Montserrat Volcano Observatory

Daily Report
Report for the period 16:00 03 August
to 16:00 04 August 1996

Activity at the Soufriere Hills volcano during this reporting period has continued in the same style seen during the previous 24 hours although the level has been slightly higher. This style is characterised mainly by the occurrence of rockfalls and near-periodic tremor episodes.

The near-periodic episodes of relatively low frequency, high-amplitude, near-harmonic, tremor continue. Five (5 ) of these, each lasting about 60 to 90 minutes, were recorded during this reporting period. While all the tremor episodes were followed by hybrid earthquakes, three were preceded by small volcano-tectonic (VT) and hybrid events and two by hybrid events only. The time interval between these tremor episodes remains about 3 to 4 hours. During the day time, large steam plumes mixed with small quantities of ash were observed to be associated with the times when the tremor level was very high. These episodes of VTs and/or repetitive hybrid earthquakes and tremor are probably associated with magma migration from shallow depths to the surface as the process of dome growth at the Soufriere Hills volcano continues. During one of these vigorous tremor episodes at around 05:00 this morning, ash clouds were observed which rose to about 10,000 ft above sea level. The ash clouds were blown westward, possibly causing light ashfalls in the Plymouth, Richmond Hill and Fox's Bay areas. These particular ash clouds were most probably due to pyroclastic flows in the Tar River Valley caused by collapses of the over steepened flanks of the lava dome.

Seventy seven (77) small- to large-sized rockfalls were recorded and several of these generated small- to moderate-sized ash clouds which were blown on the wind towards the west, possibly resulting in light ashfall in the Richmond Hill and Fox's Bay areas. The largest rockfalls occurred at 00:09, 05:31, 15:15 and 15:39 today. The ash clouds associated with the events at 05:31, 15:15 and 15:39 rose to heights of about 10,000 ft, 5,000 ft and 7,000 ft above sea level respectively. The 05:31 and 15:39 rockfalls were associated with small pyroclastic flows in the Upper Tar River Valley. Approximately 396 distinct small hybrid earthquakes and 101 small volcano-tectonic (VT) earthquakes were recorded. The VTs were generally located at shallow depths beneath the crater region.

EDM measurements were made today on the Whites to Castle Peak line and the results showed a shortening of 1.7 cm compared to yesterday's measurements. COSPEC measurements of the amount of SO2 in the volcanic plume were made at sea on the Police launch but the results are still awaited. No GPS measurements were made today. Surveys made today of the new real estate east of the Tar River valley created by the recent pyroclastic flows from the Soufriere Hills volcano indicate that it has maximum dimensions of approximately 500m and 400m in the north-south and east-west directions respectively. A maximum thickness of about 15m was also estimated for the pyroclastic flow deposits in the Tar River Valley generated by the events on 29 and 31 July.

The ashfalls due to the pyroclastic flows on 29 and 31 July have not been completely washed away by the recent rains and continue to make driving hazardous. Please exercise some care when driving and remember to wear dust masks while cleaning ash in the home or outside.

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