Montserrat Volcano Observatory

Daily Report
Report for the period 16:00 07 August
to 16:00 08 August 1996

The level of activity at the Soufriere Hills volcano during this reporting period was slightly lower than that observed during the previous 24 hours. The pattern was also slightly different.

The seismicity was dominated by the occurrence of episodes of small volcano-tectonic (VT) earthquakes and/or small hybrid earthquakes at intervals of about 3 to 4 hours. These sequences are of the same nature as those which led to the near-periodic episodes of large-amplitude, low-frequency tremor that have been a dominant characteristic for about the past one week. However, the amplitudes and rate of occurrence of the hybrid earthquakes did not increase to the point where the events merged into a continuous tremor, i.e., they remained distinct events throughout the episodes. Five of these sequences were recorded during the reporting period: two consisted of repetitive hybrid earthquakes and VTs while three were made up of repetitive hybrid events only. The duration of these sequences ranged from one hour to just over two hours. These episodes of VTs and/or repetitive hybrid earthquakes are probably associated with magma migration from shallow depths to the surface as the process of dome growth at the Soufriere Hills volcano continues. Significant quantities of ashy steam were observed being emitted by the volcano during the periods of highest intensity hybrid earthquake and may be due to the interaction of magma with the volcano's groundwater system.

Forty seven (47) small- to moderate-sized rockfalls were recorded. The largest of these occurred at 19:51 on 07 August and 04:14, 10:22 and 15:36 on 08 August. Those for 08 August were associated with small ash clouds which were blown westward over Upper Gages. Approximately 310 distinct small hybrid earthquakes and 47 small volcano-tectonic (VT) earthquakes were recorded. The VTs were generally located at shallow depths beneath the crater region. Three small long-period events were also recorded.

Visibility was generally very poor throughout the day.

EDM measurements were made today on the eastern triangle but the results are not yet available. The results of the measurements yesterday revealed shortenings of 8mm and 5 mm for the Whites to Castle Peak and Long Ground to Castle Peak lines respectively. COSPEC measurements were also made on the Police Launch but the results are still awaited.

At around 17:00 today, a series of near-continuous small rockfalls from the eastern flank of the dome led to a few small pyroclastic flows at around 17:20 in the Upper Tar River Valley. The flows travelled as far as the Tar River Soufriere and resulted in ash clouds which rose to a height of about 7,000 ft above sea level. These ash clouds were blown westward on the wind and out to sea, with only small ashfalls in the Upper Gages, Amersham and Plymouth areas.

While the current phase of activity at the Soufriere Hills volcano continues, hazards in some areas of the evacuated zone have increased significantly. In particular, we expect significant-sized pyroclastic flows to occur more frequently in the Tar River Valley area. These will obviously be associated with ash clouds which will be blown on the wind, with areas affected by ashfall being determined by the wind direction and strength at that time. The Tar River Valley and surrounding areas are now extremely hazardous, and should not be entered under any circumstances. If activity continues, there is a risk that flows or ash surges may come over Farrells' Wall and the upper reaches of some of the ghauts closest to the Tar River Valley. People are, therefore, advised not to work these areas. We also urge individuals who continue to ignore this advice to think very seriously before making trips to these highly hazardous zones.

Montserrat Volcano Observatory