Montserrat Volcano Observatory, Montserrat, West Indies

Scientific Report 15
08 May 1996


Activity at the Soufriere Hills Volcano between 02 and 08 May remained at about the same level as the previous week. The dome continued to grow, and produced a number of small rockfalls which were confined to the area around the dome. Many of these produced small ash clouds. Seismic activity was dominated by broadband seismic tremor and rockfall signals. There was, however, a brief reappearance of the intense hybrid seismicity on 02 and 03 May. Ground deformation measurements indicate that there is still only slow deformation of the volcano. COSPEC measurements show that the SO2 flux remains very low, at around 100 tonnes per day.

Visual observations

Visibility was very poor during this period and observations were only possible up until 05 May. The dome continued to grow during the period it was visibile. Growth appeared to become concentrated in the upper portion of the dome, focussed around the summit. This resulted in an apparent rapid increase in height - estimates for the height to the top of the dome were 898 m (2947 ft) on 02 May, 909 m (2983 ft) on 03 May and 907 m (2978 ft) on 04 May. Initial measurements of the dome volume indicate that the extrusion rate has remained constant.

Rockfall activity was concentrated mainly on the northeastern and eastern flanks of the dome, none of which travelled far from the dome. Many of the rockfalls were associated with small ash clouds which were blown on the wind to the west, resulting in the deposition of very small amounts of ash in the Upper Gages and Amersham areas.

Steam emissions were observed from many parts of the dome. There were near-continuous steam emissions from the eastern part of the dome on the morning of 04 May.


The seismicity at the Soufriere Hills Volcano throughout the week was dominated by the low-amplitude broadband tremor recorded at stations close to the dome and small-sized rockfall signals. The table below lists the daily counts of the different types of event recorded - volcano-tectonic, long-period, hybrid, and rockfalls. It also includes an estimate of the amount of seismic tremor.

Earthquake types 02 - 08 May 1996

Date           VT        LP        Hybrid    RF        Tremor amount
02/05          0         32        52        46        Intermediate
03/05          1         2         345       50        Intermediate to high
04/05          0         5         11        27        Intermediate
05/05          0         11        1         67        Intermediate to high
06/05          0         2         6         55        Intermediate
07/05          0         7         5         50        Low
08/05          0         21        5         64        Low

At the start of the week, the tremor was near-continuous. At 17:00 on 02 May, it became more intermittent and slightly smaller in amplitude. The small hybrid earthquakes that had dominated seismicity up until 27 April reappeared at about the same time, gradually increasing in number from one every two or three minutes to peak at about one every minute by mid-afternoon on 03 May. There was a period of near-continuous tremor from 14:30 until 16:00 on 03 May. The hybrid seismicity quickly declined after that and only a few hybrid events were recorded during the rest of the week. Intermittent low-amplitude broadband tremor was recorded up until 06 May. This then declined in amplitude until there was very little by the end of the week.

Some small long-period earthquakes were recorded during the week. The numbers varied, but there were significantly more of the long-period events on 02 and 08 May.

Rockfall events were recorded at a relatively constant rate throughout the week. The largest of these were at 15:12 on 02 May and 13:11 on 05 May.

Only one VT earthquake was recorded, at 15:38 on 03 May. This was too small to be located.

Ground Deformation

The northern and eastern triangles of the EDM network were measured during the early part of the week. No EDM measurements were possible after 05 May due to the poor visibility. The data continues to show only very small changes, suggesting very slow deformation of the volcanic edifice.

GPS measurements were made on lines in the MVO network north, south and east of the volcano. The results of these also show only small movements, within the resolution of the technique.

GPS data was collected on the University of Puerto Rico network at the St. George's Hill, Ried's Hill Estate, Tar River Estate, and Radio Antilles stations. These stations had last been occupied by the UPR group on December 30, 1995. Between October and December 1995, the deformation rates on these baselines were highest on the Tar River to Reid's Hill line, but also detectable on the Tar River to St. George's Hill line. The rates observed between December 1995 and May 1996 are lower, indicating a slowing of the deformation rate by between 30 to 50%. The total displacement on these baselines is a shortening of 1 cm, on the Tar River-St. George's Hill and St. George's Hill-Radio Antilles lines, which is at the level of detection in the preliminary analysis. No detectable change was found on the Reid's Hill-Tar River or Reid's Hill-St. George's lines. Further processing will improve the precision by about a factor of 2.

Gas Measurements

SO2 flux measurements were made daily using a COSPEC mounted in a car and driven underneath the plume at a constant speed of approximately 20 km/hr. The estimates of the SO2 flux since the start of the COSPEC program are listed in the table below. No measurements were possible on 07 and 08 May due to rain. The results show some variability but are consistently low at around 100 tonnes per day.

Date      Number    Mean SO2 flux  Standard
          of runs   (Tonnes/day)   Deviation
28/04          4              26        5
29/04          3              86        10
30/04          0              -         -
01/05          5              97        29
02/05          3              177       29
03/05          5              89        11
04/05          5              76        17
05/05          3              54        10

Montserrat Volcano Observatory