Activity this month has been dominated by small explosive and ash venting episodes, sometimes producing pyroclastic flows as far as the Tar River delta. Very little other seismic activity has occurred and deformation rates continue to be low. Gas emissions have been moderate.
Small explosive and ash venting episodes have dominated the activity this month. A total of 23 events occurred with the largest producing ash clouds reaching to approximately 20,000 feet with associated lightning and ashfall. Table 1 summarises the March events.
One of the largest events of this reporting period occurred on 1 March at 17:20. Two observers at Smokey Hill witnessed the event. The event began with a few seconds of deep rumbling followed by a vertical plume and a bursting, dark pyroclastic flow down the Tar River Valley. The flow banked high against Roche's Mountain, leaving a considerable surge deposit and then continued to the delta. The vertical ash plume rose directly above the dome simultaneously. It quickly reached 20,000 ft and formed an umbrella cloud. Thunder and lightning were noted in the plume. For the 10 minutes following the event, a continuous, yet gradually decreasing, jet engine-like noise was heard. The deposits appeared to have no surge component on the delta. Small clasts (up to 7 cm) collected from the Tar River Valley deposits are light in colour, moderately vesiculated and slightly rounded.
Later observations confirmed fresh deposits from the same event in the upper White River Valley to the base of the talus and blocky deposits in lobes (approximately 20 m long) on the top of Galway's Mountain. Fresh dry ash was deposited in White River on Fergus Mountain. There was a large fallen block seen in the notch in the dome. Also, considerable fresh ash fell in Plymouth and as far north as Salem and Runaway Ghaut. No fresh pyroclastic flow deposits were seen in the other valleys.
Another event occurred on 7 March at 12:26. A plume rose rapidly above the dome to about 20,000 ft and drifted to the west. Fresh ash was found in the Plymouth area the next day. A thin, fine-grained flow deposit was emplaced in the upper Gages Valley, reaching about 200 m below the talus. New vigorous ash venting was occurring continuously for the next 3 days near the southwest corner of the dome, visible from Plymouth. No other valleys contained fresh deposits.
Similar ash-venting episodes continued to occur with greater frequency, but less visible intensity through the middle of the month. The events exhibited more dilute and less convective plumes than that of 1 March and 7 March. The more noteworthy observations follow.
Small pyroclastic flow deposits were seen near Upper Gages and in the Galway's area on 12 March. The Gages deposit was fine-grained and ended 500 m below Gages Upper Soufriere. The Galway's deposit ended ~1.5 km from the dome: the upper part was blocky and ash-cloud surge deposits extended approximately 50 m up the bluff above Great Alps Waterfall. Observers in the White River Valley saw small pyroclastic flows develop and travel to the base of the dome in the Galways area on 24 March.
On 26 March, a sudden larger explosive event produced an ash cloud rapidly rising to 20,000 feet with associated lightning. The ash cloud travelled to the north-west depositing ash as far north as the Woodlands area. This ash was mostly sand-size and depleted in fine-grained material.
The three events on the morning of 30 March followed heavy overnight rain, and may have been partly a result of small steam explosions in or close to the dome.
Table 1. Dates and times of ash venting episodes
|Date||Time||Comments||Estimated cloud height /(ft)||Duration of seismic signal at Windy Hill (in mins)|
|1 March||17:18||Explosion and pyroclastic flows down Tar River and Galway's||20,000||29|
|7 March||12:26||Explosion and small pyroclastic flow (Upper Gages)||20,000||16|
|11 March||17:00||Small pyroclastic flows (Upper Gages and Galways)||10,000||10|
|12 March||5:16||Ash venting and loud rumbling||8|
|13 March||11:00||Ash venting||5|
|14 March||3:50||Ash venting||3|
|16 March||11:00||Ash venting||7,000||12|
|18 March||10:05||Ash venting||4|
|20 March||17:40||Ash venting||20,000||13|
|21 March||16:00||Ash venting||13|
|22 March||11:00||Ash venting||5|
|24 March||13:13||Small pyroclastic flow (Upper Galway's) and loud rumbling||19,000||12|
|25 March||12:30||Ash venting||18|
|26 March||03:00||Ash venting||28|
|28 March||05:08||Ash venting||11|
|28 March||07:38||Ash venting||7|
|29 March||03:13||Ash venting||10|
|29 March||05:49||Ash venting||10|
|30 March||04:47||Ash venting/small explosion||10,000||11|
|30 March||07:36||Ash venting/small explosion||10,000||36|
|30 March||08:39||Ash venting/small explosion||10,000||12|
Seismicity this month has been dominated by the ash venting episodes. Many of the events had impulsive starts with a gradual decline in amplitude towards the end of the signal. Otherwise the numbers of earthquakes have generally been very low (Table 2).
