Summary
The level of seismic activity increased during this reporting period with activity mainly consisting of rockfalls and hybrid earthquakes with some tremor. A cyclic pattern developed once again with peaks in seismic activity every 6-8 hours lengthening to every 8-12 hours towards the end of the period. Rockfalls and small pyroclastic flows occurred mainly on the Galways side of the dome. On several occasions in early February elevated activity and northerly winds resulted in ashfall on the inhabited northern part of Montserrat. Dome growth was still concentrated in the Boxing Day collapse scar and the dome was close to its volume prior to the December 26 collapse.
Visual Observations
Low cloud throughout the reporting period hampered observations. On 6th February observations made from the police boat offshore confirmed that the active growth area was still located in the Boxing Day scar above the White River. Vigorous ash and steam venting occurred during periods of enhanced rockfall activity and two spines were observed on the southern side of the new dome. Fresh pyroclastic flow deposits in the upper part of the White River were probably emplaced during elevated activity on 5th - 6th February. From 10th February to the end of the reporting period visibility improved. The new dome was observed to have grown considerably and almost completely filled the Boxing Day scar. The talus slope below this growth area had also grown considerably. Steam and ash venting from the dome complex continued and was particularly vigorous during periods of elevated seismic activity. A few small rockfalls were observed in the upper part of Tuitt's Ghaut but most rockfall activity occurred on the south west face of the dome complex.
Seismicity
Earthquake activity was relatively low and seismicity was dominated by rockfall events (Table 1). There were three isolated swarms of earthquakes (Table 2). The swarms were not followed by any change in surface activity. Hypocentral solutions for 21 locatable earthquakes (mainly VT earthquakes) were concentrated below the northern sector of the dome complex. Focal depths were shallow, with depths lying between 2-4km below the summit of the dome.
Recorded seismic amplitudes displayed strong periodicity. This periodicity changed from a mean value of 8.68 hours between peaks in the cycle on the February 1st to 6.17 hours on the 3rd. Peak amplitude also increased during this interval. After February 7th the period lengthened again to a mean value of 8.17 hours. Peaks in amplitude generally coincided with peaks in rockfall activity, although rockfall did occur outside these periods.
Table 1: Earthquake numbers
Earthquake counts for events triggered on the broadband seismic network each day from 00:00 to 00:00 the following day (local time)
Date VT Hybrid LP Dome RF LPRF* HYRF* 01 Feb 98 2 16 3 58 1 02 Feb 98 5 1 68 03 Feb 98 9 5 134 04 Feb 98 4 6 12 166 1 05 Feb 98 10 26 20 143 06 Feb 98 4 39 15 85 07 Feb 98 8 28 12 59 08 Feb 98 4 18 20 75 1 1 09 Feb 98 2 16 31 73 1 10 Feb 98 19 31 10 101 1 11 Feb 98 49 26 15 84 12 Feb 98 1 17 8 88 3 13 Feb 98 2 9 8 78 1 1 14 Feb 98 1 5 7 109
*LPRF: LP earthquake followed by a rockfall signal. HYRF: hybrid earthquake followed by a rockfall signal. The LP's, hybrids and rockfalls in these signals are also counted in their respective columns.
Table 2: Earthquake swarms
Start Duration (hours) Hybrid LP VT 10 Feb 98 11:54 2.40 21 3 12 11 Feb 98 1402 2.93 15 3 13 11 Feb 98 2319 0.40 1 - 7
Ground Deformation
Two separate GPS occupations of 'Leesnet' were made during the reporting period, this network includes sites at Old Towne, Waterworks, St. Georges Hill and Lees Yard. There was no sign of any movement of the pins within this network.
A long GPS occupation of Harris and Hermitage confirmed that there has been continued NNE motion of Hermitage. A fourth long occupation of Harris and Blakes enabled a good fix to be made on the position of the new site at Blakes. The occupations agreed to within a few mm in the XY position and 16mm in height.
An electronic tiltmeter was installed at Hermitage, after an initial stabilising period it is hoped that this will provide valuable real-time information about deformation on the north east flank of the volcano.
Volume Measurements
Very clear weather conditions on the 10th February allowed a theodolite survey of the dome to be carried out from Garibaldi Hill and the Delta petrol station. From these measurements it was estimated that the highest point on the dome at this time was 970m.
