MONTSERRAT

MVO Scientific Report 68

June 22-July 5, 1997

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Date: Mon, 11 Aug 1997 21:41:49 -0400 (EDT)
From: Tony Glaser 
To: Montserrat E-mail group 
Subject: MVO scientific report 68 (fwd)

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Date: Mon, 11 Aug 1997 15:27:38 -0300
Subject: MVO scientific report 68

Montserrat Volcano Observatory
Montserrat, West Indies


Scientific Report 68: 5 July 1997 This report covers the 14-day period
from 0:00 on 22 June to 24:00 (local time) on 5 July. 

Summary

During this reporting period the volcano was extremely active with major
pyroclastic flows to the north and west devastating the villages of
Harris, Streathams, Windy Hill, Riley's Yard, Bethel, Bramble and Farms.
The flows reached to within 50 m of the airport on the east. Tragically 10
people are now confirmed dead and 9 people are missing presumed dead as a
result of the activity on 25 June.

Visual observations

The volcano's most destructive act to date struck during this reporting
period. On 25 June a substantial portion of the massive lava dome nested
in the crater of Soufriere Hills collapsed, generating a series of
pyroclastic flows that swept across the northern flanks of the volcano to
the sea. The flows and associated surge deposits destroyed 150 homes,
claimed 10 lives and left 9 people unaccounted for. 

The days leading up to the event of 25 June were relatively quiet.
Activity between 21 and 23 June was limited to a few rockfalls and minor
pyroclastic flows from points high up on the east and north-east face of
the dome. The flows generally travelled no further than the base of the
dome in the Tar River Valley in the east and came to rest in the upper
reaches of Tuitts Ghaut in the north-east. The largest flow ran down the
Tar River Valley to within 100 m of the delta (~2.0 km long). In the
evenings, glow and incandescent rock falls were observed on the north-east
face of the dome. A brief daylight glimpse of the dome on 23 June revealed
a blocky summit profile punctuated by three prominent spines, and
extensive ash and steam emissions from the summit, particularly in the
area above Mosquito Ghaut. 

On 24 June the activity shifted northward slightly and a series of minor
rockfalls and pyroclastic flows (1 km in length travelled down Mosquito
Ghaut. Rockfalls continued on the east face of the dome throughout the
day. 

Between 06:00 and 08:00 on 25 June there were a near-continuous series of
rockfalls and small pyroclastic flows that fed into Mosquito Ghaut.
Sporadic rockfalls continued to occur on the east face of the dome. The
activity subsided as the morning progressed. Later in the day, at ~12:45,
a dilute steam and ash cloud billowed upwards from the summit of the dome.
At ~13:00 a dense, dark ash column shot up from the north flank of the
dome above Mosquito, reaching 45,000 feet in a matter of minutes. In rapid
succession three major pyroclastic flows pulsed down Mosquito Ghaut.
Pyroclastic surge clouds spread laterally over the ridges near the head of
Mosquito Ghaut as the flows shot past MVO reference peaks "B" and "C". The
flows fed into the Paradise River and spread laterally through Trants
Village and Spanish Point as they entered the lowlands below the town of
Bramble. A distinct lobe of pyroclastic flow material spilled out of the
ghaut and travelled into Bethel. This lobe deposited large blocks over the
area around Bethel. The upslope surges fanned westward to the head of
Dyers Valley, northward to Streatham, and eastward to Paradise Estate.
Near the town of Gun Hill, the surge was funnelled into the Dyers Valley
and travelled west along the Belham River as far as Cork Hill.

The pyroclastic flows and surges of 25 June were the largest and most
destructive ever produced during this eruption. The intensity of the event
exceeded that of the 17 September 1996 explosion. An estimated 4 to 5
million cubic metres of lava dome were unloaded during the event creating
a spoon-shaped scar with a steep back wall on the lower flanks of the dome
above Mosquito Ghaut. The resulting deposits covered ~ 4 square kilometres
to depths as great as 30 m in places. A detailed account of the 25 June
1997 event can be found in MVO Special Report 3. 

The days immediately following the event were relatively quiet. The most
notable events were two small steam explosions on the morning of 27 June,
which showered 0.5 cm-sized rock fragments over the western flank of the
volcano, in the areas of Lovers Lane, Dagenham, Richmond Hill, and Foxes
Bay. 

