Activity at the Soufriere Hills volcano during this reporting period has been at a somewhat lower level than over the past few days, although dome growth continues and associated rockfalls have increased in number.
The seismicity today has been dominated by rockfall events, with a significant lowering of the number of volcano-tectonic (VT) earthquakes. Thirty two (32) VTs were recorded during the 24-hour reporting period, which is much lower than the total recorded yesterday. These occurred in a single short swarm between 10:30 and 12:20 today (13 September). These earthquakes were again located at shallow depths (less than 2 km) beneath the volcano. A total of 129 rockfall events were recorded today, the largest occurring at 16:39 yesterday and 13:50 and 15:28 today. Twenty five small hybrid and twelve long period earthquakes were also recorded, both levels similar to the last reporting period. Broadband tremor at a very low amplitude was recorded on the Gages seismometer throughout the reporting period.
Visibility was poor again today, with only the lower flanks of the dome periodically visible from the east side. Some rockfalls were observed on the eastern flank of the dome during the day and one small pyroclastic flow was seen to reach the Tar River Soufriere around midday. The largest event during this reporting period was at 16:39 yesterday (12 September), when a moderate-sized pyroclastic flow reached almost to the Tar River Estate House. This flow moved down the most northerly channel in the Tar River Valley and, were it very much bigger, would have inundated the Estate House and areas close by. Ash clouds were produced by the bigger rockfalls and flows; these were blown slowly westwards by light winds, depositing some ash over the Plymouth area.
EDM measurements were made today on the O'Garra's - Galway's - Chance's Peak triangle. The Galway's to Chance's Peak line showed a shortening of 17 mm over the past 20 days, which is consistent with the rate over the past few months. A GPS survey commenced today on a newly instigated northern triangle. Results will not be available until after the triangle is re-occupied early next week.
Further rockfalls and pyroclastic flows will occur as the lava dome continues to grow within English's Crater and its unstable flanks undergo partial collapses. Current indications are that the pyroclastic flows will 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, and drivers should exercise caution and consideration for other road users.
The Tar River Valley and surrounding areas continue to be 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. The pyroclastic flow of yesterday afternoon came without warning and moved at between 40 and 70 miles per hour. Had it been slightly bigger, it would certainly have killed anyone in the vicinity of the Tar River House.