Summary
Activity at the volcano continued to show a cyclic pattern with short periods of elevated seismic activity occurring every 8-12 hours during the early part of the reporting period and every 14 hours or so since 22nd February. These periods of elevated activity were dominated by rockfalls during the early part of the reporting period and by hybrid earthquakes and tremor during the later part. These changes may indicate that magma was finding it progressively more difficult to reach the surface.
Visual Observations
On 15th February several rockfalls and small pyroclastic flows were observed travelling down the White River valley. Visibility was then poor until 25th February when vigorous ash venting was observed together with rockfalls in the White River valley. The new dome had grown considerably during these ten days and the top of the dome was observed to be blocky with several stubby spines.
Seismicity
Over this period there were fewer rockfalls than in the preceding few weeks and, especially towards the end of the month, comparatively more earthquakes. The most striking example of this was the swarm of earthquakes on the 26th Feb (Table 2). This swarm was unusual in that it included volcano-tectonic earthquakes as well as hybrids; it did not lead to any observed activity on the dome.
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* 15 Feb 98 2 8 17 69 8 0 16 Feb 98 7 11 5 51 3 0 17 Feb 98 14 16 8 60 1 0 18 Feb 98 5 7 14 79 9 0 19 Feb 98 6 7 1 40 0 2 20 Feb 98 6 11 5 40 0 2 21 Feb 98 14 46 7 28 0 0 22 Feb 98 6 6 7 35 4 0 23 Feb 98 17 30 11 22 0 1 24 Feb 98 2 26 23 40 0 4 25 Feb 98 27 34 11 36 4 0 26 Feb 98 35 88 3 23 0 0 27 Feb 98 2 40 2 25 0 1 28 Feb 98 9 97 14 22 0 1
*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.
Table 2: Swarms
Start Duration Hybrid LP VT 17 Feb 98 04:52 2.42 10 0 4 21 Feb 98 18:53 6.48 31 3 8 23 Feb 98 08:23 3.90 11 5 9 23 Feb 98 13:50 1.78 14 1 1 24 Feb 98 21:38 1.87 13 2 1 25 Feb 98 10:59 2.95 17 3 0 26 Feb 98 05:36 5.36 82 2 33 27 Feb 98 13:12 13.12 24 0 0 28 Feb 98 10:33 10.33 28 0 1 28 Feb 98 14:57 14.57 48 0 4
Ground Deformation
A collaborative effort has started between MVO and the University of Puerto Rico, Department of Geology to strengthen the GPS programme. Continuous GPS stations were installed by UPR at their old site at Hermitage and a new station was established at the observatory at Mongo Hill. The equipment at Hermitage had been damaged in the 17 September 1996 explosion and the site has been occupied intermittently ever since then. A GPS occupation running over four days was completed running five stations simultaneously. The sites of Harris, Hermitage, Lees Yard, Perches, St Georges Hill, Old Towne, Blakes and Lookout Yard North were occupied during this time. Hermitage and Perches continue to move slowly to the NNE, whereas the sites on the NW of the volcano and in the north of the island are relatively stable.
Electronic tiltmeters were installed at Hermitage and on Gages Mountain.
The new EDM reflector on the northern crater wall (Peak B) was shot from Windy Hill. A total of 5cm shortening has occurred since measurements began on January 25, 1998. The Lees Yard reflector was shot from MVO south and the Waterworks Estate. It has not yet shown any clear movement.
Volume Measurements
A kinematic dome survey was carried out on the 28th February. This involves taking a series of photographs of the dome from the helicopter and shooting points on the dome with laser range-finding binoculars; the position of the camera is determined using GPS. Processing of these data is still in progress. Theodolite measurements made on the 27th February from Garibaldi Hill and the old observatory in Old Towne gave a height for the highest point on the dome of 997m. More theodolite measurements were made from the South Soufrirere Hills and Perches Mountain on the 1st March. These gave a new high point of 1011m, indicating that the vertical growth rate was 14m in 2 days.
Environmental Monitoring
Sulphur dioxide diffusion tube results have been received for this reporting period (Table 3). Levels of SO2 in Plymouth, at MVO south and at Lawyers were raised since the last reporting period but levels at St Georges Hill were reduced. The Police HQ site in Plymouth is surrounded by thick tephra deposits up to 30cm thick.
Table 3. Sulphur dioxide diffusion tube analyses (levels in ppb) for the period 12 February to 1 March 1998. Recommended action level is 100ppb.
Location Police HQ Plymouth 30.2 St Georges Hill 5.25 Weekes 0.4 MVO south 8.4 Lawyers 1.35
Dust Trak monitoring has been carried out at four fixed sites, and the results are shown in Table 4.
The higher levels at the Catholic primary School are due to its proximity to a main road and the effects of resuspension of ash by cars. Sites Mango and St. Peter's, are on first floor verandas and show similar values to each other. The site at MVO north is on the third floor of the observatory which is itself at the top of a hill. This is reflected by the lower values at this site.
There has been no major ash fall during the reporting period but occasionally the diffuse volcanic plume was blown northwards (e.g. 24-25 February) causing light ash fall and hazy conditions.
Table 4 : PM10 (concentration in mg/m3)
These values are approximate 24 hr averages of the PM10 concentration.
Site 15-Feb-98 16-Feb-98 17-Feb-98 18-Feb-98 19-Feb-98 20-Feb-98 21-Feb-98 Mango 0.011 0.011 0.017 0.012 0.011 0.009 0.014 CPS 0.013 0.044 0.041 0.040 0.033 0.021 0.020 St. Peter's 0.014 0.014 0.020 0.008 0.027 0.007 0.008 MVO north 0.005 0.007 0.010 0.008 0.009 0.006 0.007 Site 22-Feb-98 23-Feb-98 24-Feb-98 25-Feb-98 26-Feb-98 27-Feb-98 28-Feb-98 Mango - 0.040 0.029 0.015 0.009 0.006 0.026 CPS - 0.049 0.050 0.050 0.018 0.009* 0.010 St. Peter's - 0.028 0.012 0.010 0.005 0.017* 0.049 MVO north - 0.023 0.008 0.004 0.005 0.005 0.007 * These readings were affected by power cuts. 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: 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.Two temporary sites at Brades Primary School (BPS) and at Lookout Secondary School (LSS) have undergone 24 hour sampling. Results are listed below in Table 5. Table 5 : PM10 (concentration in mg/m3)
These values are approximate 24 hr averages of the PM10 concentration.
Date Feb-15-98 Feb-16-98 Feb-17-98 Feb-18-98 Feb-19-98 Feb-20-98 Feb-21-98 BPS - 0.035 0.052 0.057 0.054 0.046 0.027 LSS 0.005 - 0.044 0.063 0.030 0.014 - Date Feb-22-98 Feb-23-98 Feb-24-98 Feb-25-98 Feb-26-98 Feb-27-98 Feb-28-98 BPS - 0.057 0.074 0.035 - 0.017 - LSS - 0.036 0.020 0.027 - - -
MVO Staff Changes
Arrivals:
Keith Rowley
Departures:
Chloe Harford (University of Bristol)
Ouchi Osuji (Seismic Research Unit)
Richard Luckett (British Geological Survey)
Mahar Lagmay (Cambridge University)
Gill Norton (British Geological Survey)
Visitors
Alberto Lopez-Venegas (University of Puerto Rico)
Audeliz Matias (University of Puerto Rico)
Glen Mattioli (University of Puerto Rico)
Lizzette Rodriguez (University of Puerto Rico)