The level of volcanic activity has been low, and characterised by continued dome growth on the eastern side and repetitive swarms of volcano-tectonic and hybrid earthquakes, with a few episodes of tremor.
The dome has been stable during this period. There have been few changes to the dome, and only limited rockfall and pyroclastic flow activity from the eastern face. On several days during the week, visibility was again restricted by low cloud.
On 10 February, near-continuous rockfalls were observed from the steep face of the January 20 dome in the Castle Peak area, and it seemed as if the rate of erosion was matched by the rate of extrusion. There were two clearly defined chutes. Blocks attached to the eastern side of the extrusion appeared to be moving downhill, although no movement was detected from 10 to 11 February.
The largest pyroclastic flow of the week occurred on the morning of 12 February, and flowed to about half-way across the delta. Another flow in the evening of 14 February reached as far as the Tar River Estate House.
By the end of the period, the eastern side of the dome had steepened, with a flower-shaped structure of radiating pinnacles at the top of the face.
There has been further collapse of the Galway's Wall, and more dome material has fallen over the top of the wall into the upper reaches of the White River. On 10 February, fresh rockfall deposits were seen, which extended to about 50 m above the road at Galway's Soufriere. The deposit had been emplaced dry. However there was no scorching of vegetation or ash-cloud deposits, and so the dome material involved could not have been very hot. Further flow deposits were noted on 13 February, which had not extended as far as the earlier rockfall, but were more voluminous close to the base of the wall. The flow deposits had a lobate morphology with well-defined, gravelly levees. Several small, slow-moving, dusty flows were observed from the helicopter, coming from three well-defined gullies in the top of the wall. On 15 February, a small rockfall over the wall was detected by the St Patrick's seismometer, and resulted in a small ash cloud.
The level of seismic activity has been low, and similar to previous weeks'. The activity was dominated by swarms of volcano-tectonic and hybrid earthquakes, with only a few episodes of continuous tremor.
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 09 FEB 97 66 2 8 30 10 FEB 97 77 0 15 25 11 FEB 97 96 2 35 24 12 FEB 97 129 0 33 23 13 FEB 97 96 2 35 8 14 FEB 97 20 0 25 12 15 FEB 97 33 0 15 6
The duration of the swarms was similar to the previous activity, while the time interval between the swarms lengthened during the week and averaged about 22 hours (Table 2). The number of volcano-tectonic earthquakes per hour decreased from about 10 in the previous reporting period to 8 per hour. The number of hybrid earthquakes in the swarms was variable, and generally higher than previously. Only three episodes of volcanic tremor were recorded, and these were of lower amplitude than previous episodes.
Table 2: VT / hybrid earthquake swarms and tremor episodes
Date Swarm Durn VTs Hybrids Tremor Durn start (hrs) start (hrs) 09 FEB 97 10:01 8.25 65 8 10 FEB 97 01:29 9.68 75 15 11 FEB 97 01:00 7.45 64 29 11 FEB 97 21:37 9.87 110 29 07:30 1.33 12 FEB 97 20:42 9.26 89 33 06:30 1.00 13 FEB 97 17:35 6.15 48 7 14 FEB 97 12:18 2.77 15 0 18:10 0.87 15 FEB 97 07:44 8.82 27 15
Ground Deformation and Gravity Measurements
Static gravity measurements were made on several days on the flanks of the volcano. Surveys were made on St. George's and Garibaldi Hill, between Brodericks and the foot of Chances Peak and from Trant's to above Tuitt's Estate. A microgravity survey between Hermitage and Whites was completed.
The cracks on Chances Peak were measured on 11 February. Since the previous measurement on 28 January, the more easterly of the two cracks (Crack 1) had opened by a further 44 cm (2.75 cm per day) and a dextral shear displacement of 23 cm. The original crack had split and several bifurcations and smaller subparallel cracks had developed. For safety reasons, no further measurements will be made on this crack. Crack 2 had opened by a further 2.6 cm with a shear displacement of 6 cm. The extensometer across this crack transmits real-time data to the observatory, and showed continued slow opening of the crack at a steady rate.
No GPS surveys were carried out this week.
Dome Volume Measurements
A dome volume survey was carried out on 15 February covering the presently active face on the south-eastern part of the dome. A complete set of fixed-location photographs were taken from Whites, Windy Hill and Harris Lookout. Measurements from these photographs will be incorporated with data from the kinematic GPS / laser-range finding survey.
Volume and mapping surveys were carried out on 12 February of the pyroclastic flow deposits in the lower regions of the Tar river valley. The volume of material accumulated here is in the order of 1.5 million cubic metres but as the original topography in this area has been severely eroded this figure is difficult to constrain.
COSPEC measurements were not made during this period as the COSPEC machine has malfunctioned.
Sulphur dioxide diffusion tubes were collected on 9 February, and have been sent to the UK for analysis. No further results have been received.
Rain water samples were collected at 4 locations around the volcano on 9 February, and results are in Table 3. One sample was also collected from run-off water by the side of the road to Upper Amersham on this date. These results show that the rainwater directly west of the volcano continues to be highly acidic and has high concentrations of certain anions.
Table 3: Rain and surface water geochemistry, 9 Feb 97
Units: conductivity (mS/cm), total dissolved solids (g/l), sulphate (mg/l), chloride (mg/l), fluoride (mg/l),
Upper Amersham pH Cond. TDS Slphts Chlrds Flrds 9-Feb-97 2.50 2.020 1.000 54 380 1 Lower Amersham 9-Feb-97 2.90 .987 .494 162 194 .95 Police HQ, Plymouth 9-Feb-97 3.00 1.202 0.602 200 232 0.95 Weekes 9-Feb-97 5.40 0.131 0.065 20 15.2 0.75 Amersham Stream 9-Feb-97 3.60 2.90 1.44 130 450 >1.5
Ash was collected from trays at 10 locations to the west of Plymouth. These samples will be dried and weighed to get an estimate of the mass of ash emitted from the volcano in the preceding weeks.
Lloyd Lynch, SRU
Mark Davies, BGS / Open University
Willie Scott, USGS
John Power, USGS
Richie Robertson, SRU
Tim Druitt, BGS
Brian Baptie, BGS