Montserrat Volcano Observatory, Montserrat, West Indies

Scientific Report 80
21 December 1997

This report covers the 14 day period from 00:00 on 7 December to 00:00 (local time) on 21 December, 1997.


Activity has been at a relatively low level during this reporting period. The October 22nd dome has continued to grow on the southern side while the northern flanks remained quiet. Seismicity has been dominated by rockfall and long period signals.

Visual Observations

Visibility during this period was reasonable with good views of continued dome growth in the Galway's Area. Near continuous rockfalls occurred throughout the reporting period in this sector and these started to cut into the southern margin of Chance's Peak. The talus slope above the Galway's Soufriere area has been extended significantly to the south-east, and the summit area of Galways Mountain has degraded slightly. The growing talus apron has built out over the remnants of the Galways wall so that there is now a large volume of material outside the old crater margins. The remnants of the pre-September 1997 explosion crater above Gages and Tar River continued to degrade and a small amount of material started to spill down the southern margin of the Tar River. Theodolite measurements from Jack Boy Hill indicated that no movements had occurred on the northern flanks of the volcano. A few small rockfalls occurred down Tuitts Ghaut on 10 December.


The whole of this period was dominated by rockfall signals and long-period earthquakes. In many cases it was difficult to distinguish between these two event types. A number of the events classified here as long-period earthquakes were, in fact, more like short bursts of harmonic tremor.

These lasted up to a few 10s of seconds and were nearly monochromatic at roughly 2 Hz, although there were often a few wavelengths at 1 Hz present at the start of an event. Almost all of the signals classified here as rockfall signals also had a dominant frequency of approximately 2 Hz, even those clearly correlated visually with rockfalls at the dome. Events are thus classified on the relative importance of the dominant peak, rather than its existence, and so a grey area exists between the types. It seems likely that the source of signals recorded as rockfalls is not just the falling debris on the dome but also the ash venting or degassing that causes these falls to occur. This is supported by the signal recorded during vigorous ash venting after an explosion having a corresponding monochromatic seismic signal near to 2 Hz.

Table 1: Earthquake types

These earthquake counts are of events that triggered the broadband network's event recording system between 00:00 and 00:00 each day (local time).

Date		VT	Hybrid		LP	Dome RF		LPRF*	HYRF*

08 DEC 97	0	27		35	139		7	9
09 DEC 97	0	10		25	121		9	2
10 DEC 97	0	9		65	86		14	0
11 DEC 97	0	12		31	102		4	0
12 DEC 97	0	14		26	84		6	0
13 DEC 97	2	15		24	123		2	1
14 DEC 97	2	14		42	89		9	0
15 DEC 97	3	9		51	67		15	0
16 DEC 97	4	22		56	78		16	1
17 DEC 97	0	37		8	57		1	0
18 DEC 97	1	17		9	165		0	0
19 DEC 97	1	7		6	43		0	0
20 DEC 97	0	20		16	88		3	1
* 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

Two GPS occupations of LEESNET (Old Towne, WaterWorks, St. Georges Hill and Lees Yard) were completed. Clear trends in the motion of any of these sites have not yet been identified.

EASTNET (Harris, Whites, Long Ground, Windy Hill and Hermitage) was occupied on 18 December. The baseline Harris-Whites underwent a 4.5cm shortening between April and September this year. Since September the line has returned to within 2cm of its pre-April mean. It appears to have stabilized for the moment, and the last three measurements of the baseline have been within 4mm of one another. Hermitage continues to move to the northeast. It has moved by a total of 10.7cm since mid-January 1997 of which 6.2cm has been in the last three months, representing a marked increase in the rate of movement.

