MONTSERRAT

MVO Special Report #3, final

---------- Forwarded message ----------
Date: Mon, 11 Aug 1997 21:46:29 -0400 (EDT)
From: Tony Glaser 
To: Montserrat E-mail group 
Subject: MVO special report 3, final version (fwd)

Via MNI-INFO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


MONTSERRAT VOLCANO OBSERVATORY

GOVERNMENT OF MONTSERRAT

PYROCLASTIC FLOW ACTIVITY ON JUNE 25 1997

MVO staff

MVO Special Report 3

Montserrat Volcano Observatory
PO Box 292
Montserrat
West Indies

Tel: 1 664 491 5647
Fax: 1 664 491 2423
Email: mvo@candw.ag


Any use of trade, product or firm names is for descriptive purposes only
and does not imply endorsement by the Government of Montserrat. 


1. Introduction

At about 12:55 (all times within this report are local time) on Wednesday
25 June, 1997, a pyroclastic flow commenced from the lava dome in the
crater of the Soufriere Hills volcano (Figure 1). In the following 25
minutes, a series of devastating flows swept the northern flanks of the
volcano, down Mosquito Ghaut and followed the Paradise River almost to the
sea. The flows and associated surge clouds damaged or destroyed between
100 to 150 houses, with the villages of Streatham, Dyers, Harris, Bethel,
Bramble, Trants, Farms and Spanish Point being severely affected. At the
time of writing 8 people are confirmed dead, and a further 11 are missing.
Five people also suffered serious burns. 

The pyroclastic flows were the largest produced during the current
eruption, and the intensity of the activity exceeded that of the explosion
of 17 September 1996. An estimated 4 to 5 million cubic metres of the lava
dome were unloaded during the event, and the flows and surges covered an
area of 4 square kilometres. An ash cloud rose to about 10 km, and ash
fell over western Montserrat. 

2. Precursory activity in the previous weeks

Observations Rockfalls and rapid degradation of the north face began on 14
May after approximately two-and-half months of relative stability. The
rockfalls intensified over a few days and by 19 May material had
overspilled into the head of Tuitt's Ghaut. Pyroclastic flows into the
northern ghauts began on 29 May with a minor flow into Tuitt's Ghaut. The
size of subsequent flows into Tuitt's Ghaut increased in early June and
the activity was followed by flows into Mosquito Ghaut and Gages Valley in
mid-June (Figure 2). Many of the rock samples collected from the 17 June
pyroclastic flow into Mosquito Ghaut were moderately vesicular and are
interpreted to be juvenile material. 

Observations in the week prior to 25 June showed that dome growth was
concentrated in the summit area. Unfortunately, the extrusion rate for
June is poorly known due to low visibility, although it is likely to have
been elevated because the volume of the dome as determined from a survey
conducted at the end of May was higher than usual (65 million cubic
metres; extrusion rate of 3.5 cubic metres per second). The dome had
filled English's Crater, threatening the northern slopes of the volcano
for the first time. 

Seismicity Hybrid earthquake swarms occurred during 13 to 27 May, with
swarms comprising about 100 earthquakes per day of varied sizes. Unlike
previous hybrids swarms , the activity did not consist of repetitive,
identical events. Each swarm of earthquakes was followed immediately by a
period of enhanced rockfall activity. When the earthquake swarms ended on
27 May, the pattern of regular periods of enhanced rockfall activity
continued (Figure 3). 

Following pyroclastic flow activity on 5 June in Tuitt's Ghaut the
character of the seismicity changed slightly, with more long-period
earthquakes recorded. The number of earthquakes remained quite low, not
exceeding 40 per day, and the long-period earthquakes returned to normal
levels after 13 June. 

Hybrid seismicity restarted suddenly on the morning of 22 June. This
followed a moderate pyroclastic flow in the Tar River valley, and a small
swarm of volcano-tectonic earthquakes. VT earthquakes have been rare in
recent months, usually occurring in single swarms. 

There were 7 hybrid swarms between 22 and 25 June. The swarms gradually
increased in duration and numbers of earthquakes. Within each swarm, the
earthquakes were repetitive and had generally similar magnitudes, with a
few larger earthquakes in each swarm. The maximum magnitudes were
relatively small; much higher magnitudes had been recorded previously. The
swarms on 24 and 25 June showed increasing intensity, reaching a state
where repetitive events merged into continuous tremor that was difficult
to distinguish from rockfall signals on the drum records. 

