During this period the October 22nd dome continued to grow although the focus of growth switched from the north to the southern side. Growth of the new dome against the southern wall of the crater weakened the crater wall over the Galway's area and by the 2nd November two large vertical cracks had appeared on the outside of the wall. By the 4th the wall was weakened such that a collapse occurred which removed much of the explosion crater wall (pre-explosion dome complex material). Continued growth of the October 22nd dome in that area led to further collapse on the 6th November which removed a significant amount of the new dome as well as old material. The pyroclastic flows produced during these collapses reached as far as the sea and the fan at the end of the White River valley has been significantly extended. During this period dome growth was associated with large hybrid earthquake swarms. After the collapse on the 6th the hybrid swarms subsided but seismicity levels still remained at a relatively high level.
The October 22nd dome continued to grow during this time although this was accompanied by vigorous ash and steam venting. The ash venting was often seen to occur in pulses coincident with hybrid earthquake swarms. Small rockfalls occurred from the October 22nd dome down Tuitt's Ghaut and also from the outside, of the old dome, down Galway's. Degradation of the outside of the Galway's wall occurred over this period, possibly due to physical shaking of the dome during the hybrid swarms as well as the pressure of the new growth from within the crater. An observation flight on the 2nd November revealed two large vertical cracks had appeared in the centre of the crater wall over Galway's and by the 3rd two deeply incised gullies had appeared.
The collapse of the 4th started at 12.06pm after a period of elevated rockfall activity from the southern side of the dome. Observations from the helicopter while the collapse was in progress suggest material was collapsing off a wide area in the Galway's section. The pyroclastic flows generated were continuous for 70 minutes and several of the flows reached the sea at O'Garra's building up a small delta at the coast. Ash clouds rose to about 10,000ft above the dome. Observations of the collapse scar suggested the collapse of the 4th had removed a large part of the pre-explosion dome complex wall but very little of the October 22nd dome.
During the morning of the 6th November the dome was clear such that a view was obtained of the top of the dome. It was evident that the northern and southern lobes of the October 22nd dome were separated by a small crater from which the ash venting was occurring. The northern lobe over Tuitt's appeared not to have grown significantly after the 2nd of November with a height remaining at 937m a.s.l. A second Galway's collapse occurred at 2.30pm after an 18 hour period of high amplitude tremor. Although the collapse was only sustained for a period of 35 minutes observations after the event confirmed that a larger volume of material had been involved in that event than during the collapse of the 4th. During the collapse rockfalls also occurred down the Tar river valley and in the Gages area.
Vigorous pulse-like ash venting occurred just before and for a few hours immediately after the event. The ash plume rose to about 15,000ft and drifted west.
Visibility was poor between the 6th and the 9th and continuous ash venting obscured good observations of the collapse scar but short glimpses of the dome confirm that new growth in that area had occurred. A narrow 'fin' shaped lobe of material had grown in the position of the old crater wall. This appeared to be growing almost vertically. The outer face of this material had a coarse blocky appearance while the back had a smooth slab-like appearance and could be seen to be extruding out of a cleft in the centre of the dome. Vigorous degassing and ash venting was still occurring and this was focused in the area of the cleft. The morphology of the dome during this period is clearly of two discrete lobes that were being extruded to the north and south with a central cleft between. This morphology is very reminiscent of the dome during July 1996 and April 1997. During both these periods discrete lobes with smooth curved backs were extruded from central clefts. These clefts commonly run east -west across the dome and the lobes tend to be extruded northwards and southwards. In this case the northern lobe was extruded first and when it reached a given size growth relaxed and activity moved to the southern side. As the crater wall in this area was tall and unstable renewed growth against the wall caused massive instability and collapse ensued.
The reporting period was very seismically active with intense hybrid swarms (table 2). These swarms at times merged to tremor of similar frequency to that of the individual hybrids. On the 6th , 7th and 8th November, in particular tremor, was at a high level. Individual hybrids were discernible on the paper records that did not trigger the networks because of the elevated background. The lower number of hybrids reported for these days is thus not an indicator of lower activity, which was, in fact, at a very high level. Throughout this period tremor episodes and hybrid swarms were both associated with vigorous ash venting at the dome. On the 4th November at 11:56 and on the 6th November at 14:29 there were big pyroclastic flows down Galways. In both cases the seismicity - whether hybrids or tremor - stopped soon afterwards.
