Montserrat Volcano Observatory, Montserrat, West Indies

Scientific Report 10
7 February 1996

There was some variation in the rate of dome growth and the frequency and type of seismic events at the Soufriere Hills volcano over the past week. Unfavourable weather conditions adversely affected visual observations and EDM measurements and enhanced the formation of light volcanic fog in the Gages valley.

There was a further extension to the series of hybrid (dominant frequencies of 3 to 5 Hz) earthquakes that had commenced on the evening of 23 January. These earthquakes were occurring at a rate of 5 or 6 per minute and due to their very small size (vast majority about magnitude 0.5) could only be seen on the nearest stations to the volcano (Gages and Hermitage). Because of the poor in quality of phase arrivals no instrumental locations could be constrained for these earthquakes. On 2 February the amplitudes of these earthquakes began to slightly decrease and could only be seen on the seismometer station at Gages. Magnitudes increased again on 3 February, with the vast majority of events having a magnitude of about 0.5. This pattern remained constant until 6 February when amplitudes again began to decrease to a size ( about magnitude 0.0) where they could barely be seen on the Gages seismometer. By the early evening of 6 February they had completely disappeared.

Long-period earthquakes had been occurring at a rate of about 10 per day at the start of the week and started to occur more often around the 4 February , peaking at 34 on 5 February. Most of these earthquakes were located beneath the crater area at depths of 0.1-2.0 km, and with magnitudes between 1.3 to 2.0. Two larger long-period earthquakes were recorded well throughout the seismic network on 1 February at 21:14 and 2 February at 15:26. Both had magnitude s between 2.0 and 2.5. On 6 February the long-period earthquakes started to become less frequent and began to diminish in size. By the early evening they were only occurring very occasionally and were very small (less than magnitude 1.0).

One small solitary volcano-tectonic earthquake was located beneath St Georges Hill on 5 February at a depth of about 3.0 km and had an approximate magnitude of 1.5. There was very little broad band tremor (dominant frequencies 2 to 10 Hz) during the week. However, intermitant episodes of low amplitude tremor occurred between 18:00 and 22:00 on 2 February. An episode of low to moderate tremor was recorded on the Gages seismometer between 14:00 on 3 February and 08:00 on 4 February. Seismic signatures that are thought to be from rockfalls and small eruptions were recorded through the week with about 10 per day being observed on the nearest stations to the crater. These signals became more frequent by 6 February with around 45 being recorded on that day. Many more smaller signals of these type are seen frequently only on the Gages seismic station. Some of these signals correlated with light ashfalls observed by scientists either in the helicopter or on the crater rim. A seismic signal of moderate amplitude was observed only on the Gages seismic station at 01:43 on 7 February, and lasted for about 10 minutes; this was probably caused by a small mudflow down the Gages Ghaut.

The period of seismic inactivity that commenced on the evening of 6 February and continued until the end of the week under review, is the longest since mid-January.

Low level cloud and generally bad weather throughout most of the week severely affected measurement of EDM lines. The Dagenham-Gages-Amersham triangle could not be measured despite several attempts. At least one measurement was made at all the remaining EDM sites with the Long Ground-White's Yard-Castle Peak triangles being measured several times. Daily changes on all lines continue to be less than 5mm, well within the error of the instrument. The most significant result obtained during the week was a shortening of 1.5cm on the Tar River-Castle Peak EDM line. This line was previously occupied on the 21 January and showed a 2.5cm change from its last occupation on 23 December 1995. The very slight increase in rate of shortening on this line may be related to the apparent increase in dome growth rates during the week.

Visual observation of English's Crater on 1 February indicated that activity continued to be concentrated in two main areas; the northern and southern parts of the dome. Continued vertical growth of the northern parts allowed the top of this area to become visible from the western side of the volcano beginning on the 31 January. Clear conditions on 2 February allowed good views of the crater and indicated that the northern part of the dome had become as high or higher than Castle Peak. The southern area continued to grow horizontally and steam emission was greatest from the top central part of the dome. Observations during the remainder of the week were hampered by cloud cover which obscured the active areas. Brief views of the northern and southern parts of the active area on the 4 February revealed that dome growth was continuing but at a slower rate the earlier in the week. Two late afternoon helicopter flights on the 6 February indicated that growth on the southern part had stopped. Growth of the northern part had further declined but was sufficient to bring its top to the altitude of the crater wall at Farrells. The talus deposits from the north part of the dome have filled the moat area and begun to extend up the crater wall. Incandescent material was visible during the later flight.

Light, very fine-grained ash continued to be deposited at the top of Chances Peak and on the upper western flank of the volcano from convective clouds produced by large rockfalls. A large rock avalanche from the southern part of the dome was observed from Chances Peak on 1 February. The avalanche produced a small convective cloud which deposited fine-grained ash on the top of Chances Peak. Favourable atmosphere conditions during the latter part of the week resulted in higher than usual concentrations of acid aerosols in the Gages valley. From about 4 February a persistent light blue haze was evident in the Gages valley. Vegetation damage (brown acid burns), due to the presence of this volcanic fog, extended as far as Plymouth and Richmond Hill (5km downwind). Some minor irritation of eyes and nasal passages were reported by people living in the affected area during the latter part of the week.

In summary, this week began with a continuation of rapid dome growth within English's Crater, accompanied by small, repetitive hybrid events. Higher than normal levels of acidic aerosols in the Gages valley caused damage to vegetation and some discomfort to the local population. The week ended with significantly reduced levels of seismic activity and an apparent reduction in the rate of dome growth.

Montserrat Volcano Observatory