Montserrat Volcano Observatory

Daily Report
Report for the period 16:00 30 July
to 16:00 31 July 1996

Activity at the Soufriere Hills volcano during the reporting period continued at the same relatively elevated level seen during the past several days. A series of pyroclastic flows in the Tar River Valley were associated with significant ash clouds and reached the sea. Several episodes of harmonic tremor were recorded.

The most outstanding event during this reporting period was a series of pyroclastic flows in the Tar River valley . These started around 11:50 on 31 July with a series of small- to moderate-sized rockfalls, which gradually led to the first pyroclastic flow at about 12:23. There were four (4) flows which occurred within a period of four (4) minutes, with the last three flows reaching the sea about 3 km way. These pyroclastic flows generated an ash cloud which, according to Bramble Airport Control Tower, attained an estimated maximum height of 21,000 ft above sea level. These ash clouds were blown by light and variable winds towards the north and northwest, resulting in significant ashfall in most areas of central Montserrat (Lees, Gages, St. George's Hill, Cork Hill, Salem, Old Towne, etc), and as far north as Woodlands. Ashfall in Plymouth and the southern areas was lighter while no ash fell in the northern and eastern parts of the island. Visual observations from the helicopter confirmed that the pyroclastic flows were essentially confined to the Tar River Valley area, with no overspill beyond the valley boundaries.

Five (5) episodes of sustained, high-amplitude, low-frequency, harmonic tremor were recorded at intervals of about four (4) hours during the reporting period. These episodes started with either a series of small VTs followed by small, near-repetitive hybrid earthquakes or with just the small, near-repetitive hybrid events. These hybrid events then increased in frequency and amplitude until they formed a continuous signal. After a period of sustained peak amplitude, the signal then decayed first into smaller hybrids and then background. These signals may be related to movement of magma at shallow depth as the process of dome growth at the Soufriere Hills volcano continues.

Approximately 93 small- to moderate-sized rockfalls, 88 small VTs and 178 small hybrid earthquakes were recorded. A few of the large rockfalls were associated with small ash clouds. Visibility was generally poor throughout the whole day.

GPS measurements were made today on the western part of the volcano but the results are still not available. COSPEC measurements of the amount of SO2 in the volcanic plume were also made but the results are still awaited. No EDM measurements were made today because of poor visibility.

While the current phase of activity at the Soufriere Hills volcano continues, hazards in some areas of the evacuated zone have increased significantly. The Tar River Valley and surrounding areas are now extremely hazardous, and should not be entered under any circumstances. Visits to the evacuated zone should generally be kept to a minimum. We urge individuals who continue to ignore this advice to think very seriously before making trips to these highly hazardous zones.

Montserrat Volcano Observatory