Volcanic activity during this period was at a high level with a large hybrid earthquake swarm occurring throughout Christmas Day leading to a debris avalanche down the White River early on 26 December. This triggered a series of very large pyroclastic flows which destroyed a wide area to the south-west of the volcano and a very energetic surge cloud which may have been associated with a lateral blast.
At the beginning of the reporting period, the activity followed the pattern of the previous 6 weeks, with near continuous rockfalls from the Galway's sector of the dome extending the talus slope well into the Soufriere area. However, there was an onset of major activity at about 03.00 on 26 December. Reports from the police checkpoint in Salem at 03.15 and 03.25 suggested that there had been explosions at these times. Observers on Garibaldi Hill at 04.00 reported light ash fall and burning buildings to the south of Plymouth. A pilot in the region observed an ash plume heading southwards at 36,000 feet.
Helicopter flights later in the morning of 26 December revealed extensive damage in the area between Aymer's Ghaut, south of Kinsale to the South Soufriere Hills. Deposits from a large debris avalanche had travelled down the White River to the sea and added considerable amounts of material to the pyroclastic fan. Pyroclastic flows and surges had covered all of the area between Kinsale and almost to the summit of the South Soufriere Hills. The area closest to the White River, including St. Patrick's and Morris' had been completely destroyed and there was considerable tree blow down away from the main path of the flows. A small pyroclastic flow, similar to the Belham valley flow on 25 June 1997, had travelled down Dry Ghaut towards Sweeney's Well reaching to within 300 m of the sea on the south-east coast. Brief views of the void at the top of the White River suggested that a large scoop of material had been excavated from the area around the Galway's Soufriere and that a proportion of Galway's wall had also been removed. The newly formed talus apron and southern lobe above Galways wall had been excavated, although the back limit of the scar could not be discerned.
Further flights and visits to the affected areas resulted in measurements of thicknesses and temperatures of the deposits (Table 1). A special report on this event and a preliminary study of the deposits is currently under preparation and will be released by the MVO shortly.
Observations made at Old Road Bay suggested that a large wave had come ashore immediately after the eruption. An observer in the vicinity of the north end of the bay at about 03:00 reported that the sea was sucked backwards before coming onto land near the jetty. Measurements of wash-up of debris on the shore implied that the wave must have been about 1 m higher than the level of the road in this area, but little evidence for substantial waves elsewhere along the coast existed. This wave was probably a result of the debris avalanche entering the sea, and was most likely focused in the Old Road Bay area because of the shape of the bay.
On 1 January there was an increase in activity with at least one large pyroclastic flow reaching within 1 km of the sea down the White River, and much rockfall activity from the area above Galway's Wall. A new talus apron had started to build up at the base of the remains of Galway's Wall, but no good views were obtained into the void left by the 26 December collapse. Some large wall-parallel cracks were also observed in the Chance's Peak side of Galway's Wall.
Throughout the remainder of the reporting period, rockfalls continued to create diffuse ash clouds which mostly drifted westwards over Plymouth.
Date Site Type of deposit Thickness Temperature /Deg C 30 Dec 97 Dry Ghaut (end of flow) Pyroclastic flow 30 cm 48.7 at 20 cm depth 30 Dec 97 Dry Ghaut (3m from end) Pyroclastic flow 40 cm 138.0 at 25 cm depth 30 Dec 97 Dry Ghaut (10m from end)Pyroclastic flow 40 cm 122.4 at 35 cm depth 1 Jan 98 South White River Delta Surge 70 cm not measured 1 Jan 98 O'Garra's Quarry Surge 10 cm not measured 1 Jan 98 O'Garra's Quarry Co-ignimbrite ash 7 cm not measured
For the first few days of this period the seismicity remained relatively quiet. There were slightly more hybrids than in the preceding weeks and noticeably fewer rockfall signals but no indication of any great changes. The quiet period ended on 24th December when a hybrid swarm began which led up to the large collapse on the 26th December.
The swarm began at 14:20 on 24th December. It started out sparsely, with events every 20 minutes or so, and slowly increased in intensity until approximately 20:00 on 25th December. The individual events also generally increased in amplitude as the swarm progressed, but even the largest were relatively small in amplitude (an order of magnitude smaller at MBWH than those recorded in early November, for example). At 20:00 the hybrids were occurring too frequently to trigger the networks and from this time onwards the signal being recorded was effectively a tremor signal. The amplitude of this tremor increased until about 23:00 and then declined until 3:00 when the collapse occurred.
At approximately 3:00 a continuous, high amplitude signal was recorded which lasted about 16 minutes and included several pulses. The signal then continued at reduced amplitude for a further 9 minutes and in this time a roaring was heard, sounding similar, but louder than that heard after explosions and associated with ash-venting. Intervals of monochromatic seismicity at 1.9Hz were recorded during this time, as was also the case after explosions. The combination of the roaring noise and monochromatic seismicity indicate vigorous de-gassing after the main pyroclastic flows had finished - suggesting that the top of the conduit had been exposed.