The tendency for enhanced rockfall activity at the end of the hybrid
earthquake swarms continued, although the intensity of these events
generally decreased. 

Deformation 

Deformation monitoring at the Soufriere Hills Volcano by the MVO involves
a combination of Total Station measurements (combined electronic distance
measurement (EDM) and theodolite) and global positioning system (GPS).
Crack dimensions in the crater walls are monitored by frequent
measurements between fixed points on opposite sides of the cracks.
Continuous telemetered data are collected from two tiltmeters and one
extensometer installed on Chances Peak and a tiltmeter in Long Ground.

Deformation of the northern crater wall was first observed in early March
1997 by GPS surveys. The FT3 station on the crater wall (Peak C on Figure
2) had moved 15 cm to the north-west between 13 January and 3 March.
Subsequently, more regular monitoring indicated continued motion to the
north-west, with the total displacement reaching 21.5 cm by 12 May (Figure
4a) after which the site was considered too dangerous to visit. GPS
occupations on the summit of Chances Peak (Figure 2) since July 1996 have
also shown sustained motion away from the dome. Total displacement is
currently 16 cm. 

An EDM/GPS station at Farrells has shown slow movement to the north away
from the dome complex (Figures 4b and 4c). Shortening of 5 cm occurred in
the early stages of the eruption (up to 30 November 1995). Through 1996
two lengthening and shortening cycles developed, but since Dec 1996
movement has been sustained shortening on baselines to Windy Hill and
Harris. The shortening has been at an increasing rate until the last
measurement on 10 June. Long Ground underwent a slow eastwards movement of
around 30 mm up to late April, after which it underwent a relatively
abrupt displacement to the north-north-east by 25 mm. Displacement vectors
for Farrells and points on the crater wall have been reported in the MVO
Scientific Reports series (e.g., Scientific Report 64). 

Crack measurements have been made on Chances Peak and Galway's Mountain
since 4 December 1996 and March 25, 1997, respectively. The Chances Peak
cracks showed rapid shear in early December (crack I only), mid-February
and through May (Figure 4d). The total shear is currently 31 cm on Chances
crack II. The Galways crack sheared by 29 cm between 3 May and 16 June.
The sense of movement indicates displacement of the Galway's Wall region
away from the dome complex and demonstrates the extreme stress placed on
the crater walls in an area of the dome that showed no surface activity at
the time.

Prior to 16 June the Chances Peak tiltmeter showed a regular pattern of
inflation and subsequent deflation, directed at the dome, with a period
between 12 and 16 hours and an amplitude between 18 and 25 microradians. 

>From early morning 16 June to late evening 17 June, the
inflation-deflation cycle flattened to an amplitude between 5 and 10
microradians. At approximately 16:00 on 17 June, the inflation increased
steeply, peaked at 21:00, and then rapidly deflated. This deflation
preceded a collapse at 23:30 that sent pyroclastic flows 2 km down Gages
Valley and 3.5 km down Mosquito Ghaut. The pre-16 June pattern returned
for approximately 1.5 days after this event. On 19 June, the flattened
pattern returned and persisted until the morning of 22 June. 

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. Thereafter the amplitude decreased and the period increased
until there was no discernible cycle after 12 July. No major pyroclastic
flows occurred after 5 July (until the time of writing, 27 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. 

3. Events of 25 June 1997

The hours prior to the Event:

At 03:00 a hybrid earthquake swarm began, which was similar in character
to the swarms of the previous four days (Figure 5). At the maximum
intensity, 4 to 5 events occurred per minute. The earthquakes were of
moderate amplitude, reaching saturation on the Gages and Windy Hill drum
records. The Chances Peak tiltmeter continued the previous pattern, with
relative inflation of the crater area accompanying the hybrid swarm
(Figure 5). The tilt levelled out at 05:20, and the volcano started to
deflate at about 06:10. The swarm diminished gradually after about 06:15,
with the hybrids giving way to low-level tremor at 07:05. 

Rock falls and minor pyroclastic flows commenced, fitting the established
pattern. Between 06:15 and 07:15 the activity was dominated by
semi-continuous pyroclastic flows travelling down Mosquito Ghaut with run
outs of ~ 1 km. There were also simultaneous rockfalls and small
pyroclastic flows from the south-east and east face of the dome. 