Environmental Monitoring
Sulphur dioxide diffusion tube results have been received for this reporting period (Table 3). Levels of SO2 in Plymouth and on St Georges Hill were raised but levels at Weekes, MVO south and Lawyers were low.
Table 3: Sulphur dioxide diffusion tube analyses (levels in ppb) for the period 24 January to 12 February 1998. Recommended action level is 100ppb.
Location Police HQ Plymouth 12.1 St Georges Hill 10.8 Weekes 0.6 MVO south 0.0 Lawyers 0.0
Between January 28th and February 7th there were several episodes of ash fall over inhabited areas of northern Montserrat. The array of ash collecting trays distributed across the island enabled the mass loading of the ash to be calculated. In total more than 1kgm-2 of fine ash was deposited in northern Montserrat. Most of this ash was produced during episodes of ash venting and rockfall activity. At most ash collection locations the ash fall between 3rd and 5th February accounted for more than 50% of the local monthly ash accumulation.
Dust Trak monitoring has been carried out at four fixed sites, and the results are shown in Table 1. All the sites recorded elevated values during the February 4th and 5th ash fall events. Rain overnight on the 6th lowered values slightly. However, PM10 levels were greater on the 7th than during the actual ash fall event, due to resuspension of the ash. Ashy conditions in the north were perceived to be worse than the south but the data show similar or worse PM10 levels in the south. This perception may be due to the "openness" of the north, where the haze can be seen over a larger distance, compared to further south. The commonly raised 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.
Table 4 : PM10 (concentration in mg/m3)
Site 1-Feb-98 2-Feb-98 3-Feb-98 4-Feb-98 5-Feb-98 6-Feb-98 7-Feb-98 Mango 0.021 0.019 0.014 0.153 0.123 0.054 0.266 CPS 0.024 0.03 0.029 0.265 0.38 0.084 0.425 St. Peter's 0.023 0.074 0.084 0.041 0.212 MVO north 0.021 0.032 0.009 0.048 0.037 0.112 Site 8-Feb-98 9-Feb-98 10-Feb-98 11-Feb-98 12-Feb-98 13-Feb-98 14-Feb-98 Mango 0.191 0.032 0.031 0.022 0.017 0.021 0.015 CPS 0.242 0.062 0.062 0.055 0.039 0.051 0.018 St.Peter's 0.111 0.029 0.023 0.017 0.003 0.01 0.016 MVOnorth 0.059 0.027 0.016 0.01 0.005 0.007 These values are approximate 24 hr averages of the PM10 concentration 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 Cheapend, 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 on Mango Drive in Woodlands.Note: Not all averages are 24 hour.
In addition to the daily averages at the permanent sites several other Dust Trak samples have been taken (Table 5).
Table 5 : PM10 (concentration in mg/m3)
Date Sample Description Duration Average (mg/m3) 4-Feb-98 Olveston 40 mins 0.451 Woodlands 30 mins 0.136 Palm Loop 30 mins 0.165 Fogarty 20 mins 0.563 Davy Hill 20 mins 2.18 5-Feb-98 Scientist 1 hr 30 mins 0.981 6-Feb-98 Salem Checkpoint 40 mins 0.091 6 Feb 98 CPS-Principal's Office 24 hrs 0.081 7 Feb 98 CPS-Principal's Office 24 hrs 0.232 8 Feb 98 CPS-Principal's Office 24 hrs 0.128 9 Feb 98 CPS-Principal's Office 24 hrs 0.094 10 Feb 98 CPS-Principal's Office 24 hrs 0.07 12 Feb 98 PWD Engineer 1 hr 20 mins 0.061
Two temporary sites were set up for 24 hour monitoring at Brades Primary School (BPS) and at Lookout Secondary School (LSS). Results are listed below
Table 6 : PM10 (concentration in mg/m3)
Date 10-Feb-98 11-Feb-98 12-Feb-98 13-Feb-98 14-Feb-98 BPS 0.108 0.046 0.032 0.037 0.014 LSS 0.009
Several Cyclone samples were taken during the reporting period, but results are not available as yet.
MVO Staff Changes
Arrivals:
Brian Baptie (British Geological Survey)
Sue Loughlin (British Geological Survey)
Departures:
Simon Young (British Geological Survey)
Visitors:
Christian Antenor-Habazac (Guadeloupe Volcano Observatory)
Paul Cole (Luton University)
Mahar Lagmay (Cambridge University)
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)