After the collapse, seismicity settled into a cyclical pattern with peaks in RSAM every 6 to 8 hours. Hybrid earthquakes occurred exclusively during the periods of raised background amplitude but in most cases there were not enough in any given cycle to constitute a swarm. Exceptions to this are listed in Table 3. Two days after the collapse the number of hybrids had decreased to a similar level to that before Christmas but the cycles in RSAM continued to the end of the reporting period.
Table 2: 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* 21 DEC 97 0 28 23 47 3 0 22 DEC 97 0 44 45 50 1 0 23 DEC 97 0 54 29 54 1 0 24 DEC 97 4 70 36 53 1 0 25 DEC 97 0 498 22 58 2 0 26 DEC 97 0 298 0 22 0 0 27 DEC 97 0 135 5 4 0 0 28 DEC 97 8 34 12 23 0 0 29 DEC 97 2 31 10 34 1 1 30 DEC 97 1 36 8 32 0 0 31 DEC 97 8 15 2 17 0 1 01 JAN 98 1 25 17 87 0 2 02 JAN 98 2 48 5 37 0 0 03 JAN 98 0 23 1 15 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 3: Swarms
Start (local) Duration Hybrid LP VT 24 DEC 97 14:20 36.3 617 29 2 26 DEC 97 05:29 3.8 28 0 0 26 DEC 97 10:49 6.5 152 0 0 27 DEC 97 00:24 3.1 44 1 0 27 DEC 97 06:42 3.2 28 0 0 27 DEC 97 12:15 2.7 47 1 0 29 DEC 97 14:06 .5 14 1 0
GPS occupations of LEESNET (Old Towne, WaterWorks, St. Georges Hill and Lees Yard) were completed on 22 and 28 December. Clear trends in the motion of any of these sites have not yet been identified and they appear to be stable.
A volume survey was carried out of the fan at the end of the White River valley on the 4th Jan. The results show that the fan itself was not extended into the sea significantly. The steep underwater shelf in this area means that the bulk of the debris avalanche and pyroclastic flow material that reached the sea will have slumped off the edge of the fan and have been lost into deep water where it can not for the moment, be surveyed. A survey of the material in the White River valley itself has been hampered by the ashy conditions and also for safety reasons. This will be attempted as soon as possible however rough estimates of the amount of material ponded in the valley are between 20 - 30 million cubic metres. The size of the scars in the dome are other volume constraints but due to poor visibility this has also been hard to determine. Work is still being carried out on the nature of the deposits and the volumes involved in the Boxing day event but best estimates so far are between 30 and 60 million cubic metres for the amount of material lost (this includes old Galway's Wall material).
Dust Trak sampling has been carried out at four fixed sites. Each value is an average of the PM10 concentration over approximately 24 hours. The values at the fixed sites have been low over this period except for the values at the Catholic school which sometimes recorded raised levels, this is due to the large amount of human activity at this site and the fact that it is near a main road. Also, the last value at St. Peter's is inconsistently high and thought to be influenced by human activity.
Table 4 : PM10 (concentration in mg/m3)
Site 21-Dec 22-Dec 23-Dec 24-Dec 25-Dec 26-Dec 27-Dec Mango 0.015 0.026 0.031 0.014 0.010 0.011 0.012 CPS 0.019 0.032 0.025 0.021 0.019 0.014 0.015 St. Peter's 0.014 0.026 0.021 0.014 0.009 0.010 0.013 MVO north 0.009 0.030 0.023 0.013 0.008 0.010 0.013 Site 28-Dec 29-Dec 30-Dec 31-Dec 1-Jan 2-Jan 3-Jan Mango 0.036 0.032 0.018 0.015 0.017 0.014 0.017 CPS 0.035 0.033 0.030 0.055 0.020 0.015 0.028 St. Peter's 0.033 0.029 0.016 0.014 0.017 0.014 0.323 MVO north 0.030 0.029 0.016 0.014 0.017 0.009 0.000 These values are approximate 24 hr averages of the PM10 concentration. 24 Hour averages: <0.05 mg m-3 Low 0.05-0.1 mg m-3 Raised 0.1-0.3 mg m-3 Very High >0.3 mg m-3 Alert Locations: St. Peter's: The Dust Trak is outside on the balcony of a villa in St Peter's, called St. Peter's Place. CPS: Catholic Primary School The Dust Trak is outside in the area under the roof where some of the children have classes. The school is in Palm Loop near Woodlands. MVO north: The new Volcano observatory on Mongo Hill near St.John's, the Dust trak is on the third story of the building with the sampling tube stuck out of the window. Mango: The Dust Trak is outside on the verandah of a villa in Mango Drive in Woodlands
MVO Staff Changes
Lucy Ritchie (Luton University)
Hayley Duffell (British Geological Survey)
Jurgen Neuberg (Leeds University)