Volume Measurements

A volume survey of the dome was undertaken on 8 December. The western, northern and eastern flanks of the dome have remained mostly unchanged. The southern lobe has continued to be the active area with growth occurring over the whole face rather than a localised spot. Growth still appears to be occurring by the southward extrusion of large slabs and blocks from the central cleft located between the northern and southern lobes. The talus apron at the base of the southern lobe has been extended significantly since the last survey, increasing by 26 million cubic metres. Most of this new material has been accumulating in the Galway's Soufriere region just on the southern side of the where the Galway's wall used to be. Deposits in this area are now over 140m thick. The dome volume now stands at 102 million cubic metres, with the total erupted volume for the entire eruption at 232 million cubic metres.

Environmental Monitoring

Results from sulphur dioxide diffusion tubes were received during this period (Table 2). The sets of tubes have been re-sited as a result of the volcanic activity. These results show that in the north of the island there are no significant concentrations of sulphur dioxide. On 17 December reports were received of scorching of leaves and grass in the Woodlands area. This was most probably a result of light ashfall coupled with an unusual wind direction to the north-north-west.

Table 2: Sulphur dioxide diffusion tube analyses (levels in ppb)

Location	18-Jun-97 to 4-Oct-97	12-Sep-97 to 22-Oct-97	4-Oct-97 to 25-Oct-97
Weekes			2.35			n/a			2.15
St. George's Hill	n/a			n/a			2.25
Lawyers			n/a			2.15			n/a
Fogarthy		n/a			1.00			n/a
MDF Base		n/a			0.90			n/a
MVO (S)			n/a			2.80			n/a
Control			0.00			0.00			0.00

Dust Trak sampling has been carried out at four fixed sites (Table 3). Each value is an average of the PM10 concentration over approximately 24 hours. The values at the fixed sites have been low over this period except for the values at the Catholic school which sometimes recorded raised levels, this is due to the large amount of human activity at this site and the fact that it is near a main road.

Table 3: Dust Trak Results. PM10 concentrations in mg/m3.

Site		7-Dec 	8-Dec 	9-Dec 	10-Dec 	11-Dec 	12-Dec 	13-Dec 	
Mango 			0.011	0.009	0.009	0.007	0.009	
St. Peter's		0.025	0.014	0.009	0.013	0.014	0.007	
CPS			0.106	0.086	0.130	0.046	0.010	
MVO north		0.021	0.008	0.007	0.007	0.006	0.006	

Site		14-Dec 	15-Dec 	16-Dec 	17-Dec 	18-Dec 	19-Dec 	20-Dec 	
Mango		0.006	0.014	0.013	0.010			0.008	
St. Peter's	0.006	0.007	0.013	0.090			0.008	
CPS		0.010	0.017	0.018	0.021			0.024	
MVO north	0.006	0.015	0.013	0.029			0.007	

24 Hour averages:
<0.05 mg m-3	Low
0.05-0.1 mg m-3	Raised
0.1-0.3 mg m-3	Very High
>0.3 mg m-3	Alert

St. Peter's:    The Dust Trak is outside on the balcony of a villa in St Peter's,
			called St. Peter's Place.
CPS:            Catholic Primary School The Dust Trak is outside in the area under the
			roof where some of the children have classes. The school is
			in Palm Loop near Woodlands.
MVO north:      The new Volcano observatory on Mongo Hill near St.John's, the Dust trak is
			on the third story of the building with the sampling tube stuck
			out of the window.
Mango:          The Dust Trak is outside on the verandah of a villa in Mango Drive in Woodlands
MVO Staff Changes
Professor Steve Sparks (Bristol University, UK)
Lutchman Pollard (Seismic Research Unit)
Dr. Jenni Barclay (University of Berkeley, California)
Dr. Richard Luckett (MVO) after a short visit to the USA

Dr. Willie Aspinall (Independent)
Mr. Graham Ryan (Lancaster University, UK)
Chandradath Ramsingh (Seismic Research Unit)
Godfrey Almorales (Seismic Research Unit)
Sayyadul Arafin (Seismic Research Unit)

Montserrat Volcano Observatory