Volcanic activity at the Soufriere Hills Volcano between 23 and 29 May continues to be dominated slow growth of a lava dome in English's Crater. Excellent views for most of the week allowed good observation to be made of the changing conditions at the volcano. Rockfall signals associated with dome growth continued to dominate seismicity. Most of these events had associated ash plumes and some developed into small pyroclastic flows which entered the Upper Tar River Valley. Ground deformation measurements show changes which are consistent with small scale deformation of the volcano caused by continued slow growth of the lava dome. Gas measurements using COSPEC and FTIR show that the levels of SO2 remains very low at around 127 tonnes per day. Clear conditions allowed measurement of dome surface temperatures which have given results in the range 200 to 250 degrees Centigrade.
Visibility was generally poor at the beginning of the week, improving significantly during the latter parts. The volcano was covered in low cloud on 23 and 24 May and no views were obtained of the summit. Conditions improved slightly on 24 May and there was a brief period around 12 noon when partial views were possible from several locations. At least three spines were observed on the top of the dome, none of them more than 50 ft high. Vigorous steaming was seen in the northwest moat, and from several areas of the dome. A mudflow deposit, which had descended the Upper Tar River Valley during heavy rainfall on the night of 24 May, was observed by a helicopter flight. A clear scar, about 3 ft deep and perhaps 15 to 30 ft wide, was visible on the lower northeastern flank of the dome. Several rockfalls were seen travelling down this feature.
Viewing conditions improved markedly on 26 June and excellent views were obtained of English's Crater from the helicopter, Brambles Airport and from several areas around the volcano. The active areas of dome growth were noted to be the eastern, northeastern, western and southern parts of the dome. Several small spines with morphology quite similar to the old Castle Peak were observed on the top of the dome; none were higher than 20 m above the main part of the dome. Vigorous steaming was seen from several areas of the moat, with the southern, northeastern and western parts being most energetic. Rockfalls on the northeastern flank of the dome continue to occur from a large scar which is close to the contact between the old Castle Peak dome and the new dome. Rockfalls on the southern flank of the dome were beginning to deposit material into the old July 28 vent.
Visibility was again very good on 27 May and clear observation of the dome were possible. The active areas were essentially the same with steam emission from the dome continuing from several areas. Moderate amounts of blue smoke, possibly SO2, were being emitted from the south moat and the eastern flank of the dome. There was a new area of steam production, on the saddle between Castle Peak and the new dome; this however may have been related to the rainfall on the night of 26 May. Rockfall activity was quite high with almost constant small-scale rock falls occurring from the eastern and northeastern flanks of the dome. Few of the rockfalls were large, although two of them, at 09:14 and 12:07 were seen to produce small pyroclastic flows which entered the Upper Tar River Valley area. A large highly-fractured block was observed high on the eastern flank of the dome with a large fracture approximately 50 ft long, on its northern margin. There was almost continuous ash emission from this fracture during a helicopter inspection in the morning of 27 May. Incandescence was seen in this fracture, and in a small depression to the north of it. There was much less ash being produced from the fracture during an afternoon inspection, although the incandescence was still present.
Visual observations on 28 May suggested no major changes in shape or size of the dome, although semi-continuous rockfalls were occurring from the northeast and eastern flanks of the dome. Some increase in rockfall activity was noted from the northern and southwestern flanks of the dome. Several rockfalls during the day produced light ashfalls in the Amersham and Plymouth areas.
Overcast conditions and light intermittent rainfall did not allow clear views for most of 29 May. Observations made during the late afternoon when conditions improved showed that there were no major changes in the dome. Semi-continuous rockfalls were still occurring from the northeastern and eastern flanks of the dome. Vigorous steaming was observed from the moat area and from the southwestern and western parts of the dome. Incandescent material was observed from several areas in the east and northeast of the dome. Several small pyroclastic flows were observed from the top parts of the dome. These flowed down the eastern flank of the dome and into the upper parts of the Tar River Valley. None of these flows reached further than the Tar River Soufriere.
Seismic activity at the Soufriere Hills Volcano during the week has been dominated by variable amounts of small- to moderate-sized rockfall events. The table below lists the daily counts of the different types of events 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 23 - 29 May 1996
Date VT LP Hybrid RF Tremor amount 23/05 0 12 0 64 Intermediate to high 24/05 0 19 0 50 Low 25/05 0 17 1 104 Low 26/05 0 12 8 114 Intermediate 27/05 1 13 5 85 Intermediate 28/05 1 13 4 86 Intermediate to high 29/05 0 12 3 83 Low to intermediate
Rockfall signals associated with movement of unstable rock material from the surface of the actively growing dome continued to dominate the signals recorded by the seismic network during the week. Most of these events had associated ash plumes the size and distribution of which depended on the wind conditions and how much rock was mobilized.
