Montserrat Volcano Observatory

Weekly Report
Report for the period 5 p.m. 21 May 1999
to 5 p.m. 28 May 1999

The level of activity at the volcano was generally at a similar level to over the past few weeks, but there was a marked increase in activity on 22 to 23 May.

A total of 138 rockfall signals, 1 long period, and 237 volcano-tectonic earthquakes were recorded over the last week.

There was a distinct increase in activity on 22 May when a volcano-tectonic earthquake swarm was recorded. This was the first earthquake swarm since early December 1998. The swarm started at 6:55 a.m. local time and continued until 5:01 p.m. It contained 141 volcano-tectonic earthquakes with peaks in activity occurring between 7 a.m. and 8 a.m., and between 1 p.m. and 4 p.m. Shortly after 1 p.m. a small ash cloud was produced reaching no more than 7,000 feet.

A large pyroclastic flow occurred early on the morning of 23 May at 2:42 a.m. This travelled mainly down the eastern flanks of the volcano, reaching the sea at the Tar River delta. A small surge deposit was also observed in the upper reaches of the White River valley. No other new pyroclastic flow deposits were observed in other ghauts. An ash cloud to about 19,000 feet was produced, and this drifted westwards, depositing ash to the south of St. George^s Hill.

Two other minor ash venting episodes occurred during the week on 24 and 26 May. Both only produced low level ash clouds which drifted westwards.

Correlation spectrometer measurements have continued this week. Values for the sulphur dioxide flux on 22, 24, 25, 26, 27 and 28 May were 189, 583, 255, 552, 302 and 117 tonnes per day respectively. Higher values tend to occur after ash venting episodes, but these higher fluxes are approximately at the average level for the whole eruption.

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.

Montserrat Volcano Observatory