Montserrat Volcano Observatory

Daily Report
Report for the period 16:00 05 August
to 16:00 06 August 1996

Activity at the Soufriere Hills volcano during this reporting period has been at about the same level as during the previous 24 hours. The style of the activity has also remained the same.

Five (5 ) episodes of high-amplitude, near-periodic, relatively low frequency, near-harmonic, tremor were recorded during this reporting period. One of the tremor episodes was preceded by both volcano-tectonic and hybrid earthquakes while four (4) were preceded by hybrid events only. All were followed by hybrid earthquakes. 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. Given the significant amounts of steam that seem to be pumped out during the times of large tremor amplitude, the interaction of the magma with the volcano's groundwater system may also be involved.

Forty eight (48) small- to large-sized rockfalls were recorded. The largest of these occurred at 17:44 on 05 August and 08:43, 12:06 and 13:59 on 06 August. The events of 08:43, 12:06 and 13:59 were associated with small pyroclastic flows in the Upper Tar River Valley area which resulted in small ash clouds which were blown westward over Upper Gages. About 556 distinct small hybrid earthquakes and 22 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. Low amplitude broadband tremor was recorded at the stations closest to the volcano from about 13:00 to the end of the reporting period.

Although visibility was generally poor throughout most of the day, there were brief periods during which the clouds lifted from some parts of the dome. Visual observations from the ground and the helicopter confirm that the new dome which is growing in the collapse structure created by the pyroclastic flows of 29 and 31 July has almost filled this structure and some parts of it look over-steepened and unstable. Most of the observed rockfalls were originating from this area which is just slightly north of Castle Peak.

EDM measurements were made today on the eastern triangle and the results show a shortening of 3mm and 2 cm for the Whites to Castle Peak and Long Ground to Castle Peak lines respectively. The change for Long Ground to Castle Peak refers to the period since 03 August while that for Whites to Castle Peak is for the period since 04 August. Fourier Transform infrared spectroscopy (FTIR) measurements of the gases emitted from the volcano were also made but the results are not yet available.

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