Dome growth continued during this period, with consequent rockfall activity concentrated on the southern flank of the dome, with material being deposited mainly in the southern fork of the Tar River. Seismicity was at a relatively low level, with rockfall signals dominating the records.
Visibility was very poor from 27 to 29 June. Visibility was good on 30 June, allowing a number of observations of the summit crater and dome. Rockfall activity was rare, but dome growth clearly was continuing in a number of areas, most noticeably on the southern flank of the dome because considerable new material had been deposited in the upper part of the south fork of the Tar River. There was still almost no new material over Gages Wall indicating that dome growth had almost ceased in the northern parts of the dome. Steam production was concentrated in the western moat.
Visibility was poor for the rest of the week and almost no observations were possible. Parts of the dome were visible at times on 01 July when some rockfall activity was seen on the southern flank of the dome.
The larger rockfalls throughout the week produced small ash clouds which were generally blown westwards by the prevailing winds; any ashfall to ground level was very light.
Seismicity during the week was dominated by small- to moderate-sized rockfalls from the lava dome. Long-period (LP) and hybrid events continued to occur at low levels. Broadband seismic tremor was recorded at stations near to the crater area, but at low levels. Volcano-tectonic earthquakes were mainly located at depths of 1 to 2 km beneath the crater, although two were located at greater depths offshore of Montserrat.
Table 1 lists the daily counts of the different types of seismic activity. These counts are taken from the triggered digital records for the daily period between 00:00 and 00:00, and are slightly different from the counts given in daily reports.
Table 1: Earthquake types, 27 June - 03 July 1996
Date VT LP Hybrid RF Tremor Amount 27/06 1 15 11 68 Low 28/06 3 4 6 58 Low to intermediate 29/06 1 5 0 41 Low 30/06 2 7 3 49 Low 01/07 1 9 12 61 Low to intermediate 02/07 2 9 26 64 Low 03/07 0 1 21 35 Low
The Castle Peak EDM reflector was cleaned and re-attached on 29 June. It had become obscured by ash deposits and appeared to have rotated on it's mount. A measurement taken on the same day showed that the lines had lengthened by 7 mm since it was last measured 10 days previously. Measurements on 30 June showed no movement outside the uncertainties in the technique. No other measurements were taken due to poor visibility. The Amersham-Dagenham-Chances Peak EDM lines were measured on 29 June and 1 July. These showed only small changes in line lengths. The northern triangle was measured on 27 June and showed no significant change from previous measurements.
The MVO GPS survey was curtailed again due to a recurring equipment fault. Occupation of a number of sites by the University of Puerto Rico GPS team continued throughout the week. Initial processing of data suggests that no significant changes in line lengths across the volcano have occurred over the past few weeks. Construction work was started on the continuous GPS monitoring site at Reids Hill, on the southwestern flank of the volcano.
The electronic tiltmeter at Long Ground did not record any tilt events during this week, continuing the trend of the past 10 months.
SO2 flux measurements were made on two days in the week using a COSPEC correlation spectrometer which was mounted in a car and driven between Cork Hill and St. Patrick's under the gas plume. The results of the measurements for the week are given in the table below, as the mean and standard deviation of all the runs made each day. The results show some variability but are consistently low, with a mean value of 141 tonnes per day.
Table 2: COSPEC measurements of SO2 flux
Date Number Mean SO2 flux Standard of runs (tonnes / day) deviation 28/06 5 142 32 30/06 28 76 37
The large number of COSPEC runs carried out on 30 June formed an experiment to determine if there was any systematic variation in the amount of SO2 flux during the day, which may have accounted for some of the variability seen in our measurements. 28 runs were carried out between 06:45 and 16:45. It was unfortunate that wind speeds were low, the wind direction was variable during the experiment and the flux was close to the lower detection limit of the technique. The final results suggest that there are no strong systematic variations, although another experiment is required to confirm this.
Micro-gravity surveys were initiated during the week. Micro-gravity techniques for tracking changes in the status of any magma body have been developed over the past few years and Montserrat will serve as a useful test-bed for the technique, although no results are expected until there is a good baseline of data.
Results from SO2 tube analyses over the past few weeks show that the level of SO2 at ground level even at the closest station to the volcano on the leeward side (Upper Amersham) remains below WHO health limits for average exposure. The results also suggest major control on ground-level SO2 by atmospheric conditions, with results on successive periods (11 to 24 June and 25 June to 3 July) showing the highest and lowest SO2 levels yet recorded by the SO2 tubes, which have been in place intermittently since December 1995. This major fluctuation is not matched by the total flux measurements made by the COSPEC (see above).
Analysis of rainwater collected beneath the main gas plume at Upper Amersham shows continued high acidity (pH between 2.5 and 3) and elevated levels of dissolved sulphates, chlorides and fluorides, although none of the latter are above WHO limits.
Jane Toothill, Lancaster University, UK
Glenn Thomson, Leeds University, UK
Richard Luckett, Leeds University, UK
Jean-Pierre Viode, Martinique Volcano Observatory
Neil Dyer, Lancaster University, UK
Glenn Mattioli, University of Puerto Rico
Alan Smith, University of Puerto Rico
Paul Cole, University of Luton, UK