Pyroclastic flow activity picked up again on the 28th. Between the hours
of 10:24 and 11:42, moderate collapses on both the north and north-west
flanks of the dome sent 5 pyroclastic flows down Mosquito Ghaut ((1 km
long) and 2 flows down Gages Valley/Fort Ghaut into central Plymouth ((
3.5 km long). Significant pyroclastic flow activity continued through 5
July in Mosquito Ghaut, Gages Valley/Fort Ghaut, and to a lesser extent,
in Tuitts Ghaut and over Galways Wall. A listing of the flows observed
(there may have been others) over this very active period is given below: 

28 June	3.5 km flow down Gages/Fort Ghaut
29 June	3.5 km flow down Mosquito Ghaut
30 June	3.2 km flow down Mosquito Ghaut
2.2 and 3.6 km flow down Gages/Fort Ghaut
1 July	2.0 km flow down Mosquito Ghaut

Two 1.0 km flows down Mosquito Ghaut that overtopped the ghaut at 1500
feet elevation level and proceeded ( 500 m to the NW towards Rileys
Estate. 
	1.1 km flow down Tuitts Ghaut
	0.5 km flow down Galways Wall
2 July	0.8, and 1.5 km flows down Mosquito Ghaut
	1.1 km flow down Gages/Fort Ghaut
3 July	1.5, 2.5, and 3.0 flow down Gages/Fort Ghaut
4 July	1.0 km flow down Mosquito Ghaut
	2.8 km flow down Gages/Fort Ghaut
5 July	0.5 km flow Mosquito Ghaut


Many of the flows between 28 June and 5 July started with a resounding
"BOOM" and a vertical ash column that shot upwards of 35,000 feet. On
occasion, the event ended with large ash emissions and was not followed up
by pyroclastic flow activity. Ash column rise rates of 9-17 m/s were
measured (unfortunately, there are no measurements on the most robust of
the columns occurring over this period). 

A rare viewing window on the morning of the 29 June showed a large
collapse scar visible on the northern flanks above Mosquito Ghaut. This
scar extended back as far as the central point of the dome with a classic
spoon-shape and steep back-wall. The uppermost ca. 100 m of the pre-June
25 dome had been removed and the eastern and north-eastern flanks of the
dome appeared remarkably unscathed with all the pyroclastic flows being
funnelled down Mosquito Ghaut from the collapse area. A relatively small
active growth area was visible in the central/rear floor of the scar and
was generating rockfalls that overspilled into Mosquito Ghaut. An
observation flight on the 1 July showed that the growth area within the
scar had increased considerably with ca. 65 % of the void being filled
particularly in the central and rear portions of the scar. The central
part of the growth area had several small spines on the top and large
rockfalls generated from the base of this area were commonly spilling down
the chute that had developed at the head of Mosquito Ghaut. Both the
visual observations of the dome and frequent pyroclastic flow activity
seemed to confirm the relatively high extrusion rate that is believed to
have predominated during this period. 

Seismicity

The 22 of June showed a marked change in volcanic seismicity. The previous
20 hour cycles of rockfall activity were replaced by 8 hour cycles of
hybrid swarms. This change in activity coincided with a small swarm of
volcano-tectonic events. Hybrid swarms with large numbers of events were
recorded until the large pyroclastic flow of 25 June. After 25 June the 8
hour cycles continued, but at the end of the reporting period most cycles
corresponded to a hybrid swarm of relatively few events changing to tremor
before a pyroclastic flow. Throughout the two weeks rockfall activity
continued, but not with any particular periodicity. 

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*
22 June 97	9	96 	18	180	12	7
23 June 97	3	452	5	84	2	2
24 June 97	0	421	1	60	0	1
25 June 97	0	130	0	48	0	1
26 June 97	0	55	3	38	1	1
27 June 97	0	106 	0	40	0	6
28 June 97	0	152	0	72	0	4
29 June 97	0	117	0	60	0	2
30 June 97	0	82	0	54	0	3
01 July 97	0	43 	0	29	0	2
02 July 97	0	73 	2	77	1	0
03 July 97	0	4 	7	63	3	0
04 July 97	0	14 	1	26	1	0
05 July 97	0	86 	1	65	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. 

Ground deformation

GPS occupations of the volcano's BIGNET (Harris, Whites, Long Ground,
Dagenham and Brodericks) and EASTNET (Harris, Whites, Long Ground, Windy
Hill) were completed on 22 and 24/25 June respectively. Both networks were
cut short by one line due to high seismic activity. 1 cm shortening on the
line Harris to Windy Hill was noted. The movement of direction of the Long
Ground station appears to have changed abruptly. Through mid 1996 to end
April 1997 the station moved 3 cm to the east. Five measurements to the
station since April indicate an abrupt movement of 3.4 cm to the
north-east which has produced a 2.5 cm shortening on the line to Harris.
Future measurements of these network will be severely hampered due to
problems of accessibility and the necessity for full helicopter support
during each occupation. 

