Volcanic activity during this reporting period was sustained at an very high level. Activity was dominated by vulcanian explosions which occurred every few hours producing vertical eruption columns and collapsing fountains of material which fed pyroclastic flows. The pyroclastic flows were pumice-rich and travelled down all the ghauts around the volcano reaching the sea at Tar River valley, White River valley and at Spanish Point. After a large explosion on 11 October a pyroclastic flow in White River valley destroyed the O'Garra's road bridge along with the Radio Antilles installation. Ash and pumice fall continued to affect much of Montserrat and Antigua also received occasional light ash fall.
A total of 32 explosions occurred during the reporting period with an average interval between events of 10 ? hrs, although the interval varied from 4h:56min to 33h:43min. The length of the interval between explosions appeared to follow a cyclical pattern. The first explosions after the collapse were separated by around 11hours. The interval fell to 7 hours over the next 7 days, rose to 14 hours in the next 4 days, dropped to 7h:30min over the next 2 days and then increased to 16 hours in 6 days. Explosions occurring after shorter intervals appeared to be weaker, the plumes paler and less vigorous and the pyroclastic flows tend to have shorter run-outs.
Pyroclastic flows were concentrated in Tuitt's Ghaut and the Tar River Valley, although regular activity also occurred in the White River, Fort Ghaut and over the Farrell's Plain. Many of the Tuitt's Ghaut flows had runouts of 4 -4.5km and have now built a 300m wide fan around the Farms and Trants area. In several places these flows over-spilled the ghaut walls and spread over a wide area travelling ENE and passing through Spanish Point where they reached the sea. Many flows in the Tar River area reached the sea travelling down the southern side of the valley and down two chutes on the eastern face of the dome. The surface of the fan was almost entirely covered by the new flows and they built a pronounced hump along its central axis. Pyroclastic flow activity in Whites River was more limited with a few flows reaching the area where the Great Alps Falls had been. However, pyroclastic flows from the 01:05 explosion on 2 October covered the road bridge at O'Garra's reaching the sea at the mouth of the White River. The large explosion at 17:57 on 11 October covered the field at O'Garra's with flow deposits, destroyed the Radio Antilles installation and completely buried the road bridge. The incandescent blocks and glowing cloud of this event were seen from Antigua. Flows in Fort Ghaut spread NW around the Gages fan, westwards around Upper Amersham and down into Plymouth. The Plymouth flows spread through Dagenham and got to within 300m of the sea. Occasional flows have occurred in Tyer's Ghaut although the longest runouts were only around 2km (150m below the Dyer's road bridge).
Plume heights from the explosions varied from between 12,000-25,000 feet (3.6-7.6km). Pumice fallout in inhabited areas only occurred on 1 and 2 October, although ashfall was heavy on many occasions throughout all the island. Antigua received light ashfall on three occasions.
Very good views of the dome have been obtained throughout the reporting interval. The dome has a large horseshoe-shaped scar 300m across leading out into Tuitt's Ghaut. At the base of the scar there is a bank of pumice and talus, the back wall consists of a steep, rather consolidated looking, pumice rampart. The lowest point on this wall is 860m a.s.l. and behind it is the circular explosion crater also 300m in diameter. The crater rim is fairly constant in height around 950m a.s.l. and the highest point on the dome is a spine on the rim above Galways at 975m a.s.l. Glimpses down into the explosion crater suggest its base is 100-150m below the level of the pumice wall, probably close to the level of the original English's Crater basement. The crater and scar have both been enlarged slightly by rockfalls off the inside walls and presumably by erosion and shaking during the explosions. Small modifications are also occurring on the outside of the dome. Chutes are developing either side of a large consolidated area of the dome above Galways and the SE flank in the Tar River Valley is degrading slowly.
Large amounts of erosion have occurred around the margins of the dome on the old crater wall. Peak 'B' on the northern crater wall has been lowered by 30m due to the passage of pyroclastic flows this summer. A 90m wide chute has been cut down the Gages face of the dome immediately south of the original Gages Wall against Chances Peak. This chute is up to 50m deep and in places cuts at least 30m into the original basement.
