Montserrat Volcano Observatory

Daily Report
Report for the period 4 pm 28 July
to 4 pm 29 July 1997

The alert level system has been revised by zones

Small to moderate sized rockfalls signals have again dominated activity at the Soufriere Hills Volcano. However there was a period of more intense activity this morning with moderate pyroclastic flows reaching the Gages Lower Soufriere. Overcast conditions persisted for most of the day and no views were obtained of the dome.

144 rockfalls and 5 long period earthquakes were recorded today. This is an increase on the numbers from yesterday. Four of the long period events triggered rockfalls. Broadband tremor, mainly of low amplitude was recorded on the St George's Hill and St Patrick's seismographs for variable periods during the past 24 hours. At between 6:00 am and 8:30 am there was a more intense period of activity with several pulses of pyroclastic flows down Gages valley. The volcano was overcast at the time, so an assessment of their run-out distances could not be made immediately. A field team at St. George's Hill later in the day reported that the flows has reached Gages Lower Soufriere. This period of activity was not preceded by any earthquakes and there was no perceptible increase in rockfall activity prior to the initiation of pyroclastic flows. This emphasises that as the dome continues to grow, parts of it may become unstable at any time and pyroclastic flows can occur with no warning. Further small pyroclastic flows occurred throughout the day, one of which was observed in Tuitts Ghaut at 3:24 pm.

The Jack Boy Hill repeater for the short period seismic stations in the east of the island is now operational, so the signals from Long Ground and Roche's Yard can be received at MVO.

EDM measurements were conducted today on the north-western triangle between the MVO, Garibaldi Hill and Lees Yard and on the Waterworks to Lees Yard radial line. The results of these measurements are not yet available. GPS measurements were made on the new network on the north-western flanks of the volcano today. The survey is still ongoing at the time of reporting, and results will be processed later today.

Measurements of sulphur dioxide flux were made today by scanning the plume vertically from Old Road Bay. The results from these measurements will be released later in the week.

The dome continues to grow and the potential for large pyroclastic flows onto the northern and western flanks of the volcano remains significant. The top of the Gages valley and Mosquito Ghaut have become substantially filled with debris from the growing dome. Residents should note that pyroclastic flows and ash eruptions have occurred during the past few weeks with no direct association with seismicity. This means that people entering the designated exclusion zone put themselves at extreme risk. In addition, people in the northern and central zones should stay alert and listen to Radio Montserrat.

The Belham River valley provides a pathway for mudflows which may develop rapidly following a period of prolonged rainfall. It therefore remains an area for anxiety over safety since it is also a potential pathway for pyroclastic surges. Mudflows can travel very fast and may be quite close to boiling point. In addition, they may extend much further along the river valley than the pyroclastic surges. Pyroclastic flows and surges retain their high temperatures for several weeks after they were deposited. Residents are therefore urged not to approach, attempt to handle or walk on these deposits since they could sink in, due to the uncompacted nature of the deposit, and become severely burnt. Dust masks should always be worn when there is ash in the air.

Montserrat Volcano Observatory