The hybrid earthquakes recorded on the 1st and 2nd November had particularly high amplitude at MBWH - the only broadband network station continuously recording since October 1996. The only events previously recorded with higher amplitude than these hybrids were hybrids recorded on the 24th June. Some of the events on the 1st and 2nd November were reported by SRU to have triggered their stations on Dominica, Antigua and Nevis.
Table 1: 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* 26 Oct 97 0 8 28 11 1 0 27 Oct 97 3 109 32 29 2 0 28 Oct 97 2 233 40 24 0 0 29 Oct 97 6 175 36 86 3 0 30 Oct 97 3 190 26 72 6 0 31 Oct 97 3 313 23 58 2 0 01 Nov 97 2 487 16 28 0 0 02 Nov 97 1 557 14 35 0 0 03 Nov 97 1 727 23 1 0 0 04 Nov 97 4 549 31 1 0 0 05 Nov 97 0 318 9 7 0 0 06 Nov 97 2 79 5 1 0 0 07 Nov 97 1 31 3 1 0 0 08 Nov 97 0 293 22 0 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 number of lps 27/10/97 06:07 27/10/97 08:34 0 27 7 27/10/9718:45 27/10/97 21:25 2 68 9 28/10/97 01:21 28/10/97 04:33 2 46 5 28/10/97 08:56 28/10/97 11:51 0 58 6 28/10/97 15:05 28/10/97 20:33 0 117 21 29/10/97 00:09 29/10/97 04:27 0 100 18 29/10/97 07:14 29/10/97 08:51 0 41 1 29/10/97 18:36 29/10/97 20:20 0 12 6 29/10/97 23:53 30/10/97 01:06 0 20 2 30/10/97 06:19 30/10/97 07:58 0 25 4 30/10/97 10:32 30/10/97 11:47 0 23 1 30/10/97 12:45 30/10/97 15:35 0 92 2 31/10/97 06:06 31/10/97 10:58 0 158 0 31/10/97 16:04 31/10/97 21:43 1 148 8 01/11/97 03:08 01/11/97 11:28 0 223 1 01/11/97 17:03 02/11/97 05:32 0 392 7 02/11/97 10:12 04/11/97 12:57 5 1037 29 04/11/97 16:48 05/11/97 19:58 0 497 11 06/11/97 19:35 07/11/97 01:47 0 86 0 08/11/97 12:53 08/11/97 23:23 0 283 5
No surveys were undertaken during this time.
A dome survey was carried out on the 6th November just before the Galway's Collapse occurred. Theodolite points were obtained from Jackboy Hill, the new Centre Hills observation point and Flemings. A new GPS point was set up at the Centre Hills observation point and this will be used in future surveys as an additional static photo point. Survey photographs were obtained from the helicopter of most areas around the dome although the Galway's side was poorly exposed. Good coverage of points was obtained for the northern side so that the October 22nd dome growing northwards out of the scar was well surveyed. This area was seen not to have changed since the 3rd November so that the volume calculated (5 million cubic metres) was taken to be the volume on the 3rd rather than the 6th. This enabled the collapse volumes to be calculated separately and therefore gave an average extrusion rate over the first 11 days of 5m3/s. From earlier calculations we know that during the first few days of growth that the rate was as high as 8m3/s. Visual observations suggest that the collapse of the 4th involved more material than the collapse of the 6th and a rough estimate of the volume of material in the 4th was 1-2 million cubic metres. The bulk of the material from both the collapses is thought to have accumulated in the fan at the end of the valley. A fan survey to better constrain the collapse volume will be carried out when the levels of ash in that area subside.
Dust Trak sampling has been carried out at four fixed sites. Each value is an average of the PM10 concentration over approximately 24 hrs. The values at the fixed sites have been low over this period except for the Dust Trak at the Catholic school which occasionally records raised levels due to the large amount of human activity at this site.
Table4.PM10 (concentration in mg/m3)
Site 26 Oct 27 Oct 28 Oct 29 Oct 30 Oct 31 Oct 01 Nov Runaway 0.016 0.013 0.01 0.01 0.011 St. Peter's 0.02 CPS 0.026 0.029 0.03 0.05 0.06 0.047 MVO north 0.02 0.008 0.008 0.009 Site 02 Nov 03 Nov 04 Nov 05 Nov 06 Nov 07 Nov 08 Nov Runaway 0.009 0.015 0.009 0.01 0.028 0.035 St. Peter's 0.022 0.039 CPS 0.02 0.063 0.027 0.032 0.08 0.059 MVO north 0.008 0.01 0.007 0.006 0.018 0.017MVO Staff Changes Departures:
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.