Seismicity
Seismic activity over the last week has been low, continuing the recent trend. The seismicity is dominated by a variety of long-period events that are probably mostly caused by surface processes on the active dome. There is no strong evidence of deeper long-period earthquakes accompanying upward magma movement. The number of volcano-tectonic earthquakes continues to be small.
Up to 40 large long-period events were recorded per day, but very few of these can be located with any confidence. A variety of types were observed, with variations in frequency content with time and different types of onset. Good visual observations from the crater rim on March 11 allowed several large-amplitude signals to be correlated with rockfalls from the southwest of the dome. The long-period events are probably due to a variety of processes that are poorly understood at present. Many smaller long-period earthquakes are recorded at the closest station, on Chances Peak.
Only four locatable volcano-tectonic earthquakes were recorded by the seismograph network during the week under review. All of these were located to the northeast of the crater, at depths of 2 to 4 km.
A single regional earthquake was recorded by the MVO seismic network during the week, occurring at 08:40 on 7 March. The event was magnitude 4.8 and was located c. 120 km north of the island; it was felt in the US Virgin Islands.
Deformation measurements
Almost all of the EDM lines have been measured over the last week, despite generally poor visibility. The results for the eastern and southern triangles are consistent with slow movements of Castle Peak and Chances Peak of about 1 mm/day and 0.5 mm/day respectively, continuing the trends established in late November. Re-measurement of the Amersham to Chances Steps line on March 11 showed a surprising 3 cm lengthening since the line was last measured on February 19. This movement is a reversal of the shortening that occurred from October to late December on this line. An attempt to measure the Gages Wall target on March 11 was not successful.
The electronic tiltmeter at Long Ground did not show any deformation events during the period under review.
Visual observations
A combination of helicopter and static viewing of the crater area enabled reasonable visual observations to be made during the week. Early in the week, rock falls were most common from the central and northwestern parts of the dome, depositing material into the moat between the Chances Peak and Farrell's lookouts. Late in the week, a change in focus of dome growth occurred, and activity in the form of rock falls and glowing material seen at night switched to the eastern and southwestern parts of the dome. A spine appeared in the southwestern part of the dome which showed red hot material within cracks and around its base.
A rough contour map of the new dome was constructed using basic surveying techniques from the Farrell's lookout during the week and a digital terrain model made. The comparison of this new DTM with one made prior to eruptive activity at the volcano revealed that the new lava dome has a volume of c. 6.7 x106 m3. This compares well with cruder estimates made in the field, and suggests that the growth rate of the dome has increased considerably over the past 2 to 3 months from 0.2 to 0.3 m3/sec in late November and December 1995 to something closer to 2 m3/sec at present. The highest point on the new dome was measured from the observatory at 875 m above sea level on 12 March, an increase of c. 30 m since the previous measurement on 3 March. Despite this vertical growth, all rock falls from the dome remain confined to the crater area.
Larger rock falls during the week produced several small ash clouds which deposited fine ash on the upper western flanks of the volcano. Steam production remained high, mainly from an area on the east side of the new dome, whilst SO2 and other volcanic gases were being produced from all over the dome, also at apparently high levels.
Gas sampling
Collection of gas and particle samples from a number of sites around the volcano was initiated on 11 March and will continue for 6 days. Results from SO2 diffusion tubes placed around Plymouth and on the higher western flanks of the volcano in late February show that SO2 levels in inhabited areas remain well within accepted health standards. The weekly gas sample from the Galway's Soufriere was collected as normal and sent to the French observatory on Guadeloupe for analysis.
Radiocarbon dating
A number of dates from samples submitted for radiocarbon dating were received during the week. Initial indications are that there may be more than one phase of pyroclastic flow generation during the growth of the Castle Peak Dome between c. 450 and 250 years ago. A widespread ash deposit on the southeast coast proved not to be related to this most recent phase of explosive activity at Soufriere Hills, and there is no evidence to suggest pyroclastic flows towards the west during this last dome growth episode.
Summary
The week has seen continued lava dome growth in the summit crater of the Soufriere Hills volcano at a low level of seismicity and accompanied by very slow deformation of the volcanic edifice. Dome growth rate has probably been as high during this period as at any other time in the past, leading to vertical growth of c. 100 feet in a ten day period. Dome volume is estimated to be c.6.7 x106 m3 and volume eruption rate probably c. 2 m3/sec.