A new GPS network has been set up covering the north-western flank of the
volcano that is still accessible by road. The 4 sites on this network
(LEESNET) are Lees Yard (on the northern flanks of Gages Mountain), M40
EDM (on the Waterworks Estate), St. George's (the south-west flanks of St.
George's Hill) and M27 (in Old Towne). The sites at Garibaldi and a pin on
the MVO pool deck will also be used for theodolite measurements during
clear periods using the pole on the tip of St. George's Hill as the point
of reference. 

A permanent reflector has been installed near the GPS pin at Lees Yard and
measurements to this reflector will be shot with the Total Station from
the M40 EDM site on the Waterworks Estate. Measurements to this reflector
will also be attempted using the Total Station from the Garibaldi site.
The fixed reflector at Farrells was probably destroyed during the 25 June
activity. 

Prior to 22 June the Chances Peak tiltmeter showed a flattened pattern. At
05:30 on 22 June, a sharp increase in the rate of inflation occurred.
Subsequent, sharp deflation at 06:30 was coincident with sustained
pyroclastic flows which travelled approximately 1 km down the Tar River
Valley. This event marked the beginning of a new pattern in the
inflation-deflation cycles. The periodicity of the cycles shortened to 8
hours. and the amplitude increased to approximately 40 microradians. The
change was accompanied by a short volcano-tectonic earthquake swarm which
preceded the resumption of hybrid earthquake activity (see above). The
number of hybrid earthquakes varied nearly exactly in phase with the
inflation-deflation cycle, with the maximum number of hybrids occurring at
the peak inflation. 

Following the 25 June pyroclastic flow activity, the inflation-deflation
cycle continued with the same period and amplitude which began 22 June
until 5 July. The overall trend of the tiltmeter, prior to 25 June, showed
a general inflation to the north or deflation to the south. Since 25 June,
the trend shows a general deflation toward the centre of the dome. 

Volume measurements 

No dome volume measurements were possible during this period due to poor
visibility. The volume of deposits of the 25 June flows was estimated at
4.61 million cubic metres, with the total volume of deposits in the area
affected estimated at 5.42 million cubic metres as of 25 June. This volume
was obtained using data from a kinematic GPS survey of the area on 5 July
for the more distal areas of the region unaffected by flows in the
intervening period, along with rougher estimates of deposit thicknesses in
the higher regions of the ghaut immediately following the 25 June event.
Representative deposit thicknesses after the event were 5 m in Mosquito
Ghaut, 14 m in Pea Ghaut and 6 m in the Farm's area. Estimates of the
volume of the scar left by the 25 June eruption were difficult due to poor
visibility, but a brief glimpse allowed a poorly constrained estimate of
about 6.5 million cubic metres. There is a discrepancy between this volume
and the pyroclastic flow volume, and the better constrained flow volume
will be used in future calculations. The volume of the anomalously mobile
flow down the Belham valley on 25 June is estimated to be around 90,000
cubic metres. 

The total volume of deposits on the 5 July in the Mosquito, Paradise,
Farms and Farrell's area from the kinematic GPS survey is estimated to be
9.24 million cubic metres, indicating accumulation of 3.82 million cubic
metres since 25 June. The Gages valley was surveyed using the kinematic
GPS technique on 7 July and a volume of 3 million cubic metres estimated.

Kinematic GPS surveys of the northern ghauts prior to the reporting period
gave a volume of 0.42 million cubic metres for Tuitt's Ghaut on 9 June,
and 0.81 million cubic metres for Mosquito ghaut on 20 June. These
represent the first significant flows down each of these ghauts on 5 June
and 17 June respectively. 

Environmental monitoring

Rain and trough water was collected from several sites around the volcano
on 22 June and the results are shown in Table 2. The samples from Upper
and Lower Amersham are extremely acidic and have high concentrations of
total dissolved solids (TDS).

Table 2 Rainwater geochemistry, 22 June 1997


pH
Conductivity
TDS
Fluoride
Chloride
Sulphate


mS/cm
g/l
mg/l
mg/l
mg/l
Weekes
6.34
0.090
0.044
0.30
14
nd
Upper Amersham
3.05
2.080
1.030
1.10
450
>200
Lower Amersham
3.22
1.060
0.530
1.20
218
79
Amersham cattle trough
6.53
0.487
0.243
1.15
72
33
Trials reservoir
7.51
0.916
0.459
0.35
106
33
Cattle trough, SS
7.76
0.285
0.143
0.10
41
nd
MVO
6.50
0.093
0.046
0.25
16.6
nd
Molyneaux
6.61
0.190
0.095
0.25
25.0
nd

nd: not detected

The miniCOSPEC was used to measure the sulphur dioxide flux from the
volcano. Results are shown in Table 3. These values show a marked increase
in flux before the 25 June. Since 25 June, no measurements have been
possible along the roads along the central corridor or through Plymouth
because of the extreme risk in these areas. However, experiments have been
attempted by using the helicopter (29 June) and from static scanning
methods at the observatory (5 July). Results from these experiments will
be released at a later date.

