Summary
The level of seismic and volcanic activity during the reporting period was very low. There was no evidence for dome growth and rockfall activity was minimal. Deformation of the northern and eastern flanks continued.
Visual Observations
The appearance of the dome did not change much throughout the reporting period apart from a slight degradation of the upper flanks and the continued development of small rockfall chutes on the eastern and south western flanks of the dome. There were few rockfalls during the reporting period and most of these were small and caused by degradation of the dome. The disintegration of a steep rocky buttress in the dome complex above the Tar River valley produced several small pyroclastic flows. The largest of these was observed by a field team at Windy Hill, it travelled down a narrow ravine and reached as far as the Tar River Estate house. There were also some small rockfall deposits below the area of fumarolic activity on the eastern flanks of the dome complex. Ash venting was absent throughout this period.
Seismicity
Seismic activity over this period remained at a very low level. Numbers of recorded earthquakes were low. Epicentral solutions suggest that all the earthquake activity is still concentrated below the area of the dome. Similarly there were very few dome rockfalls and the size of these events was small.
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 Mar 98 6 1 - 3 - - 16 Mar 98 1 2 - 8 - - 17 Mar 98 5 11 - 12 - - 18 Mar 98 11 3 - 4 - - 19 Mar 98 1 7 1 10 - - 20 Mar 98 - 5 1 7 1 - 21 Mar 98 4 1 - 7 - - 22 Mar 98 4 5 - 5 - - 23 Mar 98 6 6 1 8 - - 24 Mar 98 8 12 3 3 - - 25 Mar 98 2 2 - 4 1 - 26 Mar 98 1 1 - 5 - - 27 Mar 98 1 - - 1 - - 28 Mar 98 - 3 - 2 - -
*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.
Ground Deformation
GPS occupations of Blakes, Drummonds, Old Towne and Dagenham were made during the reporting interval, running the new base station at Harris. The sites appear stable with respect to Harris. Data from the University of Puerto Rico station at Hermitage show that that site is still moving to the north east.
The new reflector on Peak B was shot four times during the reporting interval, although the line continues to shorten (8cm total shortening since January 25th 1998), the last two measurements suggest the shortening rate may be slowing slightly.
Volume Measurements
There are no volume measurements to report for this time period. There have been no observations to suggest that the dome is still growing, and therefore we can assume that growth has ceased for the time being.
Environmental Monitoring
The air quality monitoring programme during this period involved 5 permanent monitoring sites and one temporary site. These were located at:
Table 2 : PM10 (concentration in mg/m3)
These values are approximate 24 hr averages of the PM10 concentration. Aerosol values measured at each of the monitoring sites are accompanied by a note on the level of volcanic activity (VA) and prevailing weather conditions.
Date 1 2 3 4 5 6 7 VA wind rain other 15-Mar - 75 63 54 83 36 - Low 320 Breezy Dry 16-Mar 75 64 60 76 44 35 - Low 290 Breezy Dry Holiday 17-Mar 65 68 61 81 38 31 - Low 300 Breezy am Holiday 18-Mar 29 54 24 36 17 48 13 Low 300 Breezy Periodic 19-Mar 18 23 15 31 8 33 7 Low N-S Gusts am,eve 20-Mar 20 23 16 45 15 29 7 Low Light Night 21-Mar 19 25 16 17 15 26 7 Low 280 Moderate Night 22-Mar 24 32 21 25 - 29 12 Low 300 Gusty dry 23-Mar 21 24 18 52 - 30 22 Low 290 Breezy am 24-Mar 50 44 43 70 - 56 - Low 310-320 Breezy am 25-Mar 20 27 17 39 - 27 16 S 280 Light dry 26-Mar - 10 8 37 - 21 8 Low N-S Light Periodic 27-Mar - 16 14 42 - 25 13 Low 300 Moderate pm 28-Mar - 23 17 22 - 29 18 Low 300 Strong Night 29-Mar - 18 16 19 - 24 17 Low 320 Light pm
S = increase in volcanic seismicity overnight.
Low volcanic activity and occasional rains have generally kept aerosol levels low. Although volcanic activity was low, there was a small increase in volcanic seismicity between 10pm on 23rd March and 4am on 24th march. This coincides with comparatively higher aerosol concentrations on Tuesday 24th march.
24 Hour averages: <50 mg m-3 Low 50-100 mg m-3 Raised 100-300 mg m-3 Very High >300 mg m-3 AlertMVO Staff Changes
Departures:
Richard Luckett (British Geological Survey)
Visitors
Peter Day (British Geological Survey)
Dawn Pirie (Edinburgh University)
Dave Stewart (British Geological Survey)