Update 8 December 2021
The alert level has been lowered to Yellow. To date no eruption has followed the jökulhlaup or increased seismicity. As Grímsvötn may erupt with little warning, the situation remains closely monitored.
Original Post 6 December 2021
The alert level for Grímsvötn has been raised to orange following the draining of the caldera lake over the past few days, the resulting jökulhlaup, and increased seismicity today.
The ice-cap has subsided by c. 77 metres. The water drained via the Gígjukvísl river; the discharge in Gígjukvísl river yesterday around noon was 2800 m3/s, reducing later in the day.
At the time of writing, there have been 25 earthquakes in the last 48 hours in the vicinity of Grímsfjall. A magnitude 2.3 occurred at 06:15, followed shortly afterwards at 06:16 by a magnitude 3.6. There have been a few aftershocks. No volcanic tremor has been detected.
Volcanic eruptions have occurred after draining of the caldera lake in the past, attributed to the sudden decrease in pressure destabilising the system; IMO cite the 2004, 1934 and 1922 eruptions.
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Icelandic Met Office: IMO
Icelandic Volcanoes: Icelandic Volcanoes (icelandicvolcanos.is)