Montserrat Volcano Observatory, Montserrat, West Indies

Scientific Report 11
14 February 1996

The focus of dome growth shifted during this week to the western, central and south-eastern parts of the new lava dome within English's Crater. Poor weather affected EDM measurements and visual observations but some measurements were possible and good views obtained. Magmatic gas emissions continued at a relatively high level. Seismicity has been at a generally low level through most of the week, although a swarm of volcano-tectonic events beneath Windy Hill was the first swarm of this type for some time.


Seismic activity during the week was dominated by a swarm of volcano-tectonic earthquakes which occurred on 11 February beneath the Windy Hill area, at depths of between 2 and 5 km. Forty of these earthquakes were recorded with magnitudes of 1.3 or greater and several were felt across central parts of the island. The largest recorded earthquake of the swarm had a magnitude of 3.2 and a further five had magnitudes of between 2.5 and 3.0; all were reported felt. Many smaller volcano-tectonic earthquakes occurred as part of the swarm activity but were too small to constrain an instrumental location. This swarm activity ceased as quickly as it began apart from two further small volcano-earthquakes detected on 11 February (19:25) and on 12 February (04:52).

Further hybrid earthquakes (dominant frequencies of 3-5 Hz) were observed on 11 and 12 February, with magnitudes less than 1.0, on seismic stations nearest to the volcano. These hybrid earthquakes occurred at a rate of about one per hour. This is the first occurrence of these hybrid events since their disappearance on the 6 February. On 13 February, at around 03:00, the rate of these hybrid earthquakes accelerated to approximately one per minute. No instrumental locations were possible due to their small size. Larger hybrid earthquakes (about magnitude 1.0) were observed on 14 February on the nearest stations to the volcano. These occurred at a rate of about one every few hours but were still too small to give an instrumental location.

Intermittent episodes of low to moderate broadband tremor were seen on the seismographs from the Gages station during the past week between 8 and 14 February. On 13 February the amplitude of this broadband tremor increased, at approximately 18:30 until 19:30, then reverted back to its low to moderate level thereafter.

Several long period earthquakes occurred on 8, 9 and 10 February but because of their small size (less than magnitude 1.0) no instrumental locations were obtained. These earthquakes were only recorded on the two seismic stations nearest to the volcano. Seismic signals associated with rockfalls and small eruptions were also recorded on the stations nearest to the volcano throughout the week. Slightly larger signals of this type were observed on 13 and 14 February, at 22:56 and 10:32, respectively.

Three regional earthquakes were recorded by the local seismic network during the past week. The first occurred on the 8 February at 12:02, with a magnitude of 2.0, and had a location approximately 110 km from the island. On 9 February, at 05:27, a further earthquake occurred about 60 km southeast of Montserrat with an approximate magnitude of 3.3. The final regional earthquake to occur during this period was at 15:41 on the 12 February (magnitude 2.7); this also had a location about 60 km from the island.

A new seismic station (MCPT) was installed near to the crater rim at Chances Peak and was fully operational by 8 February. Hopefully this station will improve instrumental earthquake locations as well as providing a back-up for the vital Gages station should the latter become damaged in any way.

Deformation measurements

Low level cloud and generally bad weather for most of the week severely impeded the measurement of EDM lines. The presence of volcanic mist in the upper Gages valley prevented measurement of the Dagenham-Gages-Amersham triangle. All other EDM triangles were measured at least once during the week with the Long Ground - White's Yard - Castle Peak triangle being measured three times. Daily changes on most lines continue to be less than 5 mm, although there is a continuation of a long term slight shortening on the eastern triangle to Castle Peak. On 13 February, the O'Garra's to Chances Peak segment of the south-western triangle showed a shortening of 1.4 cm since it was last occupied on 1 February.

The Long Ground electronic tiltmeter continues to show no measurable deformation.

Visual observations

Low cloud cover also hampered visual observations for most of the week. Brief views obtained from Chances Peak on 9 February indicated that activity had slowed down considerably on the northern and southern parts of the dome. Visual observations from Farrells and from the helicopter on the 10 February confirmed that the focus of dome growth had shifted to the western part of the dome. Several rockfalls were seen and heard to occur from this area. Brief views were obtained of a prominent spine which had been extruded in the western part of the dome during the previous three days. Observations made on 11 February from the helicopter and several areas around the volcano revealed that the spine located in the top central parts of the dome had attained a height equal to the northern areas of dome growth. Talus deposits from numerous rockfalls into the moat were seen to be in contact with the crater wall on all sides of the dome. A late morning flight on 12 February allowed observation of a small pyroclastic flow as debris from the central parts of the dome avalanched to the south. Clear views of south-eastern part of the dome towards the end of the week being reported indicated that growth was also occurring in this area. Several small rockfalls were observed from this area, and two new protrusions were noted in the west central part of the dome; these new features having grown during the previous two days.

Vigorous venting of magmatic gases from several parts of the dome, coupled with favourable weather conditions, channeled most of the emissions into the Gages gap for much of the week. The light blue haze which had been visible in the Gages valley since 4 February lifted briefly during the latter part of the week and no longer extended beyond the upper portion of the valley. However, a visit to Chances Peak on 13 February indicated that the levels of acidic aerosols were still relatively high at the top of Chances Peak. Initial results from analysis of rain water samples collected between 1 and 8 February in the Gages valley indicated that the concentration of sulphates (< 3 mg/l), fluorides (1.5 mg/l), chlorides (106 mg/l) and the pH level (2.57) were below levels which could have a deleterious effect on human health, although all of these levels have increased since mid-December when testing first took place.

Results from geochemical analysis of new dome rock as well as several ash falls were obtained during this period. The new dome rock, collected in early January, proved to be very similar in composition to the old Castle Peak Dome and is a hornblende, two pyroxene, crystal-rich andesite. Bulk analysis of ash samples from small explosive eruptions on 13 and 21 December are of dacitic composition and appear to be phreatomagmatic in origin, with accretionary lapilli and a definite juvenile component. The ash samples appear to be representative of whole-rock composition, although further work is required to confirm this.

In summary, this week began with a slowing down of dome growth on the northern and southern parts of the dome; the foci of growth shifted to the top central, western and south-eastern parts of the dome later in the week. Seismicity was relatively low throughout the period, although a notable volcano-tectonic earthquake swarm occurred beneath Windy Hill. Deformation of the volcanic edifice continues to be at a very slow rate. Acidic aerosol levels within the Gages valley continue to be relatively high, although well below levels of concern for human habitation.

Montserrat Volcano Observatory