Montserrat Volcano Observatory, Montserrat, West Indies

Scientific Report 59
29 March 1997

This report covers the 7-day period to 24:00 on the 29th March.


Activity was low for the early part of the week. However the activity picked up on Wednesday and remained at a slightly elevated level for the remainder of the week. Activity continued to be focused in the summit area with localised growth and rockfall activity occurred in the southern side of the dome. Further small pyroclastic flows have occurred over the Galway's Wall and into the White River valley.

Visual Observations

The southern face of the dome has continued to be the most active area of the dome complex throughout the reporting period. Incandescence observed at dusk on Sunday was particularly strong over the whole of the south side of the dome (January 20 material) and also parts of the pre-September 1996 dome material in the area above the Galway's Wall. Toward the end of the period the south-western region of the dome has been dominated by a large block extruding out and tilting to the south-south-west.

Two distinct peaks began to develop in the summit area of the dome, with the higher of the two being on the south side of the dome overlooking the Galway's Wall. A distinct cleft formed between the two peaks, with material being extruded upwards and to the south-west from the southern summit region. Growth of the dome above the Galway's Wall is such that there is now no barrier between the new material and the Galway's Wall. Small rockfalls and pyroclastic flows over the Galway's Wall were recorded intermittently throughout the week. Many of the pyroclastic flows were preceded by long-period earthquakes (see seismicity section). At 16:30 (local time) on 29 March the largest pyroclastic flow over the Galway's Wall to date was recorded. This event produced a dark ash cloud which rapidly convected to approximately 1500 metres. The darker colour and convective nature of this cloud indicates that fresh, hot dome material is now freely making its way over the Galway's Wall. This pyroclastic flow travelled ~400m further than previous flows in this region.

Fresh cracks were observed running through the eastern shoulder of the Galway's Wall on 25 March. The shoulder area is highly fractured in the area close to the main landslide activity on the Galway's Wall. No further increase in the size of these features was noted for the rest of the week.

Fresh landslide scars and cracks were observed in the Gages Wall on 26 March. This appears to have occurred over the few days prior to them being observed. No fresh activity was noted in the dome behind and above the Gages Wall. Further evidence of landslides was noticed on 28 March.


The level of seismic activity remained low during the week. The number of volcano-tectonic and hybrid earthquakes was low, and only one swarm was recorded. This swarm was shorter and less intense than recent swarms, and the maximum earthquake magnitude was significantly lower. There has also been a slight increase in the level of rockfall activity.

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
23 MAR 97	4	15	13	16
24 MAR 97	4	6	5	28
25 MAR 97	1	4	4	24
26 MAR 97	16	10	20	21
27 MAR 97	10	12	15	35
28 MAR 97	5	12	1	24
29 MAR 97	0	11	3	25

The main feature of the week was the increase in number of long-period earthquakes. Long-period earthquakes, as defined by the MVO classification scheme, have been rare during this eruption. The signals have a strong spectral peak at 1 Hz, and usually exhibit almost no initial high-frequency component, even at close stations. There have been occasional periods when LPs have been present for periods of a few days to weeks, and the maximum number per day has been about 40. The LP earthquakes have been small, with durations of about 30 seconds, and only 10-12 per day triggering the short-period network. About 50% of the LP earthquakes have been immediately followed by rockfall signals. This pattern has been observed before, in October and December 1996. It is possible that the LP signals are caused by a process within the dome, such as gas venting or a sudden spurt of growth, which cases can sometimes either shake or push the surface of the dome and cause a collapse.

There was only one shallow earthquake swarm, which was shorter and less intense than swarms in the previous week.

Table 2: VT / hybrid earthquake swarms.

Date    	Start time      Durn (hrs)      VTs     Hybrid 
26 MAR 97	19:34   	8.25    	24      27

Ground Deformation

EDM measurements were made on the northern triangle (Windy Hill - St. George's Hill - Farrells). It is now possible to recognise a very slow shortening trend of the slant distance of the lines from Windy Hill and St. George's Hill to the reflector at Farrells. Total shortening over the past 15 months is of the order of 15mm. The slant distance of the line between Windy Hill and St. George's Hill remains stable.

20-Hour GPS occupations were made of the stations at FT3 (Farrells Crater Wall), Hermitage and Perches with a base station at Harris. The displacement of FT3 has increased and has now reached 17.6cm since 18 January, a rate of 2.7mm/day. Its motion is directed to the NW (away from the dome). Hermitage shows no significant movement since the last measurement on March 17th, whilst Perches shows a 1.6cm displacement to the north since January 18th.

A new crack on the shoulder of Galway's Mountain was measured for the first time on March 25th, with nails hammered into trees on either side of the crack. Two arrays of nails are in place; one is on the steep flank of the mountain about 50m from the crater wall; the second array is 90m to the south in a flatter area. All the lines were measured again on March 28th; there were no significant changes in line length since the first measurements.

Dome Volume Measurements

Clear conditions on 27 March allowed a particularly thorough GPS dome survey to be completed. The results of this survey indicate that since the last survey on 19 March the volume had increased by 0.98 x 106 m3 which yields an extrusion rate of 1.26 m3s-1 for the time period 19 to 27 March. This gives a total dome volume of 49.7 x 106m3 (44.7 x 106m3 DRE). Digital elevation models created from this survey indicate that this growth has been focused on the southern peak of the dome and that the remainder of the dome has remained relatively unchanged.

Environmental Monitoring

Measurements of sulphur dioxide flux were made using the MiniCOSPEC on 24 March (198 t/d) and 28 March (160 t/d). This is slightly lower that the previous week.

Sulphur dioxide diffusion tubes were collected from 5 sites around the volcano and sent to the UK for analysis.

A rain water sample was collected at 1 location on 23 March, and results are shown in Table 3. One sample was collected from the overflow of Trials reservoir in Fairfield. The very dry conditions in the preceding week meant that insufficient rainwater was collected at the three other stations usually sampled.

These results show that the rainwater directly west of the volcano continues to be highly acidic and has high concentrations of certain anions. The sample from Trials is within World Health Organisation levels for all analyses.

Table 3: Rain and surface water geochemistry, 23 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	2.28	2.41	1.20 	36 	211	1.45 
Trials Res.	7.63	0.675	0.388 	39	76	0.40

Ash was collected at several sites to the west of the volcano. No further results on ash analyses were received during this period.

Staff Changes
There have been no staff changes during this period.

Montserrat Volcano Observatory