Activity over these two weeks has decreased. The cyclic pattern in tilt, seismic and pyroclastic flow activity has largely disappeared, and the dome now appears to be growing at a lower rate than immediately after the 25 June event. The activity at the end of the reporting period was characterised by low numbers of rockfalls from all sides of the dome.
Due to the inaccessibility of many of the routine monitoring points as a result of the recent activity, a new GPS and EDM network has been established, a new station to make static COSPEC measurements has been identified, and new positions to take static dome photographs have been chosen.
This reporting period was relatively quiet, with only minor rockfalls travelling ( 500 m from the western and northern faces of the dome. A brief glimpse of the dome on the night of 6 July revealed incandescent rockfalls above Mosquito Ghaut and Gages Valley. A partial view during the morning of 7 July showed a steep-sided mass of post-25 June growth above Mosquito Ghaut and Gages Valley with a broad, relatively flat summit area.
On 8 July a period of fairly intense ash emissions began which persisted through 13 July, and often coincided with the peak of the tilt cycle. Some of these events preceded small pyroclastic flows into Mosquito Ghaut and Gages Valley. The largest of these flowed into Gages Valley (1.0 km long flow on 9 July; 1.5 km long flow on 13 July). The ash columns were fairly dilute and reached heights of no more than 10,000 ft before dissipating. When viewed from the air, the ash appeared to be emanating from the west side of the post-25 dome above Gages Valley.
On 13 July the clouds cleared sufficiently for theodolite measurements to be made of points on the northern and western section of the dome. The highest point of the pre-25 June dome was found to be 950 m above sea level (ASL). The new growth nested in the 25 June scar was fairly lopsided; it was 941 m ASL above Mosquito Ghaut and gradually rose to the west. Above Gages Valley, the new dome juts upward in a steep-sided spine. Visibility deteriorated before an exact height could be obtained, but it is estimated that the spine rose ~50 m above the rest of the new growth.
The period between 14 July and 19 July was relatively quiet with only minor rockfalls (( 500 m long), occurring predominantly in Gages Valley and Mosquito Ghaut, but also in the Tar River Valley, Tuitts Ghaut, and over Galways Wall. There was an unusually good interval of visibility on 17 July, which allowed scientists to take static photographs of the dome for volume calculations. Theodolite measurements were also made. The high point on the old dome (NE) measured 946 m ASL, and the high point on the post 25 June dome 957 m ASL. The spine above Gages valley observed on 13 July was no longer present.
For the first three days of this reporting period the previous pattern of hybrid swarms occurring every 8 hours continued. These swarms were not associated with any large-scale pyroclastic flows. On the 8 July the hybrid swarms stopped and seismicity from that day onwards has been dominated by rockfall signals. These rockfalls exhibited no particular periodicity and had an approximately steady amplitude throughout the two weeks. Some long period and hybrid events were recorded but such activity has been at a very low level.
Table 1: Earthquake types
These earthquake counts are of events that triggered the broadband network's event recording system between 0000 and 2400 each day (local time).
Date VT Hybrid LP Dome RF LPRF* HYRF* 05 July 97 0 86 1 65 0 0 06 July 97 0 46 1 26 0 0 07 July 97 0 89 0 61 1 0 08 July 97 0 3 0 27 0 0 09 July 97 0 0 0 81 0 0 10 July 97 0 3 0 34 0 0 11 July 97 0 2 4 89 0 0 12 July 97 0 2 4 124 2 0 13 July 97 0 2 4 63 3 0 14 July 97 0 0 1 61 0 0 15 July 97 0 1 1 53 0 0 16 July 97 0 0 2 17 1 0 17 July 97 0 2 3 15 1 0 18 July 97 0 0 2 34 1 0 19 July 97 0 0 2 29 1 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.
