Summary
Further high levels of pyroclastic flow activity have occurred throughout this reporting period. The upper part of Mosquito Ghaut has now been completely in-filled. Further occupations of EDM sites in the Lee's Yard Quadrangle indicate that movement of the Lee's target is very slow. GPS measurements indicate that lines from M18-Harris to White's Yard, Long Ground and O'Garra's confirm the shortening seen on the previous occupation. Ash and water sampling has continued with the addition of new ash collection points to replace those lost due to the events of June 25.
Visual Observations
Pyroclastic flow activity was high during this reporting period and concentrated on the northern flank of the volcano. A small amount of pyroclastic flow activity occurred in the upper reaches of Gages Valley during the first three days of the reporting interval. One and a half hours of sustained activity in Mosquito Ghaut on 30 August completely filled the ghaut at the level of the Farrell's fields in a collapse involving an estimated 1 million m3 of material. Individual flows had run-outs of only 2km and spread out over Farrell's plain building a broad fan. The walls of Mosquito Ghaut are still visible in its upper reaches, but so much material has accumulated here that it is able to spill directly out into Tuitt's Ghaut. A secondary, phreatic explosion occurred from the pond on the western side of Mosquito Ghaut, where hot pyroclastic material accumulated producing a steam explosion.
The dome was very clear during the night of 5 September with semi-continuous rockfalls occurring down Mosquito Ghaut and more restricted activity above Gages and around the back of Chances Peak. Activity increased for about an hour on the morning of 8 September, with a series of moderate pyroclastic flows travelling down the northern flank of the volcano. These were wholly unconstrained by Mosquito Ghaut and spread westwards over the Farrell's plain. Eventually flows entered the Unnamed Ghaut around Riley's Estate and travelled down the Belham drainage system. The flows were all slow moving and reached Dyer's River immediately south of Molyneux. These events generated the first substantial block and ash flow deposits in the Belham Valley - the pyroclastic flow in Belham on 25 June was a dense ash-flow and the August explosions contributed material in thin pumice flows.
Observation flights on 9 September showed that dome talus at the head of Gages Valley has extended so far that material is now flush with and can gain direct access to the top of the Unnamed Ghaut, between Peak C and Gages Mountain. A substantial scar containing four small spines was present in the dome above Mosquito Ghaut at this time. This generated continuous small rockfalls and vigorous ash venting. The collapse area was clearly isolated from the area of dome growth through August at the head of Gages, although a 40m overhanging block was seen directly above Gages. A small amount of degradation has occurred in the upper 50m of the dome on the SE side, generating rock fall debris in the upper reaches of Tar River.
Seismicity
For the first few days of the reporting period rockfall and pyroclastic flow activity continued to show an approximately 12 hour repeat pattern. This did not continue after the 4th of September and activity for the rest of the period showed no clear, long lasting patterns, although there were a number of swarms of earthquakes, which are tabulated below. On the 8th and 9th September hybrid swarms preceded pyroclastic flows but there were also hybrid swarms with no associated flows. Most of the pyroclastic flows in this period did not have pre-cursory hybrid swarms. The MVO move disrupted the short period seismic network from the 10th September for only a day or two. The broadband system, however, has had data transfer problems since that date, due to the necessity of transmitting data to the new site using telephone lines. The broadband network has only been recording one or two stations (Galways Estate and Windy Hill or just Windy Hill) since the 10th September and the count has been carried out using the short period network since this date.
Date Swarm Start Swarm End # of Hybrids # of vts 3 Sep 97 22:29 01:24 0 24 8 Sep 97 00:52 04:06 52 0 8 Sep 97 13:28 15:51 29 0 9 Sep 97 01:54 02:59 35 0 11 Sep 97 01:01 03:02 37 0
Table 1: Earthquake types
These earthquake counts are of events that triggered between 0000 and 2400 each day (local time).
Date VT Hybrid LP Dome RF LPRF* HYRF* 30 Aug 97 7 7 32 102 17 0 31 Aug 97 7 17 33 93 25 0 1 Sep 97 4 26 38 143 14 0 2 Sep 97 17 14 18 73 0 0 3 Sep 97 29 1 8 63 3 0 4 Sep 97 1 6 8 86 0 0 5 Sep 97 3 3 10 99 0 0 6 Sep 97 3 3 41 83 12 0 7 Sep 97 18 12 3 78 0 0 8 Sep 97 2 94 14 76 0 0 9 Sep 97 0 56 29 98 23 0 10 Sep 97 4 13 13 80 7 0 11 Sep 97 0 43 0 51 0 0 12 Sep 97 1 1 1 21 0 0 13 Sep 97 0 54 9 25 0 0
Ground Deformation
Further occupations of the Water Works - Lee's Yard and MVO-Lee's Yard
lines have been possible. The slant distances from each instrument site to
the Lee's Yard target are approximately stable but there is a slight hint
of slow shortening on the WW-LY line. There does appear to be some changes
taking place in the horizontal distance and height difference components on
the two lines to the Lee's Yard target. The horizontal distance between MVO
and LY is slowly lengthening and the height difference is decreasing
(target site at LY is moving down relative to the instrument site at MVO).
