The alert level was reduced from orange to amber on April 22nd following a reduction in activity from the area of the dome above the Galway's Wall. Activity has been at a low level for the reporting period with rockfalls and small pyroclastic flows from the active area of the dome dominating the seismic records. Long-period earthquake activity has continued at an elevated level. Ground deformation continues to be very slow and confined to areas close to English's Crater.
Visual observations were hampered for much of the week by poor visibility, however some observations of the dome were made. The southern lobe continued to grow and had started to spill over the eastern side of the Easter collapse scar by the end of the reporting period.
Rockfalls and long-period earthquakes were the dominant forms seismicity during the week. The number of VT and hybrid earthquakes was very low, and the individual events recorded were of low amplitude. Periods of low-level VT/hybrid seismicity have in the past been characterised by free dome growth; the last such period was during growth of the December 11 dome in 1996. Rockfall activity was at a high level during 23-26 April, the highest level, in terms of number of rockfalls, recorded since the beginning of the year. However, the largest signals were associated with only small pyroclastic flows over the Galway's Wall, and there were no large pyroclastic flows. It is possible that there is better flow-to-ground coupling for small pyroclastic flows to the south, compared with the small eastern flows which run over unconsolidated talus material, which would mean that flows to the south would give stronger seismic signals, and trigger the event recording system more readily. There were very few detected rockfalls on the eastern side of the dome.
The number of long-period earthquakes remained high, especially during the period of enhanced rockfall activity. The pattern of long-period earthquakes triggering rockfalls continued, with 40 to100% of LPs triggering rockfalls on a daily basis.
Table 1: Earthquake types
These earthquake counts are of events that triggered the broadband network's event recording system between 0000 and 2400 each day (local time).
Date VT Hybrid LP Dome RF LPRF* 20 April 96 2 0 7 30 0 21 April 96 2 1 9 35 0 22 April 96 4 1 25 115 8 23 April 96 0 0 17 163 17 24 April 96 0 0 21 138 14 25 April 96 0 0 25 86 10 26 April 96 2 2 19 78 15*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 northern triangle (Windy Hill - St. George's Hill - Farrells) in co-ordination with a long occupation GPS survey of the some of the same points. The EDM occupation again showed fluctuations in the slope lengths from both Windy Hill and St. George's Hill to the Farrell's target site. Both techniques are in agreement that there is very slow movement of the site at Farrell's away from the dome.
A GPS occupation was carried out on NORTHNET (M18 Harris Lookout - Dagenham - Lookout Yard - M27 Old Towne). This network extends to the northern tip of Montserrat and is periodically occupied to check for widespread deformation associated with the volcanic activity. No significant change in the position of any of the sites was detected.
Dome Volume Measurements
A new DEM of the White River area has enabled GPS/laser binocular data obtained earlier to be used in deposit volume estimates for this area. The results show that the rockfall and pyroclastic flow activity has deposited 2.3 x 106 m3 of material on the 2nd of April and a further 2.1 x 106 m3 by the 15th April. These figures are given as Dense Rock Equivalents, assuming a deposit density of 0.8 x dense rock. These values have helped to constrain the magma extrusion rate which appears to have increased to 4.6 m3 per second after the collapse on the 11th. This value is subject to considerable error, but does confirm field observations suggesting an increase in extrusion rate.
The MiniCOSPEC machine is currently in Arizona with Billy Darroux of the MVO, so no COSPEC measurements were made this week.
Results from the sulphur dioxide diffusion tubes collected on 6 April were received this week and are shown in Table 2. They show the same levels as over the past few months which are not above the recommended standards for occupation of these areas.
Table 2. Sulphur dioxide diffusion tube results for the period between 23 March and 6 April. Concentrations are in ppb.
Location SO2/ppb Upper Amersham 31.05 Lower Amersham 11.90 Whites Landfill 1.40 Police HQ, Plymouth 5.10 Weekes 0.00 Control 0.00
On 20 April, ash samples were collected from 12 locations to the north and west of the volcano. These samples will be weighed and selected samples will be sent back to the UK for analysis. No water samples could be collected since there was very little rainfall during the week.
Dr Anne-Marie Lejeune
Dr Peter Francis (Open University, UK)
Dr Jurgen Neuberg (Leeds University, UK)
Mr Desmond Seupersad (SRU, Trinidad)