Montserrat Volcano Observatory

Daily Report
Report for the period 16:00 08 August
to 16:00 09 August 1996

Activity at the Soufriere Hills volcano during this reporting period continued at a level which was just slightly higher than during the previous 24 hours.

Three episodes of sustained, high amplitude, low frequency, near-harmonic tremor were recorded. Two of these were preceded by small hybrid earthquakes and volcano-tectonic earthquakes while the third was preceded by small hybrid events only. Some of these tremor episodes decreased rapidly to background without developing into separate hybrids while one of the sequences did end with small hybrid earthquakes. One episode of volcano-tectonic and hybrid events was also recorded. These sequences lasted about 1 to 2 hours and were often marked by vigorous steam emission during the periods of maximum amplitude. These episodes of VTs and/or repetitive hybrid earthquakes and/or 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. The significant quantities of steam observed may be due to the interaction of magma with the volcano's groundwater system.

Sixty seven (67) small- to large-sized rockfalls were recorded. Two of these led to small pyroclastic flows in the Tar River Valley. At around 17:00 on 08 August, a series of near-continuous small rockfalls from the eastern flank of the dome led to a few small pyroclastic flows at around17:20 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 clouds were blown westward on the wind and out to sea, with only small ashfalls onland. Another small pyroclastic flow which travelled as far as the Tar River Soufriere and led to a 7,000 ft high ash cloud was generated at around 10:30 today. This led to small ashfalls in St. Goerge's Hill and Fox's Bay. Three other ash producing events occurred today at around 08:49 (4,000 ft), 09:10 (5,000 ft) and 09:57 (4,000 ft). Ninety seven (97) volcano-tectonic earthquakes were recorded and these were located at depths less than 2km beneath the Crater region. The counts for hybrid and long-period earthquakes were 147 and 4 respectively.

Although visibility was generally very poor throughout most of the day, brief glimpses of parts of the dome were obtained. Rockfalls were seen occurring mainly from the eastern and northeastern flanks of the lava dome, with several periods of near-continuous activity. Steam emissions were also observed from several parts of the dome, with the most vigorous emission occurring in the area just between Castle Peak and the new dome.

EDM measurements were made today on the eastern triangle and the results show decreases in the slope distances from Whites to Castle Peak and Long Ground to Castle Peak of 2.5 cm and 3.1 cm respectively since 07 August. The height to the top of the dome was also measured at 3047 ft above sea level, compared to 3029 ft on 30 July. COSPEC measurements of the amount of SO2 in the volcanic plume were also made on land and the data is still being processed.

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