Montserrat Volcano Observatory, Montserrat, West Indies

Scientific Report 57
15 March 1997

This report covers the 7-day period to 24:00 on 15 March.


Activity at the volcano has been quiet for the most part of the week. However earthquake swarms continue to occur. The main development has been the occurrence of avalanches and pyroclastic flows over the Galway's Wall which are often triggered by earthquakes during the swarms. The dome complex shows little signs of change. Small-scale rockfall activity indicates that it is still growing, albeit slowly.

Visual Observations

Viewing conditions have been variable through the week. During periods of cloud cover it has only been possible to observe ash clouds being carried westward on the prevailing wind. These are inferred to be the result of rockfalls and small pyroclastic flows from the eastern and southern areas of the dome complex and possible the over the Galway's Wall.

Visual activity at the volcano has been dominated throughout the reporting period by small-scale rockfalls. These were mainly confined to the eastern face of the dome complex for the early part of the reporting period. The area of activity expanded on March 11. The area from the south-east to the north-east of the dome then became the source area for the minor rockfall activity. Some small pyroclastic flows from the eastern face of the dome have developed during the week but have been restricted to the very upper parts of the Tar River Valley.

The CCTV camera at the Galway's view point is providing excellent views of avalanches and pyroclastic flows over the Galway's Wall. A direct correlation between large earthquakes and avalanches over the wall has been possible. During the week several pyroclastic flows have originated in this manner. Currently the maximum run-out distance is of the order of 1 kilometre from the crater wall. The source area of these flows is part of the pre-September 17, 1996 dome to the north of Galway's Wall. No vegetation has been ignited by any of these flows to date indicating the material must be relatively cool. Some rockfalls have also occurred outside earthquake swarms.


The seismicity during this period has again been dominated by earthquake swarms interspersed with periods of relative quiescence. Table 1 summarises the event counts for the week. Four swarms of volcano-tectonic and hybrid earthquakes occurred through the week. Whilst composite in nature they are generally dominated by hybrid events (Table 2). During the longest swarm on March 14th (20.73 hours duration) some 38 VT events and 183 hybrid events were recorded.

The maximum size of events during a swarm has increased slightly but there are generally fewer events in a swarm compared to those at the end of 1996. The locations of the events remains at a shallow depth (1-3km) directly below the crater area.

Only one long-period earthquake has been detected during the week.

Rockfall activity is mainly concentrated in periods between the earthquake swarms, although this is not an exclusive point. Some of the larger events during a swarm have been followed by rockfall and pyroclastic signals from material cascading over the Galway's Wall. Several such events have occurred during the week. The run-out distance of these flows is gradually increasing down the valley.

Table 1: Earthquake types
These earthquake counts are of events that triggered the short-period seismic network event recording system between 0000 and 2400 each day.

Date	VT	LP	Hybrid	Dome RF
09 MAR 97	29	0	39	17
10 MAR 97	62	0	60	7
11 MAR 97	26	0	80	11
12 MAR 97	38	0	94	8
13 MAR 97	2	1	16	9
14 MAR 97	43	0	184	5
15 MAR 97	1	0	4	4

Table 2: VT / hybrid earthquake swarms and tremor episodes

Date		Start time	Durn (hrs)	VTs	Hybrid
09 MAR 97	18:52		14.03		86	96
11 MAR 97	01:59		12.32		26	77
12 MAR 97	11:29		11.98		41	110
14 MAR 97	02:24		20.73		38	183

Ground Deformation

EDM measurements have been made on the western lines and the northern triangle. The line between Upper and Lower Amersham continues to show minor short-lived fluctuations but overall the slant distance is not showing any definite trend of change. The line Lower Amersham - Chances Steps continues to show a very slow trend of shortening of the slant distance. There is also a suggestion of a very slow decrease in the height difference between Lower and Upper Amersham and Lower Amersham and Chances Step.

20-hour GPS occupations were completed of the Hermitage station on March 10th and 11th with a base station at Harris. The long occupations allow a rigorous check on measurement errors. Hermitage has moved by 2.5cm to the NNE since January 18th and has risen by almost 9cm. The data collected in March agree to within 1.1mm in XY position and 8mm in height. 2-sigmas of the average values for the occupations are easting: 3.8 and 4.7mm; northing: 4.6 and 9.4mm and height: 19.2 and 15.1mm for the 10th and 11th of March respectively.

A GPS occupation of the EASTNET was completed on 15th March. The Farrells station about 700m from the northern edge of the dome now shows a total movement of 2.5 cm to the NNW since June 13th 1996, and is the only station in the wider networks to have shown significant movement.

Dome Volume Measurements

A survey was completed on 14 March using the fixed location photographic method. A growth volume of 1.34 x 106 m3 has been added to the dome since 1 March, giving an average extrusion rate of 1.08m3/s during this period.

Although rockfall activity during this period has been concentrated on the eastern side, active growth of the dome is more marked in the summit region. A large spine which first appeared on 13 March was estimated to be in the order of 30m high and from the photographic surveys the top was 970m above sea level. This spine and other smaller spines which appeared after it were generally extruded from the back of the January scar (i.e. its most westerly extent) and as growth continued these features could often be traced to move upwards and eastwards so that eventually they were located the eastern face of the dome. Extrusion in this summit region is now overgrowing the extent of the original January scar and overall the shape of the dome is changing from the flat topped structure seen at the end of February to a more conical geometry.

Environmental Monitoring

Measurements of sulphur dioxide flux were made using the MiniCOSPEC on 10 March (700 t/d), 14 March (213 t/d), and 15 March (341 t/d). The increased values were associated with the recurrence of earthquake swarms on 9 March.

Results for sulphur dioxide diffusion tubes collected on 23 February were received this week. They show values similar to those measured over the last few months. The results are shown in Table 3.

Table 3. Sulphur dioxide diffusion tube results for the period between 9 February and 23 February 1997. Concentrations are in ppb.

Location		ppb
Upper Amersham		47.70
Lower Amersham		17.30
Airport			0.80
Police HQ, Plymouth	9.00
Weekes			0.00
Control			0.00

Rain water samples were collected at 4 locations around the volcano on 9 March, and results are shown in Table 4.

Table 4: Rain and surface water geochemistry, 9 Mar 97
Units: conductivity (mS/cm), total dissolved solids (g/l), sulphate (mg/l), chloride (mg/l), fluoride (mg/l), nd (not detected)

Location		pH 	Cond.	TDS	Slphts	Chlrds	Flrds
Upper Amersham 		2.39 	2.120	1.050	25	250	>1.5
Lower Amersham		2.55 	1.162	0.582	16	115	>1.5
Police HQ, Plymouth	2.57 	0.926	0.464	nd	97	>1.5
Weekes			6.49 	0.172	0.086	nd 	27	0.3

These results show that the rainwater directly west of the volcano continues to be highly acidic and has high concentrations of certain anions.

Ash was collected at several sites to the west of the volcano, but appeared to be mostly wind-blown. No further results on ash analyses were received during this period.

Staff Changes
There have been no staff changes during the week.

Montserrat Volcano Observatory