Montserrat Volcano Observatory, Montserrat, West Indies

Scientific Report 47
21 December 1996

This report covers the 7-day period to 24:00 on 21 December


Activity during this week was concentrated at the recently extruded December 11 dome, then switched to the October 1 dome at the end of the week. Rapid extrusion of the December 11 dome and intrusion directly beneath it culminated, on 19 December, with a four-hour series of pyroclastic flows which almost reached the sea. These were the largest flows since those which led to the explosion on 17/18 September. Almost immediately after these flows, there was a period of large shallow VT earthquakes and then volcanic activity switched to the October 1 dome.

Visual observations

The week started with continued growth of the December 11 dome. Growth had been rapid since the dome was first extruded and by 15 December, the top was higher than the October 1 dome, perhaps 910 m above sea level. Growth appeared to be along a linear trend striking east-south-east. Rapid growth continued on 16 and 17 December, when it had overflowed the scar left by the 17/18 September explosion. At this stage the dome's shape was conical with a flat top and it had two small spines on the top. There were several small pyroclastic flows on 17 December which travelled to the south side of Castle Peak. The maximum runout of these was 250 m. Visibility was poor on 18 December, but rockfall activity seemed to have declined slightly. Several large steam clouds were generated during the afternoon, containing only a small amount of ash.

On 19 December, the dome seemed to have changed little in shape from the previous day. The eastern face of the dome was near-vertical and appeared very unstable. The dome was at its most active during the morning of 19 December when there were discrete pulses of rockfalls and small pyroclastic flows a few minutes apart. The largest of these had a maximum runout of 1 km. There were two periods of near-continuous pyroclastic flow generation at about 8 am.

There was major pyroclastic flow activity into the Tar River valley late in the afternoon of 19 December, which started at 5:06 pm. The collapses were from the unstable eastern side of the December 11 dome. The flows were channeled to the south of Castle Peak down the Tar River valley and travelled onto the new fan at the base of the valley, to within 40 m of the sea. Activity built up rapidly to a peak just before 6 pm. The ash clouds reached 10,000 ft above sea level and were carried to the south west by the light winds. Observations by MVO personnel at the airport suggested that fresh lava was being supplied to the dome as quickly as it was being lost in pyroclastic flows. This observation was supported by rock samples collected from the flows at a later date; they contained abundant juvenile pumice with very little dense juvenile material, and were markedly different from all previous flows from the dome. Helicopter inspections the next day showed some of the pre-September dome was also involved in the collapses.

The rapid escalation of activity led to a Red alert first being advised by MVO and then declared by the authorities. There was concern that the pyroclastic flows might trigger a vertical explosion.

The pyroclastic flow activity did not escalate but stopped after about four hours. No further volcanic activity was observed on the December 11 dome.

On 21 December, there was a reactivation of growth of the October 1 dome. There were numerous small rockfalls from this dome, some of which generated small ash clouds. Most of the rockfalls were on the northern side of the dome, although there were some smaller rockfalls from the southern side. At about 6pm there were 2 small pyroclastic flows from the October 1 dome, which travelled to just short of the Tar River Soufriere.


Table 1 Earthquake types: 15 to 21 December 1996
These earthquake counts are of events that triggered the short-period seismic network event recording system between 0000 and 2400 each day.

Date		VT	LP	Hybrid	Dome RF	Galways Wall Landslides

15 DEC 96	0	7	0	4	0
16 DEC 96	0	14	0	16	0
17 DEC 96	0	15	0	26	0
18 DEC 96	0	2	0	14	0
19 DEC 96	0	15	0	21	0
20 DEC 96	81	1	0	12	2
21 DEC 96	46	7	0	20	2

Up until 19 December, seismicity was dominated by dome rockfalls and long-period events. The counts of these were relatively low. The rockfall activity did not vary during the week and was associated with activity on both the December 11 dome and October 1 dome. The numbers of long-period earthquakes did vary, but this is not felt to be significant. The newly installed broad-band system detects more long-period earthquakes than the short-period network. There seems to have been a slight increase in the number of long-period earthquakes in the 8 hours prior to the 19 December pyroclastic flows.

The pyroclastic flow activity generated continuous seismic signals that are best analysed using RSAM data, which records the average signal amplitude in 10-minute windows. Figure 1 shows the RSAM data for two-day periods spanning the pyroclastic flow activity on 19 December and previous large pyroclastic flowss on 17 September, 29 July and 12 May. The data is taken from the low-gain short-period station at Long Ground, where the signals do not clip and thus allow comparison of the relative amplitudes of the signals. Note that the amplitude scales in this figure for the 12 May and 19 December data are smaller, by a factor of about 8, than for the other two periods.

Figure 1 shows that for the 19 December pyroclastic flows, the activity started very quickly from background and had reached its peak within two hours. Activity declined quickly after this and was almost over within 4 hours. There were a few smaller pyroclastic flows about 4 pm the next day. The 19 December activity is most comparable, in terms of the amplitude of the signals, with the 12 May activity. That however consisted of three discrete pulses, corresponding to visual pyroclastic flows. The 19 December activity was continuous and therefore contained a lot more seismic energy, which is given by the square of the area under the curves.

