Montserrat Volcano Observatory

Daily Report
Report for the period 16:00 on 05 Feb. 1996
to 16:00 on 06 Feb. 1996

Hybrid earthquakes (dominant frequencies of 3 to 5 Hz) continue to occur every 10 to 30 seconds, but have diminished considerably in energy since yesterday; they are barely detectable on the most sensitive station, MGAT, near Gages; most have magnitudes less than 0, with the largest reaching about magnitude 0.3. Long-period earthquakes (dominant frequency of 2 Hz) were again fairly frequent; about 25 were recorded in all and had magnitudes between 1.3 and 2.5; four of these could be well located and originated from between 1.2 and 1.9 km beneath the crater. One volcano-tectonic (rock-breaking)earthquake was recorded during this period, but was too small to locate. Seismic signals, of the type that can often be visually correlated with rockfalls, were even more frequent than yesterday; about 45 were recorded during this period, five of which were observed to correlate with rockfalls. Broadband tremor (dominant frequencies of 2 to 10 Hz) began at about 1600 and lasted for about 3 hours and probably was the result of strong winds and heavy rain about this time.

No EDM measurements were carried out today. Results from measurements made yesterday on the eastern triangle to Castle Peak and on the Windy Hill-Farrells-St George's Hill triangle indicate changes in slant distance which are too small to measure. Two stations were established from which angular measurements would be taken so as to provide better estimates of the rate of dome growth.

During the past two days the higher than usual concentration of acid aerosols in the Gages valley has caused some minor damage to vegetation in this valley. The brown discolouration of leaves in villages such as Webbs and Amersham is caused by these aerosols. The concentrations of these gases are still below levels that would be harmful to people living in these areas.

Two late afternoon helicopter flights afforded brief views of English's Crater. The southern area of dome growth appears essentially the same as yesterday. Spalling of material off this face continues to fill in the south moat. The north face appear to be the area of most active growth at present and exhibits nearly continuous rock falls. Growth is both vertical and lateral, toward the crater wall at the top of Farrells ridge. The height of the dome in this area is now at least as high as the crater wall at Farrells and higher than Castle Peak. During the later flight incandescent material was observed at the surface during one of the many rock falls which were observed. The northern part of the dome is now clearly visible from the roadside at Farrells and from Plymouth.

Montserrat Volcano Observatory