During this two week period 29 more explosions occurred bringing the total in the present series to 61. These were similar in nature to those of the previous three weeks producing fountain collapse and simultaneous pyroclastic flows down many of the Ghauts. In detail these events differed in magnitude, column height and pyroclastic flow runout possibly reflecting the differing time periods between them. The last explosion of this phase occurred on 21st October at 19:02. By the evening of 22nd October a new dome (the October 22nd dome) had started to grow within the crater. Estimates suggest that initial extrusion rates for this dome could be as high as 8m3/s.
The explosive 'Vulcanian' type eruptions continued to occur up until the 21st October. All of these explosions produced collapsing fountains which generated pyroclastic flows into the surrounding ghauts. The interval between the explosions varied between 2 hrs:46mins and 20 hrs:24mins with an average of 8hrs:30mins. The time interval data showed no obvious trends when plotted against other variable factors but running average plots of the interval against time did show increasing and decreasing cyclic trends. During the three day period 14th - 16th October there were 12 explosions, averaging 4 a day, while towards the end of the explosive phase the time intervals between explosions tended to be more extended. The last three explosions on the 20th October at 15:15 and 21st October at 11:9 and 19:02 appeared to have been particularly vigorous with plumes of over 30,000ft.
All produced pumiceous fallout in Salem, Olveston and light ash falls in the north. In the Cork Hill area pumices collected from the last two explosions were up to 10cm diameter. Ballistics of the explosion on the 20th were seen to impact areas at the top of the Farrell's road, almost 2km from the vent. Though this period pyroclastic flows were generated in all the Ghauts radiating from the dome. Some of the smaller explosions produced flows with relatively small runout (< 1km) and only in one or two of the Ghauts. Because the crater is open to the north the northern Ghauts tend to be the prefered drainage routes for the explosion generated flows although for the larger explosions it still holds true that the collapses are generally radial.
The pyroclastic flows produced during the 75 explosions over the past month have left significant deposits around all flanks of the volcano. Although the deposit from each individual flow is relatively thin (0.3 - 1m thick) the accumulated deposits have significantly modified and infilled the topography around the volcano. In the Tar River area the fan has increased in height, probably by a couple of metres but has not significantly changed in overall shape. Mosquito Ghaut has been completely infilled so that in that area there is now a smooth fan extending from Tuitt's Ghaut in the northeast all the way around to Tyer's Ghaut in the northwest. Tuitt's and White's Ghaut have partially filled but the bulk of the flows that traveled down that route have left fans of deposits in the Bethel, Bramble and Spanish Points area. The Gages Soufriere area has filled significantly and the fan of material stacked in front of St.Georges Hill is widening so that further accumulation will lead to flows draining to the north of the hill and into Dyers River as well as to the south towards Plymouth. Fort Ghaut in Plymouth has been completely filled so that the flows are now fanning out producing a wide devastated swathe through the town centre. In the White River a small fan is beginning to be built up at the mouth of the valley.
Incandescence was reported inside the scar on the evening of 22 October and the following day fresh, dark lava was seen that had over-spilled the tephra rampart between the scar and the deep crater. This new dome has been termed the October 22nd dome and the 'old dome' with the crater, in which the new growth is occurring has commonly been referred to as the 'pre-explosion dome complex'. During the next few days the October 22nd dome grew dramatically so that by 25th October it was occupying a substantial fraction of the scar. From the north the October 22nd dome appeared to be composed of large coarse blocky material often associated in the past with elevated extrusion rates. Like the October dome last year, the lava appeared much darker in colour than the normal Soufriere Hills lava but this is thought to be an effect of oxidation processes in the top of the conduit rather than a real change in magma composition. Samples will be collected when possible for geochemical analyses. The maximum height of the new dome on the 25th October was estimated at 910m which is only 40m below the level of the crater rim. During this period however the October 22nd dome was growing out in a northerly direction as well as vertically to infill the scar. Rockfalls produced by the growing lobe were being directed into the top of Tuitt's Ghaut although few of these traveled more than 400m. The dome appeared to grow at a fast rate in the first few days of growth and the blocky northern face above Tuitt's Ghaut seemed steep and unstable however considering this surprisingly little rockfall activity occurred during this time.
The sequence of explosions which started on the 22nd of September continued until the 21st October. In total there were 76 explosions with an inter-explosion interval ranging from 3 hours 9 minutes to 33 hours 43 minutes. Each explosion was recorded on the seismometers as a pyroclastic flow signal preceded by an approximately 1 Hz signal of varying relative amplitude. This long period energy continued throughout the pyroclastic flow signal and continued afterwards at lower amplitude as tremor. This tremor lasted between 30 minutes and 3 hours and was visually correlated with ash venting from the explosion crater. In general there was little or no precursory seismicity before these explosions. This changed on the 20th October when there was a swarm of hybrid and volcano-tectonic earthquakes associated with the second explosion of the day. There was another mixed swarm before the first explosion on the 21st and then more than 24 hours of hybrids and volcano-tectonic earthquakes following the second explosion on the 21st - the last explosion of this sequence. In the course of this swarm on the 22nd there was a pyroclastic flow down Tuitt's Ghaut. This long and sparse swarm petered out on the 23rd, although there remained a higher than usual level of long period earthquakes.
Volcano-tectonic earthquakes from throughout this period were located at between 2 and 4 km below the top of the dome.
