Montserrat Volcano Observatory

Daily Report
Report for the period 16:00 20 July
to 16:00 21 July 1996

The level of activity at the Soufriere Hills volcano during the current reporting period was higher than that observed during the past several days. The activity included the occurrence of a few very small pyroclastic flows, rockfalls, volcano-tectonic, long-period and hybrid events, and tremor. Activity was generally higher during the first part of the review period than during the second part.

Approximately 560 small volcano-tectonic earthquakes were recorded and locations for all the relatively large ones are being calculated. These events began with a concentrated burst between 18:00 and 20:00 on 20 July, and the remainder then occurred intermittently during the rest of the reporting period. These events were mainly located beneath the upper northern and northeastern parts of the volcano at depths less than 3 km.

Signals indicative of small- to moderate-sized rockfalls from the flanks of the growing lava dome were, as usual, quite common. A total of 58 rockfalls were recorded. A rockfall at 16:04 on 20 July resulted in a moderately thick ash cloud which could be seen drifting in a westerly direction over Upper Gages, Amersham and Plymouth. A relatively large signal , which lasted for about 10 minutes, was recorded at 20:17 on 20 July on all the seismic stations. This signal is similar in shape to those generated by either rockfalls or mudflows. However, it does not seem to have been associated with any major ash production since none of the communities to the west and northwest of the volcano were affected. Several rockfall-like signals recorded today at around 14:24 and 15:46 were correlated with visual observations from Bramble Airport of possible small pyroclastic flows in the upper reaches of the northern part of the Tar River valley. None of the flows seems to have reached as far as the Tar River soufriere. These small flows and other larger rockfalls were associated with small ash clouds which were blown on the wind towards the west, possibly affecting the areas around Upper Gages, Amersham and Plymouth.

Forty four (44) hybrid and 19 long-period earthquakes were also recorded during this period . The hybrid events were generally small while several moderate-sized long-period earthquakes occurred. Low- to moderate-amplitude broadband tremor was near-continuous at the closest seismic stations to the crater from 16:00 to 20:00 on 20 July. It became more intermittent afterwards. A regional earthquake of magnitude greater than 4.0 occurred at 05:40 this morning about 160 km from Montserrat.

Visibility was poor throughout the whole observation period, although views of the lower part of the dome were obtained during a mid-morning helicopter flight. Smaller additional deposits were observed in the southern and northern upper reaches of the Tar River valley. A few rockfalls were observed from the eastern, northeastern and northern flanks of the dome. No additional deposits were seen in the Upper Fort Ghaut area. The level of steam emission from the lower sections of the dome was low.

Gravity, GPS and EDM measurements were not made today. COSPEC measurements were carried out but the data is still being processed.

The increase in earthquake activity during this reporting period does not signal an escalation of activity at the Soufriere Hills volcano.

The Soufriere Hills Volcano is still considered to be highly dangerous to people and property on it's eastern and upper western flanks. Visits to the evacuated zone must be kept to a minimum. The Tar River and Long Ground areas to the east and upper Fort Ghaut, Gages Village and Upper Amersham areas to the west are all extremely dangerous. All access roads to these areas remain closed and people should not enter these areas under any circumstances. If they do, they put themselves and others at direct risk of serious injury or death.

Montserrat Volcano Observatory