******************************** Montserrat Volcano Observatory Report for the period midday 22 December to midday 29 December 2006 From: Vicky Hards <vicky@xxxxxx> ******************************** Dome growth during the period appears to have undergone a marked increase in rate, and the focus appears to have shifted entirely to the western side of the dome. A high whaleback lobe directed towards the southwest was observed on 26 December, and the whole western side of the dome has also bulked up. The dome remains contained to the north and west. This switch was marked by the onset of ash venting at around 09:40 (local time) on Sunday 24 December from a vent on the western side of the dome, possibly behind Gage's wall. This venting was accompanied by low-level tremor becoming noticeable on the MVO seismic network by 11:00. Ash clouds reached a estimated maximum of 12,000 feet during the early afternoon. At 13:30 a pyroclastic flow came over the crater rim into the upper reaches over Tyre's Ghaut (northwest) travelling slowly for a 2-300 meters. At around 17:20 another small pyroclastic flow was observed in the upper reaches of Tyres Ghaut and venting became more energetic, possibly hotter, with ash clouds rising more rapidly despite strong winds (23 knots). At this point the decision was taken to raise the alert level from 3 to 4. After dark incandescent rockfalls were clearly visible on a sector through from the northwest to west and clearly visible from the Salem/Old Towne area, suggesting a switch in the focus of dome growth. Clear views early on 26 December revealed that the western part of the dome, which has been inactive for some months, had bulked up significantly. At around 15:30 on 27 December a distinct pulse of venting was observed comprising red-brown (possible old) ash. At 11:15 on 28 December a small pyroclastic flow was observed in Gages Valley, although the run out was very limited. Low level gas [and ash] venting from vent on western side of dome has been and is still ongoing, and overnight was accompanied by a loud roaring sound, reportedly audible as far north as Olveston. The most notable seismic activity during the period was a tremor episode started at around 11:00 on 24 December culminating in a swarm of low frequency events (best described as long-period earthquakes), beginning at around 20:00. This swarm included 88 triggered events and was most intense between around 22:30 and 23:30. The swarm finally died away at around 00:30 on 25 December and seismicity returned to previous background levels. There was another short burst of tremor on 27 December at 15:18 which continued for about 20 minutes. Subsequently activity has continued at relatively low level for the remainder of the period, dominated by rockfalls and long-period rockfalls. During the period the seismic network recorded 396 rockfall signals, 57 long-period rockfall signals, 163 long-period earthquakes, 6 volcano-tectonic earthquakes and 3 hybrid earthquakes. The measured sulphur dioxide (SO2) flux rate during the period was low, ranging from a maximum of 551 tones per day (t/d) on 24 December to a minimum of 85 t/d, with an average of 200 t/d. Nevertheless, it should be noted that the emissions plume from 24 December onwards has contained significant amounts of ash, therefore these values are probably only indicative of relative highs and lows within the period, not absolute values. The alert level stands at 4. Residents of Montserrat and visitors to the island are advised to tune into ZJB Radio for up-to-date information on the volcano. Access to all areas south of the Belham Valley [excepting Isles Bay Hill] is now suspended. The daytime entry zone, comprising the top part of St. George's Hill, is closed. Access to all areas south of Richmond Hill, and south of Jack Boy Hill to Bramble airport and beyond is prohibited at all times. The maritime exclusion zone around the southern part of the island extends 4 km off shore from Spanish Point to Roches Yard on the east side of the volcano, 2 km offshore from O'Garras to Gingoes on the south-west, and 200 m offshore from Plymouth. For further explanations of the volcanic phenomena described, and information about the Soufrière Hills Volcano and the ongoing eruption, please look at the MVO website: http://www.mvo.ms <http://www.mvo.ms/> . 29 December 2006. ------------------------------------------ Dr Vicky Hards Director Montserrat Volcano Observatory P.O. BOX 318 Flemmings Montserrat West Indies. Tel: 1-664-491-5647 FAX: 1-664-491-2423 Email: vicky@xxxxxx <mailto:vicky@xxxxxx> http://www.mvo.ms <http://www.mvo.ms/> ============================================================== To unsubscribe from the volcano list, send the message: signoff volcano to: listserv@xxxxxxx, or write to: volcano-request@xxxxxxxx To contribute to the volcano list, send your message to: volcano@xxxxxxxx Please do not send attachments. ==============================================================
