The period under review has been relatively quiet, with continuing slow dome growth accompanied by low to moderate seismic activity. Deformation of the volcanic edifice continues at a very slow rate. Steam and gas production continues to be high and vegetation damage on the upper western flanks of the volcano is increasing in extent and severity.
Seismicity
Seismic activity during the week was at the same general level as during the past several weeks, with a low number of volcano-tectonic earthquakes, frequent small long period earthquakes, episodes of low to moderate amplitude broadband tremor recorded on the seismograph station at Gages, and numerous signals associated with rock falls.
Several locatable volcano-tectonic earthquakes were detected by the seismograph network towards the end of the week under review. The largest of these occurred at 07:15 on 17 February (magnitude 3.3) and was felt by the staff at W.H.Bramble Airport control tower. The instrumental location obtained was approximately 4.5 km east of Montserrat at a depth of 13 km. Other locatable volcano-tectonic earthquakes during the review period occurred at 04:45 and 11:27 on 18 February, at 18:47 on 19 February and at 14:45, 15:01 and 15:41 on 21 February. The magnitudes obtained for these earthquakes were in the range 1.2 to 2.2 and the depths were between 2 to 6 km; all were located beneath the volcano.
Episodes of broadband tremor were recorded at increased amplitude levels from approximately 17:30 until 23:00 on 17 February, from 09:00 until 18:00 on 19 February and again from about 11:30 until 21:00 on 20 February. Shorter episodes of this increased amplitude tremor occurred on several occasions during the morning of 21 February. These intermittent periods of increased amplitude tremor are probably due to an increase in steam venting from the Castle Peak dome area, although cloud cover prevented visual confirmation of this.
Long-period earthquakes were observed at a relatively high rate at the beginning of the review period from around 15 to 17 February. The majority of these earthquakes were too small to obtain an instrumental location as they were only recorded on the two nearest seismograph stations to the volcano. Instrumental locations, however, were obtained for three of these earthquakes; they all occurred on 15 February near to the crater area, at depths of 0.5 to 2 km and magnitudes of approximately 1.0. Later in the week, the rate of occurrence of long-period events decreased to a very low level.
Throughout the week, many seismic signals generated by rock falls from the lava dome were recorded on the seismograph stations nearest to the volcano. Some of these signals were visually correlated by scientists from either the helicopter or from around the crater rim. Larger rock fall events were also recorded on more distant seismometers, although none of these produced ash in sufficient volumes to be deposited outside of the crater area.
Four regional earthquakes were recorded by the local seismic network at the beginning of the period under review. The largest of these four earthquakes (magnitude 3.8) occurred on 15 February at 18:42 and had a location 180 km southeast of Montserrat. The remaining three regional earthquakes were all located northeast of the island. Two occurred approximately 10 km northeast, on 17 and 18 February at 16:56 and 08:34 respectively; both had magnitudes of 2.8 and depths of 14 km. The other one was located 40 km northeast of the island on 18 February, at 15:11, and had a magnitude of 3.0 and a depth of 13 km.
Deformation measurements
Low level cloud and generally bad weather for most of the week again severely impeded the measurement of EDM lines. The presence of volcanic mist in the upper Gages valley continues to prevent measurement of the Dagenham - Gages Wall - Amersham triangle. Measurements to the Castle Peak reflector revealed continued very slow shortening on the lines from Long Ground and White's Yard; this shortening is at a rate of about 5 mm per day. Other measurements from the Amersham instrument site to targets on the middle western flank of the volcano revealed movement below the level of resolution by this method since the last occupation in early January.
The Long Ground electronic tiltmeter continues to show no measurable deformation.
Visual observations
Low cloud cover hampered visual observations for much of the week. Rapid growth of a whaleback feature towards the southeast from the southern part of the dome occurred so that the feature was of a similar size to the original southern whaleback by the end of the reporting period. The massive nature of the rock extruded in this feature meant that rock falls were not very frequent. More frequent rock falls occurred from the central and northwestern parts of the dome throughout the week and this was the most actively growing area by the end of the week. Material from this area is filling the moat of English's Crater adjacent to the Gages Wall, which was the last area of the western moat without appreciable debris against the crater wall. A large spine was poorly seen in the central part of the crater on one occasion during the week.
Steam and gas production continues at a relatively high rate from the dome area; most of this volatile release is occurring through previously active phreatic vents, especially the area of the 18 July vent, which was filled with new dome rock during late November. The volcanic haze which has been a constant presence over the upper part of the Fort Ghaut valley for the past few weeks was slightly more dilute this week, although vegetation damage continues from acidic aerosols within the steam plume.
In summary, this week has seen continued dome growth with the consolidation of active areas in the northwestern and southeastern parts of the crater. Seismicity has been at a relatively low rate, with periods of broadband tremor mixed with rock fall signals and scattered long-period events. Deformation continues at a very slow rate all around the volcano. Production of acid aerosols and gas from the degassing magma continues to cause concern in the areas downwind of the crater.