There was a notable increase in surface activity at the Soufriere Hills Volcano during this reporting period. There were flash floods in Fort Ghaut and possibly Tar River, and an increase in rockfall activity. There were a number of small pyroclastic flows into the Upper Tar River area which originated from the southern, eastern and northeastern sides of the dome.
Seismic activity up until 03:00 on 11 July was at levels similar to the previous reporting period. There was a low-amplitude, long-duration signal recorded on the Gages seismograph station between 03:09 and about 04:10. This occurred after heavy rainfall and was similar to signals seen from flash flooding in Upper Fort Ghaut in the past. There was a similar signal recorded from 06:22 until about 07:10, following more heavy rain. This was larger in amplitude than the first and was recorded by the stations at Long Ground and Hermitage as well as Gages. There was probably flash flooding in both Upper Fort Ghaut and Tar River at this time. There was a lot of thick muddy water flowing in Fort Ghaut in Plymouth. The final few minutes of this signal were of high amplitude and corresponded with more intense rainfall.
There was an increase in activity at the dome following the flash flooding, with a sequence of moderate sized ash flow rockfall signals recorded over a two-hour period. The largest of these was at 07:50. These flows produced small ash clouds which drifted to the west-northwest on the prevailing wind and caused fine ashfall in areas to the north of Plymouth and out to sea. A second sequence of similar sized ashflows started at around 12:00 and lasted for about 40 minutes. These also produced fine ashfall downwind. There were further flows at 13:29, 14:12 and 14:26. Small and moderate sized rockfalls were recorded throughout the morning and afternoon. A total of 32 rockfalls were recorded during the reporting period. The counts of hybrid and long period earthquakes were low, at 2 and 4 respectively. 2 volcano-tectonic earthquakes were recorded, but these were too small to be accurately located.
Visibility was generally poor today and there were only a few partial views of the dome. During a morning helicopter inspection, it was confirmed that some of the early activity had generated small pyroclastic flows into the Upper Tar River Valley both to the south and north of Castle Peak. The fresh deposits were obvious against the old deposits, which were wet. Some of the fresh deposits were still steaming. A significant amount of material had come down the south side of Castle Peak. A number of small rockfalls were seen on the southern flank of the dome. In an afternoon flight, following the midday activity, there were more deposits from small pyroclastic flows which had come down on the north side of Castle Peak. There were also several erosion scars on the northeast flank of the dome, which was probably the source of the flows.
No EDM or GPS measurements were made today. Processing of the GPS data collected yesterday, from Tar River, Harris's, Ogara's and Dagenham, shows only very small changes, less than 5 mm, in all line lengths since the survey started on April 11. Three new gravity stations were established, at Ogara's, Spanish Point and Harris's. No COSPEC measurements of the SO2 level in the volcanic plume were carried out today.
The Soufriere Hills Volcano is still considered to be highly dangerous to people and property on it's eastern and upper western flanks. Visits to the evacuated zone must be kept to a minimum. The Tar River and Long Ground areas to the east and upper Fort Ghaut, Gages Village and Upper Amersham areas to the west are all extremely dangerous. All access roads to these areas remain closed and people should not enter these areas under any circumstances. If they do, they put themselves and others at direct risk of serious injury or death.