Re-inflation of the dome area began at approximately 09:00. The seismic
activity remained at low levels until 10:50, at which time a second hybrid
swarm started. The intensity of this swarm escalated rapidly, reaching
about 6 events per minute between 11:30 and 12:30. The earthquake
amplitudes were uniform, and similar to those in the earlier swarm. At
12:00 the inflation trend peaked. By 12:45 the seismic record was
dominated by tremor, and hybrid earthquakes were barely discernible. A
dilute steam and ash cloud issued from the summit area, which was carried
to the west by prevailing winds at an approximate altitude of 4,500 ft. 

The Main Event:

Between 12:40 and 12:50 the tiltmeter registered the start of a sharp
deflation. A strong seismic signal began at 12:55, with intensified pulses
of activity at 12:57 and 13:00. At about 13:00, a dense, dark ash cloud
rose vertically from the north flank of dome above Mosquito Ghaut. The
cloud reached 30,000 feet in a matter of minutes. At 13:03 the eastern
stations of the seismic network stopped transmitting data, because the
Bethel telephone exchange (and/or the phone line across the central
corridor) was destroyed by a pyroclastic flow travelling down Mosquito
Ghaut. There was a third pulse of seismic activity at 13:08. 

MVO staff positioned north of the airport witnessed the front of the flow
coming around the bend at Pea Ghaut, just up-slope of Trant's village. At
13:15 MVO observers flying over the airport found that the initial pulse
had overrun the lower part of Harris, Farm and Trant's, and had come to
within 50 m of the sea. They also reported a final pulse coming down
Paradise Ghaut and surges continuing to spread slowly westwards in the
Spanish Point area. The final pulse of activity advanced at approximately
30 m/s across the flat land near Trants, and was captured on film by a
time-lapse video recorder at the airport control tower. 

Observations of the deposits and destruction of the area (Figure 6)
include the following points: 

a) The main part of the flow in Mosquito Ghaut caused intense scouring to
the top of the steep valley walls particularly on the outside of bends,
suggesting that dense, coarse pyroclastic flows nearly filled the ghaut
during transport. The scouring, however, did not extend over the lip of
the ghaut. The deposits are not extensive in the upper part of Mosquito
Ghaut but generally thicken towards the lower end where Mosquito meets
Paradise Ghaut.

b) Flow deposits ponded to a significant thickness, completely filling Pea
Ghaut and forming a thick, broad fan emerging north-west from Paradise
Ghaut just north of Bethel. Houses 200 m from the edge of the fan are
completely buried. 

c) Of particular note is a separate lobe of coarse pyroclastic flow
material which emanated over the lip of Paradise Ghaut immediately west of
Bethel. Blocks within this coarse lobe are up to 5 m in size and caused
widespread destruction to houses in Bethel village. This is the only area
where a high concentration of coarse material spilled out of the main
ghauts. 

d) As the pyroclastic flows emerged from the gap between peaks B and C
into Mosquito Ghaut, fine-grained pyroclastic surges spread laterally onto
the ridges on either side. These surges extended as far east as Paradise
Estate, went northward to within 250 m of Windy Hill, inundated the entire
village of Streatham, and covered to the west as far as Gun Hill. They
broke and flattened trees on the ridges in the Farrell's and Paradise
area. The surges did not spill into Tuitt's Ghaut to the east, but in one
or two points drained into the unnamed ghaut to the west. In Streatham,
trees were generally not broken or flattened and charring of trees and
telegraph poles was limited to the east-south-east side. The orientation
of charring, shadow zones behind a few of the houses, and the transport of
a water tank in Streatham indicate surge movement in this area was to the
west-north-west. Blocks above 1m in size are rare in the Farrell's area,
although occasional ~0.5 m blocks are present on Farrell's road. The
deposits indicate that drainage of flow material into the Dyers river
occurred largely in the narrow area south of Gun Hill and west of Riley's
Yard. 

e) Pyroclastic flows extended into the Belham valley travelling as far as
the last of the tight bends in the valley before Cork Hill. The flow front
is marked by a pile of logs oriented perpendicular to the valley axis.
Vegetation destruction is limited, with most trees remaining standing even
near the base of the valley. Deposits along the whole length of the Belham
valley are notably fine-grained, and coarse blocks are nearly absent. In
addition, two small concrete bridges were left intact at the base of the
valley. This indicates that the flows which moved down the Dyers river and
Belham valley were fed by relatively fine-grained pyroclastic surges which
detached north-westwards from the main flow in Mosquito Ghaut. 

f) Samples of the deposit collected in the Farm River area and Spanish
Point include both dense and moderately vesicular lithologies. 

