Montserrat Volcano Observatory

Morning Report
Report for the period 16:00 28 July
to 07:00 29 July 1996

Activity at the Soufriere Hills volcano during this reporting period was higher than that observed during the previous 24 hours. Several large rockfalls and small pyroclastic flows occurred in the Upper parts of the Tar River Valley, resulting in moderate-sized ash clouds and small ashfalls in several areas of western and northwestern Montserrat.

Three periods of near-continuous small- to moderate-sized rockfall activity occurred from 16:24 to 18:50 on 27 July, 22:12 on 27 July to 00:30 on 28 July, and 02:59 to 04:04 on 28 July. From 06:39 on 28 July, the activity again increased and at 07:09, a small pyroclastic flow occurred in the upper reaches of the northern Tar River Valley area. A slightly larger pyroclastic flow occurred at 09:13. Other smaller pyroclastic flows in the Tar river valley may have occurred at 11:51, 12:36 and 13:04. Signals interpreted as representing large rockfalls were recorded at 16:29, 18:06, 20:23 and 20:26 on 27 July and 03:02, 03:54, 05:16, 06:38, 10:05 and 10:15 on 28 July. From 14:00 to 16:00, activity was slightly lower, with fewer rockfalls. The large rockfalls and pyroclastic flows resulted in a near-continuous ash cloud which was blown to the west and northwest, with small ashfall in Upper Gages, Amersham, Plymouth, Richmond Hill, St. George's Hill, Cork Hill, Garibaldi Hill and Fox's Bay.

At around 18:00 on 27 July, a seismic signal which was recorded at high amplitude by all the stations of the seismic network began and was maintained by a series of pulses, lasting until around 19:24. This signal was caused by a series of large rockfalls from the eastern flanks of the lava dome and their associated small pyroclastic flows. None of these flows travelled beyond the disused north-south trending road in the Tar River Valley. These series of events resulted in moderate-sized ash clouds which were again blown on the wind towards the west, causing ash falls in such areas as St. George's Hill, Richmond Hill, Fox's Bay and Plymouth. Between 19:45 and 19:54, another series of moderate- to large-sized rockfalls were recorded . From 19:54 to 22:28, the seismicity level was lower, consisting of small- to moderate-sized rockfalls and intermittent low-amplitude broadband tremor. There was a slight increase in level to mainly moderate-amplitude broadband tremor and moderate-sized rockfalls during the period 22:28 to 23:29 on 28 July. A slight reduction in the level of seismic activity again occurred from 22:39 on 28 July to 03:00 on 29 July, with the seismicity being characterised by intermittent low- to moderate-amplitude tremor, small- to moderate-sized rockfalls and small hybrid earthquakes. But from 03:00 to 05:30, the level increased again slightly to near-continuous moderate-amplitude broadband tremor in which was embedded several large rockfalls at 03:11, 04:05, 04:47 and 05:19. Since 05:30 this morning, the seismicity level is again slightly lower, with low-amplitude broadband tremor, small- to moderate-sized rockfalls and near-repetitive small hybrid events. Most of the large rockfalls were accompanied by small pyroclastic flows in the Upper reaches of the Tar River valley and associated ash clouds whose patterns of behaviour were identical to those already described above.

Few volcano-tectonic earthquakes were recorded during this period. Several small hybrid earthquakes occurred and at times the pattern was near-repetitive. Few long-period events were recorded. Broadband tremor was intermittent and of low- to moderate-amplitude.

Visibility was generally very poor during the whole reporting period because of low cloud cover and episodic rainfall. However, brief glimpses of the upper reaches of the Tar River Valley area revealed that the pyroclastic flows were from the eastern and northeastern parts of the lava dome and that green trees were also set alight. A pulsating glowing of the eastern flank of the lava dome was seen yesterday evening from Whites and Harris' during the period of intense activity . Visibility during the early morning period of 29 July was still very poor but the smoke created by burning trees in the Tar River valley could still be seen.

COSPEC and FTIR measurements of the volcanic gases were made but the data is still being processed. No gravity, EDM or GPS measurements were made. Water and ash samples were collected from several areas for further analysis.

Dr. J-C Komorowski of the IPG in Paris (France) left Montserrat yesterday after spending 17 days at the MVO during which he participated in the gravity and GPS campaigns.

The current level of activity at the Soufriere Hills volcano may continue episodically for several days, i.e., rockfalls, small pyroclastic flows and associated ash clouds. The areas of ash fall will obviously be determined by the wind direction and strength . It is imperative to drive carefully in areas of ash fall and to wear dust masks. However, there is no need at the moment for heightened concern. The MVO, as usual, is keeping a very close watch on the situation.

The Soufriere Hills Volcano is still considered to be highly dangerous to people and property on it's eastern and upper western flanks. Visits to the evacuated zone must be kept to a minimum. The Tar River and Long Ground areas to the east and upper Fort Ghaut, Gages Village and Upper Amersham areas to the west are all extremely dangerous. All access roads to these areas remain closed and people should not enter these areas under any circumstances. If they do, they put themselves and others at direct risk of serious injury or death.

Montserrat Volcano Observatory