Montserrat Volcano Observatory

Daily Report
Report for the period 16:00 01 August
to 16:00 02 August 1996

Activity at the Soufriere Hills volcano during this reporting period continued at a slightly higher level compared to that observed during the previous 24 hours although the overall pattern remained the same.

Seven (7) periods of sustained, low- to high-amplitude, low-frequency, harmonic tremor were recorded at intervals of about 3 hours. These episodes started with either a series of small VTs followed by small, near-repetitive hybrid earthquakes or with just small near-repetitive hybrid events which increased in frequency and amplitude until they formed a continuous tremor. After a period of sustained peak amplitude, the signal then decayed first into smaller hybrid events and then background. These episodes of VTs and/or repetitive hybrids and harmonic tremor , which usually lasted about 60 to 90 minutes, are probably associated with magma migration from shallow depths to the surface or near-surface as the process of dome growth at the Soufriere Hills volcano continues.

Forty nine (49) rockfalls were recorded, with a few of these producing ash clouds. A rockfall at 14:40 today generated an ash cloud with maximum height of 4,000 ft above sea level while one at 15:45 was associated with a small pyroclastic flow in the Upper Tar River Valley area. Approximately 250 small hybrid earthquakes, 6 small long-period earthquakes and 24 small volcano-tectonic earthquakes were recorded. The VTs were generally located at shallow depths beneath the crater region. A moderate-sized broadband signal recorded on the Gages seismic station from about 09:41 for approximately 25 minutes is typical of those associated with flash flooding in Fort Ghaut.

Periods of very good visibility occurred throughout the day and visual observations of the dome were made from both the air and the ground. The pyroclastic flows on Monday 29 and Wednesday 31 July seem to have originated from a collapse structure in the new dome which is similar in shape to English's Crater. The considerable amount of new dome material extruded into this excavation has a whale back form but has broken up into several huge blocks. Vigorous emission of steam laced with small amounts of ash was observed from the new excavation and this corresponded with one of the periods of high-amplitude harmonic tremor. Steam emission was also observed from most of the dome, from some areas of the Tar River valley and from the area of entry of the pyroclastic flows into the sea. Most of the rockfalls and small pyroclastic flows were originating from the eastern and northeastern flanks of the lava dome.

EDM measurements were made today on the eastern and western flanks of the volcano but the data is still being processed. COSPEC measurements of the amount of SO2 in the volcanic plume were also made but the results are still awaited. No GPS measurements were made today.

The ashfall due to yesterday's pyroclastic flow was not completely washed away by overnight rains and continues to make driving hazardous. Please be careful 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