Table 3 Daily average SO2 flux using miniCOSPEC (tonnes/day)

22 June	438
23 June	1157
24 June 	1933

Ash was collected at a few points around the volcano after the 25 June
eruption. However, many of the sites were destroyed or rendered
inaccessible by the activity and so full coverage was not possible. After
the small explosive event on 27 June, coarse lapilli (up to 10 mm) were
collected from Dagenham and Richmond Hill.

The risk map

During June 1997, 4 successive risk maps were published in response to the
increase in activity over the northern and western flanks of the volcano.
With the advent of each map, the A-B zone (with no access) gradually
increased in size to cover most of the south of the island. The risk
assessment was clearly heading towards the situation where consolidation
of risk zones and alert levels into a new risk map was required.

A new risk map was published on 4 Jul 1997 and was designed to simplify
the previous system where there were 7 zones on the risk map and 6 alert
levels. This meant that there were 42 different options for action
depending on location and the state of the volcano. Now there are only 3
zones: the northern, central and exclusion zones, and only one alert
level: "volcanic alert".

To decide where the boundaries between risk zones should lie, the extents
of the pyroclastic flows and surges so far were marked on a map of
Montserrat. These are shown on the risk map as black arrows. The maximum
distance that pyroclastic flows and surges were expected to reach, given
the current state of the volcano and with a similar type of activity, was
estimated. There is now the potential for flows to reach much of the south
of Montserrat, and so it was decided that an exclusion zone should be
demarcated in these areas. This involved drawing a line across the centre
of the island from Old Road Bay to north of the airport. This line is
controlled primarily by the topography of the land and distance from the
volcano.

North of the exclusion zone it is considered that the risk of pyroclastic
flows and surges is low enough to allow people to live and work as normal.
However, in the case of an increase in activity it is thought that people
who live and work in the area directly north of the exclusion should be
ready to move at short notice if necessary. Therefore a central zone has
been designated in which people should be on increased alert. The further
that you move away from the exclusion zone, the safer you get. Thus the
northern boundary of the central zone is marked as a dotted line. In the
event of an increase in alert level, citizens should move uphill and away
from the Belham River Valley. If it is considered that an evacuation of
the central zone is necessary, the wailing sirens will be sounded and
maroons (explosive fireworks) will be fired. 

So in summary, the new risk map is a simplification of the old risk and
alert system, and it is hoped that it will be the current map for at least
one month and possibly longer.

MVO staff changes
Arrivals:
Dr. Keith Rowley (independent, British Geological Survey)
Dr. Maggie Mangan (Hawaii Volcano Observatory, US Geological Survey)
Dr. Gill Norton (British Geological Survey)
Mr. Richie Robertson (Seismic Research Unit, University of the West Indies, Trinidad)
Dr. Brian Baptie (British Geological Survey)
Ms. Chloe Harford (Bristol University)
Mr. Billy Darroux (MVO)
Dr. Rick Hoblitt (Cascades Volcano Observatory, US Geological Survey)

Departures:
Dr. Ricky Herd (British Geological Survey)
Dr. Sue Loughlin (British Geological Survey)
Dr. Angus Miller (independent, British Geological Survey)
Dr. Paul Cole (Luton University)

Visitors:
Professor Steve Sparks (Bristol University)
Ms. Lizette Rodriguez (University of Puerto Rico)
Dr. Alan Smith (University of Puerto Rico)
Dr. Glen Mattioli (University of Puerto Rico)
Dr. John Shepherd (Lancaster University)

The current chief scientist is Richie Robertson (Seismic Research Unit,
University of the West Indies, Trinidad). 

6 Montserratian staff (Team Seismic: Pops Morris, David Silcott, Venus
Bass, Thomas Christopher, Levar Cabey and Grace Lewis) went to the Seismic
Research Unit of the University of the West Indies in Trinidad for a week
to take part in a training course in seismology and ground deformation
methods led by Dr. John Shepherd (Lancaster University). This was followed
by a training week at MVO practising out their new skills under Dr.
Shepherd's guidance. 


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