Table 2. Earthquake types
These earthquake counts are of events that triggered the broadband network's event recording system between 00:00 and 00:00 each day (local time).
Date VT Hybrid LP Dome RF 01 March 99 1 0 0 7 02 March 99 6 0 0 18 03 March 99 5 0 0 4 04 March 99 2 0 0 11 05 March 99 2 0 0 12 06 March 99 6 0 0 8 07 March 99 11 0 0 15 08 March 99 14 0 0 15 09 March 99 4 0 0 14 10 March 99 2 0 0 3 11 March 99 2 0 0 16 12 March 99 5 0 0 7 13 March 99 4 0 0 10 14 March 99 2 0 0 13 15 March 99 0 0 0 6 16 March 99 7 0 0 21 17 March 99 5 0 0 3 18 March 99 3 0 0 9 19 March 99 5 0 0 1 20 March 99 5 0 0 6 21 March 99 4 0 0 13 22 March 99 1 0 0 10 23 March 99 1 0 0 13 24 March 99 1 0 0 7 25 March 99 1 0 0 21 26 March 99 2 0 0 7 27 March 99 1 0 0 1 28 March 99 0 0 0 15 29 March 99 3 0 0 11 30 March 99 2 0 0 12 31 March 99 1 0 0 12
Several GPS sites around the volcano have been measured this month. None of the positions show any significant movement. A permanent receiver was installed at South Soufriere Hills and is now continuously recording data that can then be interrogated remotely by MVO.
Electronic Distance Measurement
The EDM network in Long Ground was occupied on 3, 11 and 18 March. Currently there is no consistent displacement of any of the sites that may be related to movement on the postulated fault in this area.
Microgravity measurements on lines radial to the volcano and sites for completing a Bouguer gravity map of the island were completed during March. A report on this work will hopefully soon be available.
Sulphur dioxide flux was measured using the miniCOSPEC during March, and results are shown in Table 3. Values are moderate for the current eruption. Dusty conditions throughout the month on the western flanks of the volcano as a result of ash venting prevented more measurements.
Table 3. Average daily sulphur dioxide fluxes measured by miniCOSPEC, March 1999
Date Flux (tonnes/day) Comments 6 March 560 Helicopter 19 March 570 Helicopter
Sulphur dioxide was also measured at ground level by using several sets of diffusion tubes located around the island. The results are shown in Table 4. These levels are generally lower than over previous months. However, values at St. George's Hill were slightly higher than normal; this is thought to be due primarily to unusual wind directions.
Table 4. Sulphur dioxide diffusion tube results, March 1999. Levels in ppb.
Location 25 February to 17 March Police HQ, Plymouth 22.85 St. George's Hill 19.20 Weekes 9.70 Vue Pointe Hotel 9.20 Lawyers 6.30
Changes In Reporting Procedures
From 21 March 1999, the MVO changed from a daily to a weekly activity report, issued after 5 p.m. on the Friday of each week. From April 1999, the monthly report will be replaced by a quarterly scientific report issued at the end of each quarter. The next scientific report will be issued at the end of June 1999.
MVO Staff Changes
Dr. Keith Rowley, Independent
Stephane Acounis, Guadeloupe Volcano Observatory
Richie Robsertson, Seismic Research Unit
Mark Davies, Open University
Dr. Keith Rowley, Independent
Dr. Richard Luckett, British Geological Survey
Dr. Ricky Herd, British Geological Survey
Stephane Acounis, Guadeloupe Volcano Observatory
Dr. Joe Devine, Brown University
Chan Ramsingh, Seismic Research Unit
Desmond Seupersad, Seismic Research Unit
Dr. Jean-Christophe Komorowski, Guadeloupe Volcano Observatory
Dr. Georges Boudon, Institut de Physique du Globe, Paris
Uwe Grunewald, Bristol University
Prof. Barry Voight, Penn State University
Amanda Clarke, Penn State University
Dr. Alexander Belousov, Moscow State University
Irvin Christopher, Soufriere Monitoring Unit, St. Vincent