Table 1: Earthquake types
These earthquake counts are of events that triggered the broadband network's event recording system between 16:00 and 16:00 each day (local time).
Date VT Hybrid LP Dome RF LPRF* HYRF* 12 Oct 97 5 1 0 2 0 0 13 Oct 97 1 1 1 3 0 0 14 Oct 97 8 8 3 9 1 0 15 Oct 97 10 4 2 3 1 0 16 Oct 97 3 1 22 2 0 0 17 Oct 97 1 9 13 6 1 0 18 Oct 97 5 6 5 9 1 0 19 Oct 97 26 16 3 38 1 0 20 Oct 97 21 32 11 7 0 0 21 Oct 97 35 63 14 11 1 0 22 Oct 97 67 70 8 4 0 0 23 Oct 97 16 7 16 15 0 0 24 Oct 97 3 26 29 11 1 0 25 Oct 97 0 32 10 11 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 2: Swarms
Start Stop # of vts # of hybrids 20/10/97 10:33 20/10/97 15:43 20 18 21/10/97 09:07 21/10/97 11:18 11 11 21/10/97 22:16 22/10/97 08:57 72 81 22/10/97 09:30 22/10/97 17:05 35 23 22/10/97 20:03 23/10/97 10:02 14 14 23/10/97 11:44 23/10/97 21:33 0 10
Table 3: Explosions
Explosion of Duration of LP signal Amplitude of Duration of PF Amplitude of before PF signal LP signal signal (min) PF signal starts (sec) 12/10/97 07:55 14 15306 210 25726 12/10/97 22:24 no data 13/10/97 09:32 no data 13/10/97 15:25 0 240 15813 14/10/97 01:36 39 28399 210 30044 14/10/97 13:48 18 20828 190 48731 14/10/97 23:16 13 14414 250 25354 15/10/97 05:47 0 360 6421 15/10/97 08:33 10 16538 210 25185 15/10/97 14:50 34 48463 130 34041 15/10/97 22:20 31 3252 150 45914 16/10/97 02:51 19 18674 160 15536 16/10/97 06:35 42 7061 0 16/10/97 09:44 44 32847 140 11275 16/10/97 14:20 12 39357 30 15117 16/10/97 18:48 6 22517 120 39790 16/10/97 23:19 22 12622 0 17/10/97 04:01 26 35000 140 52444 17/10/97 12:36 no data 17/10/97 16:05 15 33870 230 52215 17/10/97 23:18 26 27381 230 32620 18/10/97 06:48 8 14923 250 45512 18/10/97 15:17 10 7655 230 42377 19/10/97 05:13 22 46554 300 111028 19/10/97 21:27 55 30973 270 44074 20/10/97 05:04 17 15588 260 23411 20/10/97 15:12 18 66336 120 78478 21/10/97 11:38 21 20120 210 33765 21/10/97 19:02 no data
All values are measured on the Windy Hill broadband station. Amplitudes are peak-to-peak in counts.
A GPS occupation was attempted on 20 October. Only the sites at White's, Harris and Windy Hill could be accessed as there was so much ash on the ground on all flanks of the volcano. The line Harris-White's showed around 2cm lengthening over the previous measurement on 20 September closer to the pre-June-97 long-term mean. Although the line-length is still shorter than 2-sigma below its pre-June 97 mean, the single measurement does not indicate an acceleration in the deformation rate. The last four measurements of the line Harris-Windy Hill (since 12 August) have shown a slight shortening. The Lees Yard EDM target was shot from MVO South on 14 October. The measurement was consistent with an overall 1cm lengthening of the line since the measurements began in July 97.
Deposit volume measurements have not been attempted while the volcano is in an explosive phase of activity. The gross morphology of the pre-Sept 21 dome is unchanged since the collapse, although there have been some modifications described in the visuals section. Volume estimates of the October 22 dome have been limited to simplistic geometrical calculations as the weather conditions have not yet allowed a detailed survey. However assuming that the October dome completely filled the explosion crater by the 23rd when it was first seen overspilling the rampart then its volume would have been 1.7 million cubic metres. This gives an extrusion rate of between 10 - 8 m/s depending on exactly when it first appeared in the bottom of the crater.
Dust Trak sampling has been carried out at four fixed sites. Each value is an average of the PM10 concentration over approximately 24 hrs.
Table4: PM10 (concentration in mg/m3)
Location 12 0ct 13 Oct 14 Oct 15 Oct 16 Oct 17 Oct 18 Oct Runaway - 0.080 0.08 0.049 0.033 - 0.014 St. Peter's - 0.023 0.023 0.061 - 0.064 0.02 CPS - - - - 0.076 0.065 0.042 MVO north - 0.045 0.045 0.065 0.041 0.019 0.007 Site 19 0ct 20 Oct 21 Oct 22 Oct 23 Oct 24 Oct 25 Oct Runaway - - - 0.048 0.031 0.017 - St. Peter's - - 0.024 0.036 0.02 0.099 0.02 CPS 0.008 - 0.056 0.091 0.064 0.054 0.015 MVO north 0.031 - 0.024 0.033 0.016 0.018 0.01 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: Runaway: The Dust Trak is outside on the patio of a villa in Olveston. 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.
MVO Staff Changes
Dr Keith Rowley (Independent)
Dr Simon Young (BGS, Edinburgh)
Dr Paul Jackson (Seismic Research Unit, Trinidad)
Costanza Bonadonna (University of Bristol, UK)
Dr Joe Devine (Brown University, USA)