This period was characterised by continued dome growth within the Easter collapse scar until a second collapse occurred on April 11 shedding considerable pyroclastic flows down the Galway's Soufriere and White River region.
Visual observations during the first part of this period were aided by extremely clear weather and cloudless skies. The Easter collapse scar was observed on a daily basis. On Sunday coarse rockfall debris had completely filled the chute which had been carved by the Easter collapse pyroclastic flows. The chute had been considerably widened and resembled a talus slope about 100 m wide, but dipping at 50(. Regular rockfall activity was observed some of which were started by rock-bursts from the active face.
A GPS-laser range finding binocular survey of the dome on the 6th showed that the steep headwall representing the active face of the new lobe had become higher but had decreased in angle since the 3rd. On the 3rd, it was about 90 m high, 120 m wide and at 80( to the horizontal. By the 6th it was 100 m high, 150 m wide and at 60(, however, the summit altitude had decreased from 968 m to 950 m. The headwall had also advanced by about 20 m southwards toward the top of the chute. In comparison, the northern peak of the January dome did not show any movement and remained at about 965 m a.s.l. The top of the lobe maintained its smooth, slabby, arcuate surface but appeared more fractured. Tension fractures in the side of the lobe suggested that lower portions were extruding faster than the upper regions. The head wall area also showed intense internal fracturing.
The southwards and downwards movement of the lobe created a horseshoe shaped scarp around it. The scarp is up to 15 m deep and joins the Easter collapse scar to the northern peak. Vigorous degassing and milky ash-laden steam jets have been observed in the centre of the scarp between the two peaks.
The pyroclastic flows from the collapse on April 11th travelled almost 500 metres further than any previous flows down the White River valley. There is now only 500 metres of the valley remaining before the flows reach the sea at O'Garra's. The deposits in the valley are considerably thicker and the Great Alp Falls, originally some 35 metres high, have been completely buried. Two distinct flow paths have developed in the area around the Galway's Soufriere; a main channel in the White River valley itself and a path over flat ground directly west of the main valley, over the road leading to the soufriere and then back into the main valley at a point half way along the White River valley. A considerable area in the upper half of the valley has been affected by surge clouds. The topography in this area is gradually being covered by ponding deposits.
The level of seismic activity was low until the dome collapse on 11 April. For most of the week, only small rockfall signals and occasional earthquakes were recorded. The numbers of long-period and hybrid earthquakes were low, and there was a slight increase in the level of volcano-tectonic earthquake activity. VT earthquakes have been uncommon since the start of the recent dome growth, with most VTs occurring in rare swarms. The number of VT earthquakes was quite low, without any swarms occurring.
On April 11th, the level of rockfall activity increased from about 9:30 am. The collapse began with sustained low-amplitude signals on all stations. The first pulse of high-intensity activity was recorded at 11:07 am, and this was followed by a longer pulse at 11:55 am which lasted for about 15 minutes. At the end of the second pulse, a short, high-amplitude signal was recorded at the stations at St Patrick's and Galway's Estate. This is interpreted as a pyroclastic flow travelling down the lower reaches of the White River valley. The pyroclastic flow activity ceased abruptly following this episode.
Fifteen hours after the end of the pyroclastic flow activity on April 11th, there was a hybrid earthquake swarm. This swarm was short-lived and less intense than recent swarms.
Table 1: Earthquake types
These earthquake counts are of events that triggered the broadband seismic network event recording system between 0000 and 2400 each day (local time).
Date VT LP Hybrid Dome RF LPRF* 06 April 96 0 1 0 10 0 07 April 96 4 13 1 40 12 08 April 96 6 10 1 31 7 09 April 96 0 7 1 38 4 10 April 96 4 5 0 36 1 11 April 96 1 15 0 58 13 12 April 96 2 2 46 37 1
*LPRF: LP earthquake followed by rockfall signal. The LPs and rockfalls in an LPRF signal are also counted in their respective columns.
