Montserrat Volcano Observatory, Montserrat, West Indies

Scientific Report 92
01 June 1998

This report covers the 8 day period from 00:00 on 24 May to 00:00 (local time) on 1 June, 1998.


Volcanic activity has remained at a very low level during this reporting period. No significant changes in dome morphology have occurred and seismic activity is limited to small numbers of volcano-tectonic earthquakes.

Visual Observations

Activity has been at a low level during this reporting period. There have been no pyroclastic flows and only an occasional small rockfall down the upper flanks of the eastern side of the dome complex or small events originating from the summit area of the Galway's dome travelling down the south-west flanks of the complex.

The temperatures of the flow deposits at Trant's formed during the September 21 collapse were re-measured on May 31 and maximum temperatures of 355 degrees centigrade at a depth of 2 m were obtained highlighting that the deposits had not cooled since they were previously measured on May 13.

Poor weather conditions hindered any visual observations however no significant changes are believed to have occurred around the entire dome complex.


Seismicity was generally low for the reporting period with intervals of scattered volcano-tectonic earthquakes alternating with intervals of almost complete quiet. One exception to this was the swarm on 25th May. This swarm consisted of many small signals, most of which did not trigger the networks. The signals were called volcano-tectonic earthquakes because of their frequency content but were generally emergent and often had two or three velocity maxima, as opposed to the simple rise and decay of most volcano-tectonic earthquakes. The most striking feature of these signals was the difference in arrival times at different stations, often over 10 seconds. Also the order of arrival at different stations changed from event to event. This indicates that the signals are propagated as air-waves and that the source is different each time. Crude time-distance calculations assuming a velocity of 330 m/s show many of the source locations to be in the Farms and Upper Gages areas. It is concluded that these signals are caused by small phreatic explosions as water reacts with hot pyroclastic deposits. This conclusion is borne out by comparison with data from July 1995, when phreatic activity was underway and there were many events recorded with similar emergent starts and long inter-station arrival time differences.

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*
24 May 98	26	0		0	2	0	0
25 May 98	13	0		0	2	0	0
26 May 98	31	2		0	4	0	0
27 May 98	1	0		0	1	0	0
28 May 98	13	0		0	1	0	0
29 May 98	24	2		0	0	0	0
30 May 98	3	0		1	4	0	0
31 May 98	0	0		0	3	0	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.

Table 2: Swarm

Start (local)		Duration	Hybrid	LP	VT
25 May 98 20:26		13.1		1	0	40

Ground Deformation

Some of the MVO GPS kit was on loan to the Seismic Research Unit (Trinidad) who were conducting surveys on St Vincent during this period. Thus surveys at Montserrat were reduced. The fixed sites were run and the motion of Hermitage is clearly slowing.

Volume Measurements

Unfavourable weather conditions did not allow an accurate GPS/Binocular survey of the summit area and upper flanks of the whole dome complex to be undertaken. This will be attempted when the opportunity arises.

Environmental Monitoring

Localities of Dust Trak monitoring sites:

The air quality monitoring programme during this period involved 4 permanent monitoring sites. These were located at:

  1. Old Towne; the Dust Trak is on the veranda of the old Montserrat Volcano Observatory (MVOsouth). Wind direction is also noted here.
  2. Central Salem; the Dust Trak is on the veranda of a house immediately opposite the old Arrows store.
  3. Woodlands, Mango Drive; the Dust Trak is on the veranda of a villa.
  4. St. John's, Mongo Hill; the Dust Trak is on the third floor of the MVO building with a sampling tube attached and leading out of the window.

Table 3 : PM10 (concentration in microgramme/m3)

These values are approximate 24 hr averages of the PM10 concentration. Aerosol values measured at each of the monitoring sites are accompanied by a note on the level of volcanic activity (VA) and prevailing weather conditions.

Date	1	2	3	4	VA	Wind (*)			rain	
24/5	22	21	22	-	Low	095/gentle breeze		None	
25/5	25	23	18	13	Low	095/gentle breeze		Heavy overnight downpour	
26/5	14	14	10	10	Low	110/breezy, moderate strength	Few brief light showers, heavy one at 08:00	
27/5	21	21	21	13	Low	090-100/still/v. light breeze	None	
28/5	21	22	13	12	Low	105/v.breezy,gusting strong	None	
29/5	21	21	14	12	Low	105/strong breeze		Heavy downpours at 04:30 and 12:30	
30/5	19	16	11	12	Low	100/strong breeze		Heavy 06:00 downpour	
31/5	20	15	11	13	Low	100/strong & gusty		Heavy overnight downpour	

Low volcanic activity has kept aerosol levels low, with heavy, typically early morning, downpours maintaining some of the lowest airborne dust and ash levels over the last month. A sulphurous haze was visible from (and was occasionally smelt at) MVOsouth over the course of the week where it is seen extending over Gages Mountain, Chances Peak and Chances Steps. The haze descends to the west over Fort Ghaut, Gages Fan and Plymouth.

24 Hour averages:
<50 microgramme m-3	Low
50-100 microgramme m-3	Raised
100-300 microgramme m-3	Very High
>300 microgramme m-3	Alert

MVO Staff Changes

Dr. Jean-Christophe Komorowski (Observatoire Volcanologique Guadeloupe)



The MVO scientific reports will now be issued at the end of each calendar month.

Montserrat Volcano Observatory