Montserrat Volcano Observatory, Montserrat, West Indies

Scientific Report 09
31 January 1996

The past week was characterised by a series of seismic events whose rate and amplitude varied slowly throughout the period under review. Dome height increased during the latter part of the week allowing clear views of the new dome to be obtained from areas on the western side of the volcano. Very good weather for most of the week permitted close observation of dome growth.

Seismic activity at the start of the week was dominated by the series of very regularly spaced hybrid (dominant frequencies of 3 to 5 Hz) earthquakes that had commenced on the evening of 23 January. Earthquakes were occurring at variable rates (between 3 and 5 per minute), with varying amplitudes. This pattern continued throughout the week with subtle changes in rates of occurrence and amplitude until 30 January. Around this date the rate of earthquakes accelerated to six or seven per minute with a noticeable increase in amplitude and remained this way until the end of the week under review. However, even at their largest amplitude, these events were still too small to give a well-constrained instrumental location owing to being sufficiently recorded on just the nearest stations. Scientists on the crater rim for visual observations reported that the largest of these earthquakes could be felt with a very slight "jolt" in ideal conditions. There was a paucity of other seismic activity over the week. A short episode of moderate-amplitude broadband tremor was recorded on the nearest seismometer to the volcano between the evening of 28 January and the morning of 29 January. Very occasional, small, long-period earthquakes started to appear on the evening of 28 January and a solitary volcano-tectonic (ie. rock-breaking) earthquake at 02:42 on 29 January was located beneath the crater area at a depth of 2.8 km. This event was of magnitude 2 to 2.5 and was reported felt in the Long Ground area. Other seismic signals were recorded on the nearest seismometers and are thought to be the signatures of small rockfalls and explosions. Some of these signals correlated with small ash emissions on 25 January at 10:30, 15:50 and 16:41. A larger such signal correlated with an ash emission caused by a major rock avalanche witnessed by scientists on the crater rim on 26 January at 11:55. The total seismic energy released during the week under review was at its highest level since early December.

Two small regional earthquakes were recorded by the seismic network. The first occurred on 27 January at 19:04 approximately 120 km from the island and was a magnitude of about 3.5. The second, which occurred on the 31 January at 03:04 between 80 and 120 km from the island, was also of about magnitude 3.5.

EDM measurements were conducted on the Galways-Chance's Peak-O'Garra's and the Long Ground-White's Yard-Castle Peak deformation triangles during the week. Attempts to measure the Dagenham-Gages-Amersham triangle was unsuccessful due to poor visibility and obscuring of the reflector by fine ash deposits. Daily changes on all lines continue to be well within the uncertainty of the method. A new tilt levelling line, oriented radially to the volcano, was established during the week. Measurements on this line will assist in the future long-term monitoring of the volcano. The benchmarks were established in 12x12x36" reinforced-concrete piers, set in reworked volcanic deposits in the Farrells area on the northwestern flank of the volcano. Baseline measurement to the benchmarks will be carried out during the next month, after sufficient time has elapsed for settlement of the piers.

Active dome growth at the beginning of the week was most pronounced in the northern parts of the new dome. New material had built up to a steep angle and continued to collapse into the moat area producing rock falls and associated ash clouds. Spillage of material from the collapsed 20 January spine to the south continued to deposit debris in the moat area. Strong glowing of the active northern part of the new dome was clearly visible on the 25 January, from several areas on the eastern flank of the volcano. Excellent views of the crater on the 26 January, indicated that a spine was growing vertically in the south, whereas growth in the north appeared to be a general swelling. The northern part of the dome produced less fragmental material from the 27 January onwards. Rock falls from this face began to decrease. Growth rate up to that time did not appear to have changed significantly and remained very low. On 28 January growth on the southern part of the dome changed to a general swelling accompanied by subsequent spalling of material into the moat. A helicopter flight on the 29 January, which enabled close observation of the dome, indicated that dome growth rate was at a somewhat increased level compared with the previous few weeks.

Beginning about 29 January, two elongate ridges, oriented northeast-southwest, became apparent on the north and south ends of the growing dome. These are rough mirror-images of each other, and resemble whale backs. The top of the north whale back became increasingly visible from several areas on the western flank of the volcano. Dome growth has decreased the width of the moat area between the dome and the wall of English's Crater. Incandescent material is still clearly visible on the north and south face of the dome. Rock avalanches, observed on 31 January from Farrells and Chances Peak, included glowing blocks which fell towards the moat. Steaming continued from several areas of the new dome throughout the week.

Geological field work on the volcano during the week involved the collection of charcoal samples from numerous prehistoric pyroclastic flow deposits at the east end of English's Crater. Dating of these samples will help in the determination of the volcanic evolution of Castle Peak and English's Crater.

Light very fine-grained ash continues to be deposited on the upper western flank of the volcano. Ash falls usually results from large rockfalls from the actively growing sections of the new dome. Several trips to the crater rim at Chances Peak during the week indicate that gas and acid production from the crater area remains high. However, dilution by the wind ensures that gas concentrations are well below levels of concern to inhabited areas on the island.

In summary, this week has seen an increase in the rate of slow dome growth within English's Crater, accompanied by a long-lived swarm of small, repetitive hybrid events similar to those which have been seen often during dome growth at Soufriere Hills volcano.

Montserrat Volcano Observatory