EDM measurements were made on the lines of the western triangle between Lower Amersham and the target sites at Upper Amersham and Chance's Peak Steps. The very slow shortening of the slant distances between the instrument site and the two target sites is continuing. There is also an indication of very slow subsidence of the two target sites relative to the instrument site at Lower Amersham. Further occupations will be made at regular intervals to keep a check on these changes. A long occupation GPS occupation of the sites at Harris Lookout (M18) and the site on the crater rim above Farrell's Yard (FT3) was performed on April 6th. The recent outward movement of the FT3 site away from the dome appears to have stopped. An occupation of BIGNET (Harris Lookout - Long Ground - Dagenham - Brodericks) on April 9th indicated that there have been no significant changes in the positions of the sites involved.
Dome Volume Measurements
The excellent visibility allowed a good GPS/laser range-finding binocular survey of the dome to be carried out on April 6th. The increase in volume in the Easter scar area since the April 3rd was calculated to be about 0.5 x 106 m3. This implies a growth rate of 2 m3s-1. On the 6th, the volume of the lobe extruding into the scar area over Galway's Wall was about 0.9 M m3. Both GPS points and photographic profiles suggest that growth of the dome is being limited to the area within the Easter scar. There has been no detectable change in the northern peak of the January dome or in the eastern sectors of the whole dome complex over the reporting period.
Measurements of sulphur dioxide flux were made using the MiniCOSPEC on April 7th (223 t/d) and April 11th (1524 t/d). This latter value was measured immediately after the collapse over the Galway's Wall and is one of the highest values ever measured at the Soufriere Hills Volcano. At least half of this value appeared to originate from the new deposits in the White River. Several COSPEC experiments were run in the early part of this week as the wind direction was not conducive to measurements along the normal traverses. These experiments included measurements from the helicopter and static measurements from St. Patricks and St. George's Hill. These techniques will explored in some detail so that sulphur dioxide flux can be estimated when the wind direction is not favourable.
Results from the sulphur dioxide diffusion tubes collected on 23 March were received this week and are shown in Table 2. They show a return to the levels of the past few months which are not above the recommended standards for occupation or habitation in these areas. The Whites Landfill site on the north-eastern side of the volcano has no detectable sulphur dioxide.
Table 2. Sulphur dioxide diffusion tube results for the period between 9 March and 23 March. Concentrations are in ppb.
Location SO2/ppb Upper Amersham 45.10 Lower Amersham 17.70 White Landfill 0.00 Police HQ, Plymouth 8.05 Weekes 0.00 Control 0.00
Rain water samples were collected at 4 locations on April 6th and April 12th. The results April 12th are shown in Table 3 together with results from 31 March. A sample of run-off water was also collected by the road to Upper Amersham on April 6th. The pH of the samples could not be measured since the electrode of the pH meter is broken. However, the samples will be stored, and the pH will be measured once the pH meter is operational.
Ash was collected at several sites to the west of the volcano after the pyroclastic flows on April 12th. These samples will be weighed and an estimate of mass of co-ignimbrite ash erupted will be calculated.
Dust sampling in Plymouth and its surroundings continues with pumps at Cork Hill School, the port, Kinsale checkpoint, Monlec and various other sites. The samples have been sent to the UK for analysis.
Table 3. Rain water geochemistry, March 31st and Apr 12th 1997.
Units: conductivity (mS/cm), total dissolved solids (g/l), sulphate (mg/l), chloride (mg/l), fluoride (mg/l), nd (not detected)
31 March 97 Location pH Cond. TDS Slpht Chlrd Flrd Upper Amersham - 1.566 0.784 39 16.2 1.5 Lower Amersham - 1.394 0.698 50 158 1.4 Police HQ - 1.553 0.727 20 16.6 >1.5 Weekes - 0.058 0.028 nd 10.3 0.2
12 April 97 Location pH Cond. TDS Slpht Chlrd Flrd Upper Amersham - 0.235 0.117 20 135.5 1.4 Lower Amersham - 0.300 0.150 32 64.0 >1.5 Police HQ - 0.243 0.121 - 28.5 >1.5 Weekes - 0.112 0.055 3 20.0 0.55
Professor Steve Sparks (Bristol University, UK)
Dr Richard Herd (BGS) is away for a short break.
Dr. Simon Young (BGS)
Dr. Peter Francis (Open University, UK)
Dr. Jurgen Neuberg (Leeds University, UK)
Mr Chandradath Ramsingh (SRU, Trinidad)
Dr Willy Aspinall (BGS)