Montserrat Volcano Observatory, Montserrat, West Indies

Scientific Report 14
06 March 1996


Seismic activity during the last week has continued at about the same general level as in previous weeks. The seismicity has been dominated by long period earthquakes, and occasional episodes of broadband tremor that were recorded only on the Gages seismograph station.

Between 5 and 20 large long-period earthquakes have been recorded per day, with magnitudes of up to 2. The number of earthquakes increased towards the end of the period under review. Many of the long-period earthquakes have signals with varying frequency with time, and often two distinct segments can be identified, with similar amplitudes but different frequencies. Usually, the start of the signal is of a higher frequency, but reversals of this pattern have been observed. These earthquakes are different from "hybrid" earthquakes that have been observed in the past - the hybrid earthquakes have a more impulsive onset and S phases (usually poorly developed). No S phases can be reliably identified in the long-period earthquakes, and they have emergent arrivals. Instrumental locations have been determined for some of the long-period earthquakes but they are poorly constrained.

Many smaller long-period earthquakes are recorded at close stations, with events occurring about once per minute at the closest station, Chances Peak. Some of these events are due to rockfalls from the dome.

The tremor recorded at the Gages station has been intermittent and of moderate amplitude. Tremor occurred in the late afternoon every day until 5 March, and also most of the day on 4 March. During periods of good visibility, there was no obvious correlation between steam production and the level of tremor. The tremor may be due to increased ground water circulation rather than steam venting as previously suggested.

Six locatable volcano-tectonic earthquakes were detected by the seismograph network during the week under review. These were located beneath the crater area, at depths of 2 to 4 km.

Deformation measurements

Good weather conditions during most of the week enabled all of the EDM triangles to be occupied at least once. A re-occupation of the Tar River to Castle Peak line was also possible. This occupation showed a continued very slow shortening of this line consistent with the shortening on the other lines to Castle Peak from Long Ground and White's Yard. The rate of shortening is c. 1 mm per day and has been constant since mid-November. The northern (St George's Hill - Windy Hill - Farrell's) and southern (Galway's Plantation - O'Garra's - Chances Peak) triangles do not show any consistent movement of the targets. The data collected this week for the Gages Wall reflector, when joined to that collected last week, suggests a consistent rate of shortening of the Dagenham and Amersham to Gages Wall lines of c. 2 mm per day; this remains below levels of immediate concern. Some upgrading of deformation measurements in the Gages Wall area is planned over the next several weeks.

The electronic tiltmeter at Long Ground did not show any deformation events during the period under review.

Visual observations

Observations from both the helicopter and the crater rim revealed continued slow dome growth, with abundant rock falls, especially from the southwestern and northwestern parts of the dome. A spine began to grow in the southwestern part of the active area; rock falls from this and other areas produced ash clouds visible from the western and eastern sides of the volcano on several occasions during the week. The new lava dome became visible from the observatory on 5 March. Rate of dome growth during this period is probably higher than it has previously been, although the lack of good visibility from high in the air has frustrated attempts to obtain stereo photographs which might enable accurate volume estimates.


Dome growth continues at Soufriere Hills volcano at a slow but increasing rate. The current phase of growth is occurring at a very low level of seismicity. Deformation measurements continue to show very small daily changes; the Gages Wall reflector appears to be moving more quickly than other target sites at present, although some further measurements are required to confirm this.

Montserrat Volcano Observatory