Montserrat Volcano Observatory

Daily Report
Report for the period 16:00 16 September
to 16:00 17 September 1996

Activity at the Soufriere Hills volcano during this reporting period has been a higher level than yesterday. Volcano-tectonic earthquakes, large rockfalls and pyroclastic flows were the most dominant seismic signals with ashfalls affecting Plymouth and its environs.

There were 45 volcano-tectonic earthquakes recorded during the 24-hour reporting period, a reduction in numbers compared to yesterday. These occurred in three short swarms separated by periods of high to intense rockfall activity. The first VT swarm occurred between 19:10 and 20:10 yesterday (16 September); the second between 02:35 and 03:30 this morning(17 September) and the third between 09:45 and 11:10 late this morning (17 September). These swarms are similar to those seen over the previous several weeks and were located beneath the crater at shallow depth (above 2 km). Rockfall activity increased markedly today with three periods of near-continuous activity being experienced. These were all preceded by short VT swarms and occurred between 20:10 and 23:30 last night (16 September); 02:50 and 05:40 early today (17 September) and from 11:30 this morning to present. The period of activity which begun at 11:30 today has been the most intense during the reporting period. Activity begun with a series of small to moderate sized events which became progressively larger and eventually developed into a sustained period of near-continuous rockfall and/or pyroclastic flow which has continued to the end of the reporting period. At least five of these flows extended as far as the new delta at the mouth of the Tar River valley and a few flowed over the sea for about 5-10m.

Five small hybrid and seven long period earthquakes were also recorded; both levels were similar to the last reporting period. Tremor levels have been variable throughout the period being continuous and broadband in nature during the heightened periods of rockfall activity.

Visibility was generally poor for most of the day with low cloud cover obscuring the summit and flanks of the volcano. Excellent views were obtained of the rockfalls and pyroclastic flows which developed in the Tar River valley during the afternoon period. The debris involved in these events appear to have originated from the area on the eastern flank of the dome which have in previous reports been noted to be over steepened. The flows developed as small discrete events which during the course of the day extended further down the Tar River valley. The method by which the events are generated continue to be gravity collapse due to over steepening of the growing lava dome. Ash clouds associated with the rockfall and pyroclastic flow activity produced ashfalls in the area of Plymouth and its environs during most of the afternoon period.

No EDM or GPS measurements were completed today due to the very dusty and overcast conditions which prevailed for most of the day

The heightened period of rockfall and pyroclastic flow activity which begun at about 11:30 has generated a large amount of ash in the areas of Plymouth and its environs. Drivers are therefore urged to exercise extreme caution and give due consideration for other road. All indications are that the pyroclastic flows will continue to be confined to the Tar River valley area. Areas affected by associated ashfalls will depend on the direction and strength of the wind at the time. Dust masks should be worn at all times in ashy environments.

The intense period of pyroclastic flow which has occurred today underscores the fact that the Tar River Valley and surrounding areas are extremely hazardous, and should not be entered under any circumstances. We urge individuals who persist in ignoring this advice to think very seriously before making trips to these potentially deadly zones.

Montserrat Volcano Observatory