. The seismicity is dominated by rockfalls signals, although three moderate pyroclastic flows occurred in Mosquito Ghaut.

Some clear views of the volcano were obtained today from the helicopter and on the ground. Several fresh rockfalls were noted on the eastern face and northern faces of the dome. Three pyroclastic flows occurred this morning at 3.28 am, 6.20 am and 7.13 am. The deposits from these extended up to less than 1 km from the dome and all occurred in Mosquito Ghaut. No major changes could be seen in the northern and western sides of the dome since 13th July except that the spine that was situated over the Gages wall had collapsed. The scar from the 25th June event is now hardly discernible, and there is still a large blocky extrusion over Gages wall. There was also considerable steaming from the top of the dome on the eastern side over Tar River. Theodolite measurements of the dome indicated that the highest point of the dome is now on the new growth within the scar and measures 948 m or 3110 feet. Photographs were taken from five points around the volcano and will be used to make an estimate of the current volume of the dome.

The number of rockfall signals is less than yesterday with 17 events being recorded. 4 long period earthquakes were also recorded, 2 of which triggered rockfalls. No hybrid or volcano-tectonic earthquakes were recorded. The low level tremor seen on St. Georges Hill seismometer appears to have decreased during the day. A new repeater site at Jack Boy Hill for seismic signals being relayed from the east of the island was established today, and so signals from Long Ground and Roche's Yard short period seismometers can now be received again at the Observatory.

The EDM line between Waterworks and Lees Yard was again surveyed twice today. The data collected from these stations have not shown any changes since they were first established on 8th July, 1997. The GPS network between Whites, Windy Hill, Long Ground, Lees Yard and Harris Lookout that was surveyed yesterday has been processed. Lines to Lees Yard cannot yet be assessed since this is first occupation of this site in the new network. However, lines from Harris Lookout to Whites and Long Ground shortened by 16 mm and 13 mm respectively and lines from Windy Hill to Harris Lookout, Whites and Long Ground lengthened by 22 mm, 24 mm and 33 mm respectively. All these measurements are in comparison to a survey made on the 24 June. There has been a very low amplitude peak in the Chances Peak tiltmeter during the past 24 hours and there has been a continuation in the long term deflationary trend.

Measurements of the sulphur dioxide in the volcanic plume were made today using the Correlation Spectrometer (COSPEC) in static mode from the top of Garibaldi Hill. These data will be processed later today, and results will be released later in the week.

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 period of heavy rain associated with the tropical depression heading towards Montserrat. 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.

Mr. George Skerritt rejoined the observatory team today after being away on a training course in Hawaii followed by a short holiday.

Montserrat Volcano Observatory