Montserrat Volcano Observatory

Daily Report
Report for the period 16:00 06 August
to 16:00 07 August 1996

Both the level and pattern of activity at the Soufriere Hills volcano during this period have been similar to that observed during the past several days.

The seismic activity continues to be dominated by small- to moderate-sized rockfalls and near-periodic episodes of large-amplitude, low-frequency, near-harmonic tremor which is preceded by volcano-tectonic (VT) and/or hybrid earthquakes and followed by hybrid earthquakes. Five (5 ) of these large-amplitude tremor episodes were recorded during this reporting period. Four of the episodes were preceded by both volcano-tectonic and hybrid earthquakes while one was preceded by hybrid events only. All were followed by hybrid earthquakes. The duration of these episodes was quite variable compared to previous ones and ranged from just less than one hour to over two hours. 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.

Thirty two (32) small- to moderate-sized rockfalls were recorded. The largest of these occurred at 17:03 on 06 August and 09:16, 10:36 and 13:56 on 07 August and were associated with small ash clouds which were blown westward over Upper Gages. Approximately 427 distinct small hybrid earthquakes and 36 small volcano-tectonic (VT) earthquakes were recorded. The VTs were generally located at shallow depths beneath the crater region. Two small long-period events were also recorded. Two episodes of low-amplitude broadband tremor lasting more than one hour were recorded at the stations closest to the volcano.

Visibility was good throughout most of the day. Visual observations from the ground and the helicopter suggest that there hasn't been any major changes in the dome since yesterday's observations. 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 but the results are not yet available. Fourier Transform infrared spectroscopy (FTIR) measurements of the gases emitted from the volcano were 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