The elevated seismic signal persisted until 13:18, and the large deflation
recorded by the tiltmeter bottomed out at 14:30. Low amplitude tremor with
hybrid earthquakes continued until 15:00, at which time the seismicity
dropped to background levels. 

The RSAM peak for the event lasted for 30 minutes, indicating shorter but
more intense activity relative to the explosion of 17 September 1996. More
than 4 square kilometres were inundated by pyroclastic flows and ash surge
deposits. The estimated 4 to 5 million cubic metres of material unloaded
during the event left a steeply-dipping, circular scar roughly 200 m
across in the north-north-west face of the dome. There was no evidence of
explosive activity on 25 June. Ash fall was limited to areas west and
north-west of the volcano. Maximum accumulations reached 2 mm.

4. Post-event activity

Following the end of the pyroclastic flows, the level of seismic activity
remained low for several hours. However, the tiltmeter showed another
inflation which started at 20:00, accompanied by a small swarm of hybrid
earthquakes. In subsequent days, the inflation and deflation pattern
continued, earthquake swarms became more intense and there was further
pyroclastic flow activity in Mosquito Ghaut and Gages valley. 

Brief views of the dome on 28 June indicated that a large part of the
existing dome had been removed during the pyroclastic flow activity, and
fresh dome growth was occurring within the scar at a rapid rate.

Small explosions on 27 June and into early July caused concern that the
level of activity was possibly still escalating. Pyroclastic flows
continued down Mosquito Ghaut and Fort Ghaut into early July, but the
frequency and run-out distances of flows decreased until after 13 July
when only small rockfalls occurred.

5. Commentary

The Soufriere Hills volcano was highly active for several weeks prior to
the 25 June event, with dome collapse over the crater wall threatening the
northern slopes of the volcano for the first time. The repetitive hybrid
earthquake swarms and inflation-deflation cycles suggested that the rate
of dome growth was high, and the conduit pressure was elevated. A large
event was not a surprise, and the effects of the pyroclastic flow were
largely as anticipated by the hazard zonation (Figure 7) and warnings
issued in MVO reports throughout June (MVO daily reports). 

The surge into Dyer's Ghaut and the Belham River valley was remarkable, in
that a relatively fine-grained, low volume (approximately 100,000 cubic
metres) flow travelled a significant distance from the main flow path. 

It is interesting that the activity continued at a high level in the days
following the June 25 event. This contrasts with previous periods of
enhanced activity at the Soufriere Hills volcano, in which significant
dome collapse events have normally been followed by a respite in activity
and a change in the eruption pattern. 


Appendix 1: Time line summary

3 March Displacement of point in north crater wall (FT3) of 15 cm since 13
Jan.

12 May Displacement of FT3 of 21.5 cm since 13 Jan. 

14 May Beginning of rock fall activity on north face of dome.

19May First rockfall spills into Tuitt's Ghaut.

29 May First pyroclastic flow activity in northern ghauts

2 June Pyroclastic flow down Tuitt's Ghaut travelling 1 km from the crater.

3 June Pyroclastic flow in Tuitt's Ghaut travelling 1.4 km.

4 June Pyroclastic flow in Tuitt's Ghaut travelling 1.8 km.

5 June Pyroclastic flow in Tuitt's Ghaut travelling 2.9 km, reaching to
250 m above where the ghaut joins Paradise River. 

7-14 June Rockfall and pyroclastic flow activity concentrated in Tuitt's
Ghaut. 

15 June Significant changes in Mosquito Ghaut. Pyroclastic flow material
advanced 500 m down the ghaut. Debris from a small rockfall in Gage's
Valley. 

16 June Pyroclastic flows in Gage's Valley travelling 1.6 km from the
crater rim. Smaller pyroclastic flows occurred in Mosquito and Tuitt's
Ghaut. 

17 June Significant pyroclastic flow activity in Mosquito Ghaut to a
distance of 4 km. Lesser activity in Gage's Valley with pyroclastic flows
travelling 200m further than previously. 

22 June Moderate pyroclastic flow in Tar River followed by a short
volcano-tectonic earthquake swarm and resumption of hybrid earthquake
swarm activity

24 June Small pyroclastic flows down Mosquito Ghaut for the first time
since 17 June, travelling 1 km from the crater rim. Focus of dome growth
confirmed at top of Mosquito Ghaut. 