GPS occupations of the new network, LEESNET, (6 and 10 July) and EASTNET (16 July) were carried out. Baseline data are still being collected for LEESNET and it is likely to be some time before stable trends can be determined. The occupation of EASTNET (stations at Harris, Windy Hill, Whites and Long Ground) indicated some significant movement. The baseline M18-Whites had shortened by 16 mm since the last measurement on 24 June and by 31 mm from its long term mean (2 sigma error on the average =3D 9 mm). The line to Long Ground showed continued shortening and the line between Long Ground and Windy Hill showed slight lengthening. All the changes were consistent with current trends although at slightly higher rates, and will require a further occupation of the network to confirm the changes.
The Chances Peak tiltmeter showed a gradual decrease in the rate of subsidence of the x-axis (oriented towards Galways wall) through the reporting interval. Superimposed on this trend were periods of cyclical inflation and deflation. Until 10 July, cycles lasted for 8 to 16 hours and were associated with hybrid swarms. The last four days of the reporting period included tilt cycles of much lower amplitude and longer periodicity (30 hours).
The volume of the dome on 17 July was estimated to be 75.5 million cubic metres using static photos from Whites, Harris and Windy Hill. The total inventory of pyroclastic flow deposits on this date was estimated to be 55.05 million cubic metres (DRE). The previous dome volume estimate on 31 May was 64.6 million cubic metres, with a deposit volume of 43.0 million cubic metres. The average growth rate between 31 May and 17 July was 5.2 cubic metres per second (DRE), and visual observations suggest that in the period following 25 June the growth rate was significantly higher.
Rainwater samples were taken at three points around the volcano on 9 July and the results are shown in Table 2. All samples which are to the north of the volcano have very low pH. This is probably due to northerly wind direction on 8 July during heavy rain. Residents in the north of the island reported unusual sulphurous smells and light ashfall at this time.
Table 2 Rainwater geochemistry, 9 July 1997
Location pH Cond. TDS Flrd Chlrd Slpht mS/cm g/l mg/l mg/l mg/l Lawyers 3.95 0.074 0.036 0.30 11.2 nd Hope 3.79 0.133 0.066 >1.50 20.5 5 MVO 4.16 0.070 0.035 0.30 11.5 nd
nd: not detected
The sulphur dioxide flux from the volcano was measured on 17 July by static scanning of the plume from Garibaldi Hill. The measured flux averaged approximately 200 tonnes/day for 10 scans. This method is an extremely quick way of measuring sulphur dioxide flux, and will be used in the future for routine monitoring of gas emissions.
No ash was collected during this reporting period.
Mr. Chan Ramsingh (Seismic Research Unit, University of the West Indies, Trinidad)
Mr. George Skerritt (MVO)
Dr. Willy Aspinall (independent, British Geological Survey)
Mr. Wilkie Balgobin (Seismic Research Unit, University of the West Indies, Trinidad)
Ms. Joan Latchman (Seismic Research Unit, University of the West Indies, Trinidad)
Dr. Anne-Marie Lejeune (Bristol University)
Dr. Rick Hoblitt (Cascades Volcano Observatory, US Geological Survey)
Mr. Matt Watson (Cambridge University)
Professor Steve Sparks (Bristol University)
Dr. Glen Mattioli (University of Puerto Rico)
Dr. John Shepherd (Lancaster University)
Dr. John Bennett (British Geological Survey)
We currently have 2 Montserratian students working at MVO whilst on vacation from university
Levar Cabey (Lancaster University)
Graham Ryan (Lancaster University)
MVO scientific staff (at 19 July 1997)
Chief Scientist: Mr. Richie Robertson (Seismic Research Unit, University of the West Indies, Trinidad)
Deputy Chief Scientist: Dr. Gill Norton (British Geological Survey)
Dr. Keith Rowley (independent, British Geological Survey)
Dr. Maggie Mangan (Hawaii Volcano Observatory, US Geological Survey)
Dr. Brian Baptie (British Geological Survey)
Dr. Richard Luckett (British Geological Survey)
Mr. Chandrapath Ramsingh (Seismic Research Unit, University of the West Indies, Trinidad).
Mr. Rob Watts (independent, British Geological Survey)
Ms. Amanda Clarke (Penn State University)
Ms. Chloe Harford (Bristol University)