Activity in this reporting period was at a low level. This seems to have been a period of relatively slow dome growth, with moderate rockfall activity being recorded and low levels of other seismic events. No material came over the Gages Wall.
Visibility was generally poor during the week. The summit was obscured by clouds from June 20 until June 23 and it was not possible to make any observations of the dome or of the upper parts of the ghauts. Good views of the lower flanks of the dome were possible from the helicopter on June 24 and 25. There were no major new deposits on the lower flanks, although some new material was noted to the south of Castle Peak in the upper reaches of the south fork of the Tar River. Little new material had fallen into the upper part of Fort Ghaut, although the full width of the Gages Wall was filled with debris from the dome. There were two short periods on June 25 when the entire dome was visible from the ground. There were no new spines observed and it was not possible to determine where the active areas of growth were. On June 26, small rockfalls on the northeast flank of the dome were seen from the Hermitage and Whites areas. The dome was visible for only a short time. It appeared wet and there was heavy steaming on the southwest flank. There was a small stubby spine to the northeast of the summit.
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. The table below lists the daily counts of the different types of seismic activity.
Earthquake types 20 June - 26 June 1996
Date VT LP Hybrid Rockfalls Tremor Amount 20/06 0 3 7 56 Low to intermediate 21/06 0 4 3 52 Low to intermediate 22/06 0 5 1 55 Low to intermediate 23/06 0 6 4 66 Low 24/06 1 16 3 45 Low to intermediate 25/06 0 6 1 28 Low 26/06 1 9 6 70 Low
Rockfall activity remained at a relatively constant rate throughout the week. Between the 20th and the 25th, the signals had small amplitude and short duration. None of these generated any visible ash clouds, although visibility was poor. The rockfall signals on the 26th were generally of larger amplitude and there were two moderate-sized rockfalls which generated small but visible ash clouds, at 11:32 and 12:48. Some of the rockfall activity on June 26th had larger signals than the rest of the week.On 26th they seemed larger than earlier in the week.
Both hybrid and small long-period events were recorded throughout the week in relatively small numbers.
The level of broadband seismic tremor recorded at stations close to the dome was generally low during the week. It was generally intermittent and of variable amplitude. There were a few spells of more continuous low-amplitude tremor. The most notable was from 21:20 on 20 June until 04:00 on 21 June.
No EDM measurements were possible on the lines to Castle Peak, probably because of ash deposits on the reflector. EDM measurements to other reflectors were hampered by the low cloud level and the reflectors could only be seen on June 25 and 26. The lines in the Windy Hill - St Georges Hill - Farrell's triangle were measured on both days. When the data was first analysed, these lines seemed to show large changes. The changes were subsequently determined to have been caused by a technical problem. There had been no movement on the lines greater than the uncertainties in the technique.
GPS lines in the Reid's Hill - St George's Hill - Radio Antilles triangle show no changes since last measured 27 days previously. Site preparation for the first of two permanent GPS stations, at Hermitage Estate, was started this week. These will be operated by the University of Puerto Rico.
The dry tilt levelling line at Amersham was reoccupied on 21 June, for the first time since 4 January. The results show that no measureable deformation has occurred in that time.
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 during 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 are given in the table below, as the mean and standard deviation of all the runs made each day. The results are consistent with measurements made since the COSPEC programme restarted on 28 April 1996, with a low SO2 flux.
COSPEC measurements of SO2 flux
Date Number Mean SO2 flux Standard of runs (tonnes / day) deviation 20/06 5 223 77 24/06 5 76 22
Joe Devine, Brown University
Alan Smith, University of Puerto Rico
Glenn Mattioli, University of Puerto Rico
Lutchman Pollard, Seismic Research Unit, Trinidad