Summary
Activity at the Soufriere Hills Volcano between 08 and 10 May remained at about the same low level as the previous week. Continued dome growth produced a number of small rockfalls which were confined to the area around the dome. Activity increased slightly on the 11 May and on the 12 May four significant pyroclastic flows occurred in the Tar River Valley. Seismic activity throughout the week was dominated by rockfall signals with a few long-period events and volcano-tectonic earthquakes. Visibility was generally poor for most of the week and only brief views were obtained of conditions at English's Crater. The main area of dome growth continues to be the northern and northeastern parts of the dome. Ground deformation measurements indicate that there is still only slow deformation of the volcano. COSPEC measurements show that the SO2 flux remains very low, at around 178 tonnes per day.
Visual observations
Visibility was generally poor during most of this period and only brief views of the lower parts of the dome were possible. Rockfalls still seem to be concentrated on the eastern and northeastern flank of the dome.
Visual observations on the 11 May revealed that a small pyroclastic flow had travelled 300 metres from the base of old Castle Peak dome into the Upper Tar River Valley just south of the area scorched by the 03 April flows. This had set fire to some trees. No significant changes were observed in the area of the moat beneath the Gages and Farrells crater wall. Several subsequent rockfalls generated small ash clouds which were blown on the wind towards the west and resulted in small quantities of ash being deposited in the Upper Gages, Amersham and Fort Barrington areas.
The most significant period of activity occurred between 06:30 and 12:20 on 12 May. At about 06:30 on the 12 May, a sequence of near-continuous rockfalls from the eastern flank of the dome started and lasted until about 07:20. This led to ash falls in the Gages, Lees, St. George's Hill, Weekes', Richmond Hill and Fox's Bay areas. From 07:20 to 09:45, the activity was characterised by intermittent, small-sized rockfalls which also resulted in ash clouds that affected the same areas. At 09:45, the first significant pyroclastic flow in the Tar River Valley occurred. Three other significant pyroclastic flows were generated at around 09:52, 11:05 and 11:53. The 09:45 and 11:05 pyroclastic flows entered the sea and travelled on the water for more than 30 m while the event of 11:53 stopped just short of the sea. These events were all associated with large ash clouds which resulted in the deposition of ash in several areas in southern and central Montserrat, including Farrell's, Rileys, Windy Hill, Gages, Lees, St. George's Hill, Weekes', Fox's Bay, Richmond Hill, Cork Hill, Garibaldi Hill, Isle Bay, Old Towne and Salem. Small amounts of ash also fell in the Tar River, Long Ground and Whites areas. The maximum ash thickness deposited was about 3 mm. Activity after about 12:20 was dominated by intermittent small- to moderate-sized rockfalls. The pyroclastic flows did not result in the destruction of any manmade structures but trees were set ablaze in the Tar river Valley area. Although visual observations of the crater area were limited throughout this period of activity, excellent views were obtained of the pyroclastic flows.
Light ashfalls associated with the largest rockfall events on 13 May affected Plymouth, Amersham, Upper Gages, Richmond Hill and areas to the southwest of the volcano. Brief views were obtained on the 15 May of the lower parts of the dome from several places around the volcano. Most of the rockfalls appeared to originate from the northeastern part of the dome, although there was one on the western flank at around 12:00 on 15 May. Small ash clouds associated with the largest rockfalls were blown on the wind towards the west, depositing ash in Plymouth and surrounding areas.
Steam emissions were observed from several areas of the dome throughout the week with the most active region being the southwest moat. On the 15 May moderate steam emissions were also observed from the base of the northern flank of the dome.
Seismicity
The seismicity at the Soufriere Hills Volcano throughout the week has again been dominated by small- to moderate-sized rockfall events. The table below lists the daily counts of the different types of event recorded - volcano-tectonic, long-period, hybrid, and rockfalls. It also includes an estimate of the amount of the broadband seismic tremor seen on stations close to the crater.
Earthquake types 09 - 15 May 1996
Date VT LP Hybrid RF Tremor amount 09/05 0 21 0 73 Low 10/05 1 16 0 97 Low 11/05 1 4 0 62 Low 12/05 0 6 0 109 Low 13/05 0 15 0 127 none 14/05 0 18 0 147 none 15/05 2 50 67 103 none
At the start of the week there was very little of the low amplitude broadband tremor which had been recorded during the previous week. Seismicity was dominated by small- to moderate-sized rockfalls with a few small long period events also being recorded.
The level of activity increased slightly on 11 May but continued to be dominated by small- to moderate-sized rockfalls. From 12-14 May signals associated with pyroclastic flows became an important aspect of the seismicity. The flows for the events of 05:35, 08:02 and 11:52 on 14 May were observed from Brambles Airport.
Variable amounts of low-amplitude broadband tremor were recorded during the earlier parts of the week (09-13 May). Tremor activity was mainly intermittent, for short episodes and was mainly recorded by the Gages seismic station.
A few long period events and VT earthquakes were recorded throughout the week.
There have been a few small hybrid events recorded since about 02:00 on 15 May.
A regional earthquake of magnitude greater than 4 was also recorded by the seismic network at 00:51 on 12 May.
Ground Deformation
EDM measurements were hampered throughout the week by poor visibility. Measurements made on the eastern triangle on the 10 May indicated that there had been no changes in the slant distances for the past six days. Measurement of the eastern triangle on the 11 May indicated that a small shortening of 3 mm had occurred on line lengths since 10 May. No other measurements were possible for the remainder of the week. The data continues to show only very small changes, suggesting very slow deformation of the volcanic edifice.
GPS measurements were made on the University of Puerto Rico network at Radio Antilles, Reid's Hill, Harris Lookout and St Georges Hill during the week. The results of these occupations are still being processed
GPS measurements on the MVO GPS network were hampered by technical problems throughout the week. In addition to faulty controllers, heavy ashfalls on Sunday 12 May damaged one of the computers used in processing the data.
Gas Measurements
SO2 flux measurements were made daily using a COSPEC mounted in a car and driven between Cork Hill and St. Patricks under the gas plume coming out from the volcano. The estimates of the SO2 flux for the week under review are listed in the table below. No measurements were possible on 12 May due to the heavy ashfall. Technical problems with the instrument prevented measurement on the 14 and 15 May. The results show some variability but are consistently low, with a mean value of 178 tonnes per day.
Date Number Mean SO2 flux Standard
of runs (Tonnes/day) Deviation
09/5 4 138 11
10/05 5 123 46
11/05 4 96 30
12/05 - - -
13/05 3 357 119
14/05 - - -
15/05 - - -
Tests of the Fourier Transform Infrared Spectrometry (FTIR), gas-measurement equipment were carried out on the 15 May. Low cloud cover throughout the week did not allow any passive measurements using the dome but attempts were made to sample the plume by application of a an active technique using an artificially generated infrared light.