During this reporting period the level of volcanic and seismic activity declined. Rockfall activity was reduced to a few small events per day. Several spines grew in the summit area of the dome during the early part of the reporting period but since the 9th March there was no visual evidence for further dome growth.
On the 1st March the summit area of the dome was seen to be blocky with a small number of rather stumpy spines, the summit was measured at 1011m. Visibility was then poor for several days but on 8th March a glimpse of the summit area revealed the presence of two prominent spines. On 9th March good visibility enabled observations of the new spines to be made. One large broad-based spine was particularly prominent and its top was measured at a height of 1027m. Since 9th March no major changes were seen to indicate further growth of the dome.
Rockfall activity declined in early March but continued at a low level throughout the reporting period. New rockfall chutes developed on the south west side of the dome complex and on the eastern side above the Tar River valley. Some of the larger rockfalls reached to the base of the talus slope in both directions. Fumarolic activity was mainly confined to a V-shaped cleft in the pre-explosion dome complex on the eastern side of the dome. The deposits from pyroclastic flows observed in the upper part of Tuitt's Ghaut on February 28th were found to be composed of older material from the base and sides of the October 22nd dome which forms a large dark mass on the northern side of the dome complex.
On 15th March temperature measurements were made in the pyroclastic flow deposits produced in the September 21st 1997 collapse. The maximum temperature was 590 Centigrade at a depth of 1.5m.
Seismic activity further declined over the reporting period. The number of recorded earthquakes (Table 1) was low, with no swarms of events. Surface rockfall activity was also low. Epicentral solutions for locatable events were all in the area of the dome.
The previously observable periodicity in seismic amplitude ended. From the beginning of February, when the cycle was at its peak, the mean period lengthened from 6.17 hours to 17.36 hours at the end of February, with a corresponding decay in amplitude. By early March, the period had lengthened and the amplitude decayed to such an extent that discrete peaks in activity were no longer recognisable.
Table 1: 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 LPRF* HYRF* 1 Mar 98 4 9 5 22 0 0 2 Mar 98 4 22 0 34 0 0 3 Mar 98 4 7 3 28 0 1 4 Mar 98 16 0 1 12 0 0 5 Mar 98 13 3 0 2 0 0 6 Mar 98 13 5 7 16 0 0 7 Mar 98 6 8 7 15 0 0 8 Mar 98 10 1 0 8 0 0 9 Mar 98 0 0 0 5 0 0 10 Mar 98 0 3 12 10 0 0 11 Mar 98 2 3 14 4 0 0 12 Mar 98 3 6 0 1 0 0 13 Mar 98 7 8 0 11 0 0 14 Mar 98 1 1 0 8 0 0
*LPRF: LP earthquake followed by rockfall signal. HYRF: Hybrid earthquake followed by rockfall signal. The LPs, hybrids and rockfalls in these signals are also counted in their respective columns.
A fixed GPS site was installed at Harris Lookout. GPS occupations of sites at Whites, Long Ground, Windy Hill, Perches, Old Towne, Lees Yard and Blakes were made during the reporting period. The occupations were processed using the new site at Harris and the University of Puerto Rico sites at Whites, Hermitage and MVO north as base stations. The NE movement of Hermitage continues and the occupations of Whites, Long Ground and Perches confirm their slow movement.
The EDM reflector on the remains of the northern crater wall was shot from Windy Hill. The line continues to shorten with a total shortening of 8cm since January 25th 1998.
Theodolite measurements were carried out on 9th March to determine the height of a new spine seen on the dome. From these measurements the top of the spine was found to be 1027m. A kinematic dome survey consisting of photos and laser range finding binocular measurements was carried out on the 10th March. Heights from these measurements correlated well with those of the previous day. The volume of the dome complex on 10th March was 113 million m3. This incorporated 29 million m3 for the talus slope and 84 million m3 for the dome itself. The volume of the dome just before the Boxing day event 1997, was estimated at 115 million m3.
There was no ash fall on inhabited parts of the island during the reporting period. Dust Trak data are recorded in Table 3.
Table 3 : PM10 (concentration in mg/m3)
These values are approximate 24 hr averages of the PM10 concentration.
Site 1-Mar-98 2-Mar-98 3-Mar-98 4-Mar-98 5-Mar-98 6-Mar-98 7-Mar-98 MVO south 12 22 25 40 60 54 55 Mango 9 17 26 40 57 48 39 CPS 12 54 19 91 * 98 72 St. Peter's 59 13 15 28 * 33 21 MVO north 5 12 13 23 34 29 20 Site 8-Mar-98 9-Mar-98 10-Mar-98 11-Mar-98 12-Mar-98 13-Mar-98 14-Mar-98 MVO south 51 31 - 19 15 14 27 Mango 19 18 - 18 11 10 17 CPS 59 94 - 80 35 72 25 St. Peter's 9 8 - 16 8 1 9 MVO north 9 12 - 13 6 4 9 *Dust Traks failed to operate for 24 hours due to powercuts. 24 Hour averages: <0.05 mg m-3 Low 0.05-0.1 mg m-3 Raised 0.1-0.3 mg m-3 Very High >0.3 mg m-3 Alert Locations: MVO south: The evacuated observatory building in Salem Olde Towne; the Dust Trak is on the veranda. St. Peter's: The Dust Trak is outside on the balcony of a villa in St Peter's. CPS: Catholic Primary School. The Dust Trak is outside in the area under the roof where some of the children have classes. The school is in Palm Loop near Woodlands. MVO north: The observatory on Mongo Hill near St. John's, the Dust Trak is on the third floor of the building with the sampling tube out of the window. Mango: The Dust Trak is outside on the veranda of a villa in Mango Drive in Woodlands.
MVO Staff Changes
Wilkie Balgobin (Seismic Research Unit, Trinidad)
Kate Moore (Bristol University)
John Shepherd (Lancaster University)
Rick Hoblitt (United States Geological Survey)
Hayley Duffell (Independent)
David Pyle (University of Cambridge)
Richie Robertson (Seismic Research Unit, Trinidad)
Dawn Pirie (Edinburgh University)