Montserrat Volcano Observatory

Weekly Report
Report for the period 5 p.m. 18 June 1999
to 5 p.m. 25 June 1999

The level of activity at the volcano was again low this week, although there were two small collapse events producing pyroclastic flows on the northeastern flank of the volcano.

The seismic networks recorded a total of 71 rockfall signals, 9 volcano-tectonic earthquakes and 3 long-period earthquakes over the week.

Two small collapse events occurred from the dome during the later part of the week. The first, at 11:32 pm on Wednesday night (23 June), produced pyroclastic flows in the Tar River valley which travelled less than a mile. An ash cloud was generated but did not reach more than 10,000 ft in altitude, and a trace of ash was deposited to the west of the volcano. The second event, at 6:46 am this morning (25 June) was slightly more vigorous, with flows reaching more than half a mile down Tuitt's Ghaut and also spreading across the Farrell's Plain into Tyer's Ghaut. The ash cloud reached about 10,000 ft, and light ash fall occurred to the west of the volcano. Both of these collapse events sourced from an area on the northeast flank of the dome close to the 5 June collapse scar.

Observations of the dome during the week suggest that further small collapses are likely from the northeast flank, and one particular area is extremely unstable and could generate a significant pyroclastic flow and ash cloud without warning.

Correlation spectrometer measurements have continued throughout the week. Values have stabilised in the 200 to 300 tonnes per day range, which now seems to be established as the background sulphur dioxide flux. Values were slightly higher following the collapse event on 23 June.

GPS measurements of volcano deformation have continued during the week; MVO now operates six continuous sites. After slightly raised rates of deformation between March and May, the rates have decreased to normal levels again.

Residents of Montserrat are advised to keep listening to ZJB Radio Montserrat for information in case of any changes in the state of the volcano. Dr Simon Young took over from Dr Gill Norton as Director of MVO on Monday 21 June.

Montserrat Volcano Observatory