The explosions, as in August, had a distinctive seismic signal, with an initial low frequency phase, a high frequency phase - assumed to be caused by the pyroclastic flows observed after each explosion, and a low frequency tremor. The low frequency start to these signals preceded any observed activity in the crater by several seconds and is assumed to continue throughout the pyroclastic flow signal and become the post-explosion tremor. Both the low frequency phase and the tremor have the same peaked spectrum with the main peaks at 1.2 Hz and 1.7 Hz. The relative amplitudes, at a particular station of the broadband array, of the two phases of each explosion are listed in table 3, along with an approximate time interval between the start of the first phase and the start of the second phase and the duration of the second phase. There is no obvious correlation between the delay between explosions and any of these four variables and no clear correlation between any pair of the four variables.
Apart from the explosions and the subsequent tremor - which on occasion lasted for several hours - seismicity was low for the reporting period.
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* 28 Sep 97 0 29 2 12 3 0 29 Sep 97 16 7 5 9 0 0 30 Sep 97 50 19 4 9 0 0 01 Oct 97 28 15 2 8 0 0 02 Oct 97 4 6 6 2 0 0 03 Oct 97 33 5 6 29 0 0 04 Oct 97 4 0 4 35 0 0 05 Oct 97 0 5 17 61 5 0 06 Oct 97 0 1 15 13 10 0 07 Oct 97 14 46 1 5 1 0 08 Oct 97 3 8 7 5 1 0 09 Oct 97 1 5 2 9 2 0 10 Oct 97 8 4 0 3 1 0 11 Oct 97 7 20 0 6 1 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 number of vts number of hybrids 29/09/97 3:38 29/9/97 11:02 0 10 30/09/97 3:39 30/9/97 3:56 1 7 30/09/97 7:30 30/9/97 17:42 14 45 01/10/97 21:12 2/10/97 0:02 14 4 07/10/97 14:10 8/10/97 07:20 17 14 11/10/97 06:29 11/10/97 16:56 5 19
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) 9/28/97 4:28 22.2 139868 231 78465 9/28/97 10:34 10.4 13179 290 17927 9/28/97 23:03 17.5 133280 290 115986 9/29/97 6:27 12.9 10142 287 30784 9/29/97 11:23 23.5 40622 216 29695 9/29/97 16:48 15 29554 251 82990 9/29/97 21:57 7 21468 160 54007 9/30/97 4:44 34 20005 220 21610 9/30/97 17:44 16 189880 170 75990 10/1/97 5:00 4 1950 240 35951 10/1/97 11:34 10/1/97 17:41 14 5442 290 32284 10/2/97 1:05 18 41329 150 38498 10/2/97 12:54 10/2/97 22:50 17 29551 210 12119 10/4/97 8:33 15 30904 360 161781 10/4/97 18:27 19 5250 250 28641 10/5/97 2:53 13 42141 300 40914 10/5/97 10:42 4 5644 150 19594 10/5/97 18:41 14 31789 190 56612 10/6/97 2:44 21 41720 130 39381 10/6/97 10:42 14 28892 120 37664 10/6/97 17:50 5 9210 130 55690 10/7/97 4:06 10/7/97 16:03 24 96213 150 49257 10/8/97 3:47 0 290 36828 10/8/97 15:10 15 31663 120 74980 10/9/97 3:03 13 34058 130 74980 10/9/97 12:31 20 21303 150 38356 10/10/97 4:13 16 42172 230 81507 10/10/97 18:40 17 23915 220 41035 10/11/97 17:57 15 39798 280 74243
All values are measured on the Windy Hill broadband station. Amplitudes are peak-to-peak in arbitrary units.
No GPS occupations were made during this reporting interval. The amount of ash on the north, east and west flanks of the volcano prevented landing in the helicopter at all but two sites. The O'Garra's GPS site (M46) was destroyed by pyroclastic flows on 11 October. EDM measurements to the Lees reflector have not been possible due to the reflector being covered in ash and ash in the air between the target and instrument sites.
Several clear days during early October allowed a detailed survey and map of the dome to be completed. Theodolite measurements were made from Jack Boy Hill, Flemings, Garibaldi Hill and the old observatory in Old Towne. Static photographs were taken from White's, Harris and Jack Boy Hill, and a series of photographs at different angles around the dome were taken from the helicopter and the position of the camera determined with the GPS.