The week began with activity at about the same level as that observed during the latter part of the previous week. Small- to moderate-sized rockfall signals were recorded, the largest of which occurred at 20:35 on 23 May and was associated with light ashfall in Lover's Lane, Ryners and Gages. The level of activity increased slightly on 24 May although overall it was still relatively low. Rockfall activity continued to dominate the seismicity on 25 May although none of the signals were very large. Light ashfall was generated on a number of occasions by the rockfalls, the largest event occurred at about 14:00 on 25 May and produced ashfall in Cork Hill and Old Towne. Ash clouds generated during the period were thinner than has recently been the case, and showed very little thermal convection. None of the rockfalls on 26 May were particularly large but most generated small ash plumes. Calm conditions and the absence of strong local winds provided ideal conditions for the development of near vertical ash plumes, some of which attained heights of up to 6000 ft above sea level. The largest rockfalls which occurred on 27 May produced small ash plumes which drifted on the wind over the Gages and Upper Amersham areas. The level of seismicity was moderate on 28 and 29 May and the larger rockfall signals were correlated with ash plumes which attained heights of up to 3500 ft and with small pyroclastic flows into the upper Tar River Valley. A slight change in the wind direction caused light ashfall to affect areas to the west and northwest of the volcano.
Low-amplitude broadband tremor recorded mainly on stations closest to the volcano, has been generally intermittent for most of the week. There were a few discrete periods of near-continuous tremor during 25, 28 and 29 May . A number of long duration seismic signals were recorded on 24 May. The signals are much longer than the typical rockfall signals, lasting for up to 20 minutes. These events show much greater variation in amplitude than the broadband tremor and have been directly correlated in the past with the occurrence of mudflows.
The number of long period earthquakes varied from about 8 to 17 per day during the week.
No hybrid events were recorded during the early part of the week. From the 26 to 29 May, there were a few hybrids with the daily counts varying from one to eleven.
There were two volcano-tectonic events recorded during the week. The first was recorded on the 25 May and was located at a depth of 2.5 km beneath English's Crater. The other event occurred on 28 May and was located at a depth of about 0.5 km to the north of the crater.
A regional event earthquake of magnitude greater than 3 was recorded by the seismograph network at 10:00 on 24 May; the event was located northeast of Guadeloupe.
EDM measurements were hindered by low cloud cover obscuring clear views to the reflectors and preventing daily measurements from being conducted on all the main lines. No EDM measurements were made on 23, 24, 25 and 29 May, due to low cloud cover. Measurements were made on the eastern and southern triangles on 26 May. The line lengths on the southern triangle shortened by 8 to 9 mm since 21 April while the eastern triangle shortened by about 1 cm since 20 May. The changes observed on these triangles are consistent with small scale deformation of the volcano caused by continued slow growth of the lava dome in English's Crater. The data obtained by the EDM technique are consistent with the recently concluded GPS measurements conducted by the University of Puerto Rico. The northern triangle between St George's Hill, Windy Hill and Farrells Yard was reoccupied on 28 May; the results of this survey continued to show no consistent changes in line lengths on this triangle.
Only one occupation was possible with on the MVO GPS network, the Reid's Hill, Long Ground, Harris Lookout and St George's Hill stations were occupied on the 23 May. For the remainder of the week the GPS programme had to be curtailed because of technical problems. Continued failure of the controllers prevented occupation of GPS stations; replacements for the faulty controllers were obtained on 29 May.
SO2 flux measurements were made during the week using a COSPEC mounted in a car and driven between Cork Hill and St. Patricks under the gas plume. No measurements were made on 23, 24 and 29 May. The results of measurements carried out during the week show some variability but are consistently low, with a mean value of 127 tonnes per day.
Gas measurements using the FTIR technique were conducted during most of the week. The active technique using an artificially generated infrared light source was used between St Georges Hill and Amersham on 23 May and in Plymouth and Fox's Bay on 24 May. Measurements were attempted directly to the dome for the first time on 26 May and again on 27 May from the school yard at Harris Village and from the helicopter. Due to the relatively cold temperatures on the surface of the lava dome (~ 350 oC), the signal strength to the instrument was too low for any measurements to be obtained. The results of FTIR measurements for the week all indicated very low hydrogen chloride (HCl) and sulphur dioxide (SO2) content in the areas sampled. Direct sampling of the volcanic plume has so far not been possible but the results obtained have proven very useful in monitoring the levels of volcanic gases in areas which are still periodically inhabited. The results obtained indicate that levels of hydrogen chloride and sulphur dioxide are still well below levels of concern for human habitation.
Estimates of dome surface temperatures were made using an optical pyrometer from the helicopter on 27 May. The surface of the dome showed temperatures in the range 200 to 250 degrees Centigrade. The highest temperature measured was about 350 degrees; this was obtained from the surface of the dome immediately after the occurrence of a rockfall.
Samples of the 12 May pyroclastic flow deposit in the Tar River valley were collected for petrological analysis on 26 May.
A network of ash collectors were deployed between Gingoes and Salem on 28 May; these collectors will enable better estimates of the volume of ash being produced from the volcano, which may help with estimates of dome growth rate.
Eliza Calder from Bristol University.
Dr Simon Young of the British Geological Survey
Dr Jenni Barclay from Bristol University.
Dr Pierre Delmelle from Brussels University.
Professor Geoff Wadge of Reading University.
Professor Andy Wood of Bristol University.
Dr Rosalyn Lopez, Jet Propulsion Laboratories.
Professor Haraldur Sigurdsson, University of Rhode Island.
Professor Wagner, University of Geneva.
Dr Laurent Stieltjes, BRGM, France.