25 June	      03:00 Start of hybrid earthquake swarm

06:00 to 08:00 Deflation accompanied by small pyroclastic flows in
Mosquito Ghaut 10:50 Start of hybrid earthquake swarm 12:00 Crater
inflation peaked 12:45 Volcanic tremor, steam and ash production 12:55
Start of pyroclastic flow activity 12:57 First pulse in seismic activity
13:00 Second pulse in seismic activity c. 13:00 First pyroclastic flow
observed in Mosquito Ghaut from MVO 13:02:12 First flow seen from airport
around Harris Hill (didn't get much further than Farms)  13:03 Seismic
signals from eastern stations lost 13:07:40 Second pulse seen from airport
(probably reached furthest run-out distance)  13:08 Third pulse in seismic
activity 13:13:49 Third pulse seen from airport (distinct pulse of
co-ignimbrite ash through cloud of second pulse)  13:20 End of seismic
activity

Appendix 2: Emergency actions on 25 June

07:00 to 08:00 Heightened concerns about possible pyroclastic flow
activity. MVO observer at airport alerted. Police HQ contacted, advised to
keep checkpoint to Plymouth closed for the time being (it later transpired
that essential services were already working in Plymouth, without the
knowledge of the duty scientist). MVO observers deployed at Windy Hill to
observe small pyroclastic flows in Mosquito Ghaut. 

08:20 Police Commissioner advised that checkpoints into Plymouth can open,
and essential services allowed access. 

12:30 Police Commissioner visits MVO, briefed by Chief Scientist (CS).
Check points into Plymouth closed. 

12:45 Deputy Chief Scientist (DCS) arrives at MVO. CS / DCS discuss
situation. Duty scientist at airport put on alert. 

12:45 to 12:55 Essential services in Plymouth advised to evacuate. Port
Authority, Monlec, Montserrat Mills contacted successfully but direct
contact with Texaco fuel tanker not made (the tanker later cut the fuel
lines to the port and successfully withdrew). Plymouth siren sounded. Live
update of situation broadcast on Radio Montserrat. Field team at Whites
advised to speed up operation. Field team requests permission to enter
central corridor area, refused by CS and advised to withdraw to airport. 

12:55 Start of pyroclastic flow activity

c. 12:58 Phased evacuation of airport recommended by MVO. 

c. 13:00 Immediate evacuation of airport initiated. Staff clear of airport
in less than 5 minutes, plane on ground takes off. HE the Governor at
airport, accompanies MVO scientists to safe viewing position north of the
airport. 

c. 13:00 Acting Governor, Chief Minister, Minister for Education and
Health, Minister for Agriculture, Trade and Environment arrive at MVO for
briefing. 

c. 13:00 Two MVO staff members take off from Vue Pointe in helicopter. 

c. 13:10 All MVO staff accounted for and confirmed safe. 

c. 13:15 Acting Governor, etc. leave MVO for Emergency Operations Centre
(EOC), accompanied by DCS and Dr Baxter. 

13:16 Helicopter reconnaissance over the airport picks up survivors in the
Trants area. 

13:20 Ash advisory issued by MVO. Due to phone line problems, fax reached
only 2 of 5 intended recipients. 

14:00 Helicopter searches for survivors in Spanish Point area. 

14:45 Survivors sighted in Harris, evacuation initiated by MVO helicopter. 

15:06 Search and Rescue team approaching from west by road report hot ash
at Dyers. 

15:14 Hot ash reported by Police in Bellham Valley near Cork Hill

16:00 CS briefs authorities at EOC of current situation

Figures

Figure 1 Location map

Figure 2 Sketch map of pyroclastic flow paths in May and June prior to
June 25 (rfs: rockfall directions) 

Figure 3 Seismicity for the period 13 May to 25 June 1997

Figure 4 Deformation (a) Map showing motion of site FT3 on the crater wall
between January and May 1997. Grid squares are 5cm by 5cm.  (b) Change in
EDM baseline length between Windy Hill and Farrells (5 point moving
average).  (c) GPS baseline length between Harris and Farrells.  (d) Shear
length of Chances Peak crack II.  

Figure 5 Chances Peak tilt data and seismic data for the period 22 to 25
June.  

Figure 6 Sketch map of the 25 June 1997 flow deposits.  

Figure 7 a.  Montserrat Volcanic Risk Map, February 1997 b. Montserrat
Volcanic Risk Map, 6 June 1997 c. Montserrat Volcanic Risk Map, 24 June
1997



. . . The Electronic Evergreen, courtesy of GEM Radio Network