Montserrat Volcano Observatory

Daily Report
Report for the period 4 pm 19 September
to 4 pm 20 September 1997

The level of volcanic activity over the last 24 hours has continued at the same relatively low. The period between 6 pm and 10 pm was dominated by rockfalls and small pyroclastic flows. Activity then diminished to isolated rockfalls until 4 am when the activity dropped to a low background level. Seismic activity increased from 5:30 am onward with a number of volcano-tectonic and hybrid earthquakes. The largest peak of activity of the day began just before 11 am when a series of pyroclastic flows developed in Tuitt's Ghaut. This lasted for about 1 hour after which the remainder of the day has been quiet with only isolated small rockfalls.

The short-period seismic network has been triggered by 10 hybrid earthquakes, 9 volcano-tectonic earthquakes, 14 long-period earthquakes and 22 rockfalls. The VT events occurred in a small low intensity swarm between 5:21 am and 5:55 am. These were followed by a small low intensity swarm of hybrid earthquakes between 6:23 am and 8:07 am. The main period of rockfall and pyroclastic flow activity began at 10:59 am and was the result of a small collapse of part of the dome above Tuitt's Ghaut. This activity lasted for approximately 1 hour after which activity has bee at a low background level.

A GPS survey has been carried out today on the sites at Harris Lookout, White's Yard, Long Ground and O'Garra's. The computational reduction of the data is currently underway.

The mini-COSPEC has been deployed today with the kind assistance of the Royal Montserrat Police Force. A series of traverses under the plume were made in the Police launch. This work was forced to end prematurely due to ashfall from pyroclastic flows. The results from today will be presented in a future report.

Visibility has been poor today due to low cloud on the volcano. A reconnaissance trip was made to the area around Bramble Airport in preparation for installing new EDM reflector targets to enhance the ground deformation monitoring program. A number of new sites were identified and installation will begin as soon as the new targets arrive on island.

The area above Tuitt's Ghaut continues to be the main active area on the dome at the moment and rockfalls have been confined to this region. Pyroclastic flows were observed extending 1.5 km away from the crater rim this afternoon. However, visual observations confirm that there is a large overhanging mass above the Gages valley which appears highly unstable. The instability and volume of dome material mean that the probability of further larger collapse episodes remains relatively high. Large pyroclastic flows generated on either the northern and western sides of the dome are likely to reach the Belham valley. The surge component of these flows could reach areas such as Frith, Old Towne, Salem and Olveston. Such flows and surges could occur with no warning and so people still living in these areas are urged to move north as soon as possible.

In addition to generation of large pyroclastic flows, explosions could also occur. If an explosion occurs, small rocks and ash can fall anywhere on the island. People should seek shelter under a strong roof as soon as possible. Helmets or other head protection should be used and it should be remembered that ash and falling rocks make driving hazardous.

Montserrat Volcano Observatory