Activity during September was low. Occasional pyroclastic flows are interpreted as gradual degradation of the lava dome. No new magma was extruded. Periods of vigorous ash venting, particularly towards the end of the month, correlated with low amplitude seismic tremor and increased gas fluxes. The passage of Hurricane Georges on 20/21 September produced large mudflows down all flanks of the volcano, inundating the Belham Bridge and depositing much sediment and debris on the airport runway and in Plymouth.
Three periods of pyroclastic flow generation occurred during September. On 7 September at 20:03, small pyroclastic flows travelled down the Tar River to the east of the volcano. A dense dark ash cloud was seen drifting westwards after the event. An observation flight the following day showed that the material originated in the back of the scar from the July 3 event. Some small pyroclastic flow lobes were evident in the scar and at the mouth of the scar. The main deposit formed a broad tongue of material to the old coastline, and consisted of some blocky, altered material. Towards the end of the deposit, finer grained facies predominated. Approximately 1 - 2 mm of ash fell on areas south of the Delta Gas Station in Plymouth.
On 19 September at 11:20, a further pyroclastic flow occurred: on this occasion the flow travelled down Mosquito Ghaut on the northern flank of the volcano, with the ash cloud once again drifting westward.
On 24 September at 14:18, a pyroclastic flow occurred in Gage's valley. The initial ash cloud from the upper part of the valley rose very quickly from the summit area before the pyroclastic flows were observed. The deposits from this event extended into the area around the former position of the Lower Gages Soufriere. Distinct flow lobes were noted in the distal portion of the deposit; the material consisted mostly of dark grey dome rock. Observations of the July 3 scar on the same day showed new deposits around the fumaroles, but these only extended just outside the limits of scar.
Heavy rain associated with Hurricane Georges on 20-21 September produced large volume mudflows down all flanks of the volcano. Visual observations from the HMS Sheffield helicopter on 21 September revealed that all of the main valleys were steaming due to water erosion into hot pyroclastic flows during the previous night. Steaming was most obvious in the White River valley, where the deposits from the 26 December 1997 eruption are still extremely hot a few feet below the surface. The Belham valley below the bridge (on the golf course) was totally inundated with water and debris. New debris several feet thick was also deposited on the airport runway and in Plymouth where a deep gully had been eroded along the course of Fort Ghaut.
The recorded levels of seismicity were low over the reporting period. Over the first 19 days of the month, the average number of earthquakes recorded each day was below 10. Seismic activity was mainly restricted to rockfalls from the surface of the lava dome. These events were mostly small, although some larger events with a duration greater than one minute were also recorded. Occasional volcano-tectonic earthquakes continued to be recorded.
The threat of the Hurricane Georges meant that nearly the entire seismic network was dismantled for a few days to safeguard equipment. For this reason, and also because of the high levels of wind and sea noise recorded on the few stations left in place, no definitive counts of earthquake types are available between 20 and 23 September.
There was a swarm of volcano-tectonic earthquakes on 26 September, which started at 10:35 local time, continued until 15:58 and contained 35 events. All the earthquakes in the swarm were small. It was not possible to locate these earthquakes because insufficient stations were operational. This swarm of earthquakes was followed, after a break of a few hours, by a period of volcanic tremor.
Volcanic tremor continued to occur almost daily, until the end of the month. Tremor duration varied between a few tens of minutes and a few hours. There was a strong correlation between the tremor and observations of ash emission from a vent on the northern flanks of the dome and also with a roaring sound, audible at several kilometres from the volcano in Salem. Spectrograms calculated for the tremor indicate the sharply peaked nature of the spectra. These peaks generally remained constant over the duration of the tremor period, but occasionally the frequency of the dominant spectral peak changed to a lower frequency as the amplitude of the tremor was observed to increase. Given the strong correlation, it seems that the venting is the most likely cause for the tremor. The vent itself may be acting as the source or it may be generating some resonance in the area of the lava dome around the vent, however this relationship is unclear. One hypothesis is that water from the recent high levels of rain is percolating through the volcanic edifice after the recent hurricane, then being boiled and subsequently released as steam.
