Montserrat Volcano Observatory, Montserrat, West Indies

Scientific Report 70
02 August 1997

This report covers the 14 day period from 0:00 on 20 July to 24:00 (local time) on 2 August.


The activity at the start of this reporting period was low, but on 31 July there was a marked increase in seismic activity with near continuous tremor recorded from 3:30 am through to 1 August. There was a resumption of hybrid earthquake swarms on 1 August, and these were generally followed by a period of more intense pyroclastic flow activity.

Visual Observations

The level of volcanic activity was relatively low during the start of this period with rockfalls and small pyroclastic flows. Activity was concentrated in Mosquito Ghaut and Gages valley, with the upper parts of the valleys being progressively filled up. Helicopter observations on the 17th and 24th showed a marked filling of the upper regions of the Gages Valley from little material deposited at all, to scree covering the Gages Wall completely. Incandescent chutes observed on 22nd above Gages and Mosquito confirmed that growth continued to be in this area of the dome. There were occasional rockfalls in the Tar River, Tuitt's and White River area. The latter area originated from the very steep wall of old dome material that has been over Galways Wall since May, rather than from new growth.

Activity increased towards the end of July, with more rockfalls and small pyroclastic flows on the northern flanks, and one reaching Gages Lower Soufriere on 29th July. On 31st July activity was elevated, with semi-continuous production of ash plumes, the most vigorous of which reached over 15,000 feet asl. This activity continued throughout the day, and did not seem to be associated with significant pyroclastic flow activity. Following this episode there was little evidence of large volumes of new deposits around the volcano, although some new flows had reached as far as Gages Lower Soufriere, and views of a small amphitheatre-like scoop out of the dome above Gages were glimpsed from the helicopter. The activity was attributed to semi-continuous rockfalls above the Gages Wall area.

Activity remained elevated to the end of the reporting period with weakly convecting ash plumes and several periods of intense rockfall and pyroclastic flow activity in the Gages Valley.

Temperatures of >640?C were measured at 1.8m depth in the Trants area of the 25th June pyroclastic flow deposits on 21st July.


The first 11 days of the reporting period were dominated by rockfall signals. These rockfalls showed no particular periodicity and there were no pronounced variations in their amplitudes. On the 31st July, however there was a marked change in seismic activity. Starting at 3am and peaking at approximately 2:30 pm a continuous high amplitude, high frequency signal was recorded, which lasted into the next day. In character this signal was identical to those normally associated with pyroclastic flows but no large flows were observed, just a high level of ash emission. In this period a number of bangs were heard coming from the volcano - in at least one case a bang was correlated with an impulsive signal recorded on the broadband seismometers and explosive activity was surmised. A new sequence of hybrid swarms started on the 1st and 2nd of August but by the end of the reporting period no clear cycle had developed in the timing of the swarms. Pyroclastic flow and rockfall activity occurred towards the end of each hybrid swarm.

Table 1: Earthquake types

These earthquake counts are of events that triggered the broadband network's event recording system between 0000 and 2400 each day (local time).

Date		VT	Hybrid	LP	Dome RF	LPRF*	HYRF*
20 July 97	1	0	2	31	0	0
21 July 97	1	0	5	55	1	0
22 July 97	0	0	4	33	0	0
23 July 97	1	0	3	77	2	0
24 July 97	0	0	0	64	0	0
25 July 97	0	0	2	117	1	0
26 July 97	0	0	2	134	0	0
27 July 97	0	0	6	91	2	0
28 July 97	0	0	1	106	0	0
29 July 97	0	0	4	161	2	0
30 July 97	1	0	8	129	6	0
31 July 97	0	8	23	212	6	0
1 Aug 97	1	73	5	61	2	0
2 Aug 97	0	107	6	53	3	0

* LPRF: LP earthquake followed by rockfall signal. HYRF: Hybrid earthquake followed by rockfall signal. The LPs, hybrids and rockfalls in these signals are also counted in their respective columns.

Ground Deformation

Three occupations of the GPS network LEESNET were made during this reporting period. It is still too early to discern any clear trends in the data.

EDM occupations were carried out on the Lee's Yard Quadrangle involving Water Works - Lee's Yard, MVO - Lee's Yard, Garibaldi Hill - Lee's Yard and MVO - Garibaldi Hill. All the slant distances are generally stable. However there have been some erratic spikes on the slant measurements from Water Works to Lee's Yard. It is currently not known whether these represent real elastic ground deformation or not. Further occupations are planned to investigate this phenomenon.

Volume Measurements No dome volume measurements were possible during this period due to poor visibility, and no deposit volumes were attempted.

Environmental Monitoring No rainwater, ash samples or gas samples were collected during this period.

MVO Staff Changes
Prof. Tim Druitt, University of Clermont-Ferrand
Dr. Richard Herd, British Geological Survey
Ms. Costanza Bonadonna, Bristol University

Keith Rowley, Independent
Dr. Maggie Mangan, US Geological Survey, Hawaiian Volcano Observatory
Mr. Rob Watts, Independent

Prof. Barry Voight, Penn State University

Montserrat Volcano Observatory