On the WW-LY line the horizontal distance is stable but the height
difference is decreasing (LY down relative to WW). These components are the
least well resolved by the technique and presently the changes are only
small. Further occupations will be made to examine any further changes.
A GPS survey was carried out on September 13 involving sites at Harris,
Windy Hill, Broderick's and O'Garra's. The occupation confirmed the small
amount of shortening of a line across the volcano (Harris-O'Garra's) that
was reported in the previous report. Lines to Broderick's appear stable.
The Harris-Windy Hill line has shown some length changes since last
summer, although the changes have not been large or systematic to date. An
occupation of the sites at Whites, Long Ground and Hermitage is a priority
but attempts to land there were foiled due to the amount of ash on the
ground.
Volume Measurements
No new volume measurements were made during this interval.
Environmental Monitoring
Dust Trak sampling has been carried out at several sites around the island
showing low values in the northern and eastern area and higher values in
the Salem area. The central area is still characterised by acceptable
values. Results are shown in Table 2.
Table 2. PM10 (concentration in mg/m3)
These values represent the average reading over the sample duration. The
mean duration of a sample is usually between 15 and 30 minutes. Samples of
longer duration are labeled with the following letters:
Rain water samples were collected at 3 locations around the island on 12
September, and results are shown in Table 3. These results show less acidic
values compared to the previous week but still outside of the acceptable
range. The anion concentrations is well within WHO guidelines for drinking
water.
Table 3. Rain water geochemistry,12 September 1997.
Sulphur dioxide diffusion tubes were collected on 12 September and sent to
the UK for analysis.
A new ash collection plan has been designed after the previous ash
collection sites have been devastated by the 25th of June event and/or are
located in the currently inaccessible area. We now take into account 16
sites north of Garibaldi Hill. Thanks to the north-western wind direction
we have been able to collect interesting ash samples after a single
co-ignimbrite event (12 September). These samples have been weighed and
selected ones will be sent back to the UK for analysis.
MVO Staff Changes
Departures
Visitors
Date 31/8/97 1/9/97 2/9/97 3/9/97 4/9/97 5/9/97 6/9/97 7/9/97 8/9/97 9/9/97 10/9/97 11/9/97 12/9/97 13/9/97 14/9/97
Angelo's- - - - - - 0.098 - - - - - - - -
BC - 0.006 - - 0.048 - - - - - - - - - -
BSo - - - 0.037 0.039 - - - - - - 0.067 - - -
CB - 0.023 - - 0.045 - 0.074 - - - - - - - -
GH - - - - - - - - - 0.141 - - - - -
GPo - 0.008 - - - - - - - - - 0.029 - 0.02 -
JBH - - - - - - - - - - - - - 0.015 -
Mc - - - - 0.092 - - - 0.021 - - - - - -
MVOi - - - - - - - - - - a 0.05 - - - -
MVOo a 0.621 d 0.014 f 0.023 b 0.023 d 0.069 d 0.029 - e 0.028 d 0.032 0.03 b 0.039 d 0.039 - 0.037 0.012 0.068F
MVOpd - - - - - - - - - - 0.55F - - - -
RBC - - - - 0.057 - - - - - - - - - -
RC - 0.005 - 0.023 0.027 - 0.01 - - - - 0.04 - 0.02 -
SCP 0.026 0.041 - 0.027 0.089 - 0.019 - 0.036 - - 0.11 - - -
SPSh.i - - - 0.025 - - - - - - - - - - -
SPSh.o - - - 0.027 - - - - - - - 0.017 - - -
SSo - 0.035 - 0.022 0.06 - 0.027 - - - - 0.106 - 0.034 -
SSSo - 0.014 - 0.018 0.044 - 0.015 - - - 0.067 0.074 - 0.057 -
- No samples taken
i sample taken inside
o sample taken outside
F during fallout
P personal sample (during activity)
Key to locations:
Angelo's: Supermarket (Cudjoehead)
BC: Brades Church (North)
BS: Brades Shelter (North)
CB: Carr's Bay (North)
GH: Garibaldi Hill
GP: Gerald's Park Shelters (North)
JBH: Jack Boy Hill (eastern area)
Mc: McChesney (Olveston)
MVO: Montserrat Volcano Observatory (Old Towne)
MVOpd: MVO pool deck
RBC: Royal bank of Canada (Olveston)
RC: Red Cross (Woodland)
SP.Sh.: S. Peter's Shelter(North)
SCP: Salem Car Park
SS: Supermarket in Salem (RAMS EMDEE)
SSS: Secondary School Shelter (Salem)
Units: conductivity (mS/cm), total dissolved solids (g/l), fluoride (mg/l), chloride (mg/l),
sulphate (mg/l), ND: not detected
Location pH Cond. TDS Slphts Chlrds Flrds
Lawyer's 5.63 0.039 0.019 ND 5.6 0.10
Fogarthy 5.84 0.112 0.056 ND 21.1 0.20
MVO south 5.34 0.078 0.038 ND 14.4 0.90
Arrivals
Dr Sue Loughlin (BGS)
Brian Baptie (BGS; Returned from field trip in the Cascades)
Professor Steve Sparks (Bristol University, UK)
Levar Cabey
Jean-Pierre Viode (Guadeloupe Volcano Observatory)
Montserrat Volcano Observatory