The dome volume is currently 68 million m3. This compares to the last measurement on 28 August when it was 78.1 million m3, at which time the extrusion rate was 8.7 m3 s-1 (average 17 July - 28 August). However the difference in these volumes does not represent just the volume of the 21 September collapse as a substantial amount of pyroclastic flow activity had occurred over Farrell's plain and in Tuitt's Ghaut before the collapse.
Sulphur diffusion tubes were left at Weekes and at St George's Hill in the evacuated zone on 4th October, these will be collected after two weeks. Under normal prevailing wind conditions (to the west or north west) these two sites lie under the plume. Sulphur diffusion tubes at four sites in the inhabited area are being left for four weeks, until now there has been no detectable SO2 in the inhabited area. The tubes can only record an average background level and any peaks are thus lost.
Rainwater was collected at three sites and the rainwater is still quite acidic.
Table 4: Rainwater geochemistry 21st to 30th September 1997
pH Cond. TDS Flrd Chlrd Slpht mS/cm g/l mg/l mg/l mg/l MVO (south) 3.64 0.62 0.31 1.2 148 30 Lawyers 3.86 0.559 0.28 >1.5 94 22 Fogarthy 3.85 0.697 0.35 1.35 133 31
Dust Trak sampling has been carried out at several sites around the island showing comparatively high values in the northern and eastern area and high values in the Salem area. The central area also shows elevated values. These relatively high levels are as a result of fallout from explosive activity Results are shown in Table 5.
Table 5: PM10(concentration in mg/m3)
Location 3/10/97 4/10/97 5/10/97 6/10/97 7/10/97 8/10/97 9/10/97 10/10/97 11/10/97 Angelo's - - - - - - - - - BC - - - - - - - - - Bso - - - - - - - - - CB - - - - - - - - - GH - 0.543 0.653 - - - - - Gpo - - - - - - - - - JBH 0.04 - - - - - Mc 0.478 0.327 - - - - - MVOi - - - - - - - - - MVOo - 0.271 0.034 0.201 - - - a 0.047 b 0.495. MVOpd - - - - - - - - - MVOn,0 g 0.039 - - d 0.012 F 0.033 - b 0.009 - - RBC - - - - - - - - - RC 0.727 0.082 - 0.14 - - - - - SCP - - - - - - - - - SPSh.I - - - - - - - - - SPSh.o - 0.469 - 0.151 - - - - - Sso - 0.57 - 0..361 - - - - - SSSo - P0.234-0.152 - 0.191 - - - - - SSSi - - - - - - - - - Fogarty - - - - - 0.022 - - - Lawyers - - - - - - - - - Lookout(i) c 0.041 0.049 - b 0.093 - - - - d 0.11 Lookout(o) - - - - - - - F b 0.103 F b 0.643 MWAi - - - 0.19 - - - - - MDF 0.05 - - - - - - - - PS - - 0.06 - - - - - - CPSi - - - 0.919 b 0.035 b 0.035 c 0.12 - e 0.149 CPSo - - - 0.815 - b0.02-0.0 - - e 0.207 Runaway - - - - - - - d 0.088 -
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 labelled with the following letters:
- No samples taken i sample taken inside o sample taken outside F during fallout P personal sample (during activity) 24 Hour averages: <0.05-3 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 Angelo's: Supermarket (Cudjoehead) BC: Brades Church (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 pooldeck RBC: Royal bank of Canada (Olveston) RC: Red Cross (Woodland) SPSh.: St. Peter's Shelter (North) SCP: Salem Car Park SS: Supermarket in Salem (RAMS EMDEE) SSS: Secondary School Shelter (Salem) MVOn: (The new Volcano Observatory in the north on Mongo Hill) Lookout: (A villa in Olveston). MWA: (The Montserrat Water Authority office in Woodlands). MDF: Montserrat defence force (Gerald's ) PS: Police station (Salem) CPS: Catholic primary school in Woodlands.MVO Staff Changes Arrivals
Dr Brian Baptie (BGS)
Graham Ryan (University of Lancaster, UK) - Graham didn't like it in England and returned to Montserrat six days later.
Dr Glen Mattioli (University of Puerto Rico)