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 01 September 98 5 12 02 September 98 4 8 03 September 98 4 2 4 04 September 98 6 9 05 September 98 4 06 September 98 1 1 1 07 September 98 4 2 3 08 September 98 6 1 3 09 September 98 14 10 September 98 2 2 11 September 98 2 3 7 12 September 98 7 14 13 September 98 3 4 14 September 98 1 6 15 September 98 5 5 16 September 98 3 4 17 September 98 5 1 10 18 September 98 9 7 19 September 98 9 3 20 September 98 limited seismic monitoring - Hurricane Georges 21 September 98 limited seismic monitoring - Hurricane Georges 22 September 98 limited seismic monitoring - Hurricane Georges 23 September 98 limited seismic monitoring - Hurricane Georges 24 September 98 5 1 3 25 September 98 5 26 September 98 37 10 27 September 98 9 11 28 September 98 7 15 29 September 98 14 22 30 September 98 13 7
Table 2. Swarms
Start Duration/min Hybrid LP VT 26 September, 10:35 323 35
All the GPS sites around the volcano and in the north of the island were occupied this month. The rates of deformation remain very low. Hermitage continues to move to the northeast at about 0.5 cm/month. Long Ground has resumed its eastward movement that occurred early in the eruption. Between June 1996 - May 1997 the site moved 3 cm in an eastward direction. During the period of elevated activity between May 1997-March 1998 it moved about 8 cm in a northeasterly direction. Since that time it has slowed down and moved about 2 cm in an easterly direction. Sites on the west of the volcano at Brodericks and Galway's show no change. The University of Puerto Rico installed a new continuous station at the observatory and a temporary station at Blake's earlier this year. Both stations initially showed southeasterly movement with respect to Harris of around 1.5 cm until early June. The movement directions reversed at this time and the sites have moved slowly to the northwest by about 2 cm.
The EDM reflector on the northern flank was shot from Windy Hill three times. The line has started to shorten again by almost 4 cm in the last month.
Sulphur dioxide fluxes were measured using the miniCOSPEC machine this month. Results are shown in Table 3. The increase in sulphur dioxide flux towards the middle of the month corresponds with observations of enhanced steam and ash venting from the dome fumaroles. On 11 September, two sets of runs were made: one from the helicopter and one from the police launch. The average fluxes from these two methods show comparable sulphur dioxide levels.
Table 3. Average daily sulphur dioxide fluxes measured by miniCOSPEC, September 1998
Date Flux (tonnes/day) Comments 4 September 210 7 September 130 11 September 290 From Police Launch 11 September 360 From helicopter 16 September 730 25 September 1340 30 September 410
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. The high levels in mid August correspond to increased steam and ash venting reported in Scientific Report 95.
Table 4. Sulphur dioxide diffusion tube results, 1998. Levels in ppb.
11 to 24-Aug-98 24-Aug to 9-Sep-98 9 to 30-Sep-98 Police HQ, Plymouth 300.8 173.8 -* St. George's Hill 19.1 25.0 13.4 Weekes 25.7 12.0 15.7 Vue Pointe Hotel 5.0 2.2 3.5 Lawyers 3.9 0.0 1.6 MVO north - 4.0 -
*: Police HQ, Plymouth was inaccessible after Hurricane Georges
MVO Staff Changes
Brian Baptie (British Geological Survey)
Anne-Marie Lejeune (Bristol University)
Godfrey Almorales (Seismic Research Unit)
Paul Cole (Luton University)
Lucy Ritchie (Luton University)
Costanza Bonadonna (Bristol University)
Desmond Seupersad (Seismic Research Unit)
Steve Sparks (Bristol University)
Sarah Dornan (British Geological Survey)
Clive Oppenheimer (Cambridge University)
Dave Pyle (Cambridge University)