The pyroclastic flows on 17 September and 29 July produced seismic signals that were about 10 times as large, and therefore much more energetic. The 29 July pyroclastic flow started very quickly from the background noise in the same way as the 19 December flow. The 17 September pyroclastic flow was very different and was preceded by about 14 hours of pyroclastic flow activity. That activity was started by a pyroclastic flow episode which looks, on RSAM, very similar, to that on 19 December, although twice the amplitude.

There were two swarms of shallow VT earthquakes on 20 and 21 December (Table 2). The signals recorded from these events were very similar to those from the VT events that were recorded from 21 October up until 11 December - they contain significant long-period energy and would be more accurately described as hybrid events. The events in these, the 15th and 16th swarms in the sequence, had locations similar to those in previous swarms - beneath the crater at shallow depths (0 - 2 km). The first of the two swarms was the more intense, and had event rates similar to the most intense period of activity in the first week of December (swarm 13 - see scientific report 45). The sizes of the largest events in swarm 15 were comparable to those in the most intense periods and may have been as large as M 3.0. For the first five hours of the second swarm, the activity was only slightly less intense than the first. The rest of the swarm was, however, much less intense and the events were noticeably smaller.

Table 2 VT swarms

Swarm	Start	      End		Duration	No. quakes/hour
															(hours)		No.	Mean	Peak

15	12:21 20/12 20:24  20/12    	8.0		81	10.1	19
16	03:45 21/12 17:52  20/12	14.1		46	3.3	10

These swarms seem to have been associated with the reactivation of the October 1 dome, and occurred after the pyroclastic flows and cessation of activity from the December 1 dome. It is interesting to note that the only Galways Wall landslides recorded by the seismic network this week were during these earthquake swarms. This confirms the positive correlation between Galways Wall avalanches and VT swarms. The exact mechanism for this is not yet known, but is probably a combination of an intrusion deforming the wall and the intense VTs shaking it. The latter has already been confirmed by observers who have seen avalanches almost immediately after felt VT events.

Ground deformation

Electronic Distance Measurements (EDM) to the Chances Peak reflector were made on 16 December. This showed no significant change since it was last measured on 4 December. The EDM lines to Castle Peak were successfully shot on 18 December, and showed only 3 mm of shortening since last measured on 10 December. This is a very low rate. The EDM lines on the north flank of the volcano were also shot on 18 December and showed no significant changes since last measured on 5 November.

No GPS measurements were carried out this week.

Dome Volume Measurements

Dome growth during this reporting period was highly variable in location, style and rate. Complete dome surveys were carried out on 16, 17 and 19 December, concentrating on different aspects of this growth. The survey of the emerging December 11 dome on the 13 December showed an initially high extrusion rate consistent with visual observations at that time (~3 cubic metres per second). Points measured on the pre-existing (or pre-September) dome during this survey showed the area near the dome to have been uplifted near the December 11 dome. Consequent surveys on the 16th and 17th of December showed that this area of uplift was focused ~150m south of the December 11 dome, located in the pre-September dome closest to the Galways Wall. This uplift reached a maximum of 100 m in this region attenuating to <20m beyond 200m from this focus. This uplift could be accounted for by the introduction of 6 million cubic metres of new material into this area as an intrusion.

The survey of the 13 December showed the December 11 dome to have a volume of 500,000 cubic metres approximately 2 days after first extruding . A second survey of the December 11 dome on the 19 December produced a volume of 0.95 million cubic metres (0.8 million cubic metres of dome rock plus 0.15 million cubic metres of talus) indicating an appreciable lowing of extrusion rate during this period (0.9 cubic metres per second). No evidence of dome growth on the December 11 dome was noted following the collapses of 19 December.

Gas Measurements

No gas measurements were possible, as the COSPEC instrument is awaiting repair.

Environmental Monitoring

The routine collection of air and water samples down-wind of the volcano continues.

Other Work

A programme of gravity measurements was started on 19 December. Gravity stations will be re-measured, along with GPS locations, on a network of gravity stations established in July - August 1996. This should allow any density or mass changes beneath the volcano to be detected. The first stations measured were along a radial line on the eastern flank of the volcano, from Whites to Hermitage Estate. Some stations in this line could not be reoccupied since they were destroyed by the September 17/18 explosion. Initial processing of the data shows no significant changes since the last measurements in late July. Stations on the upper slope do show some changes, but these are felt to be consistent with the mass that has been added to the dome in that time.

A continuous-recording gravity station was deployed at Whites on 18 December, co-located with the permanent GPS station.

The cracks in Chances Peak were measured on 15 and 21 December. These showed a reduction in the deformation rate, consistent with the reduction in avalanches from the wall. An extensometer and a pair of orthogonal tiltmeters were installed over one of these cracks on 21 December. These transmit their data by radio to MVO and should reduce the need to visit Chances Peak. The extensometer measures changes in the width of the crack and the tiltmeters will detect movement on the Galways side of the crack.

Staff Changes
Mark Davies, BGS
Rick Hoblitt, USGS
Randy White, USGS
Mark Stasiuk, BGS
Simon Young, BGS

Steve Sparks, BGS
Jean-Christophe Komorowski, IPGP France
Richard Herd, BGS (short break)
Angus Miller, BGS (short break)

Anthony Langlais, Gaudeloupe Observatory

Montserrat Volcano Observatory