Grjotarvatn Volcano-Volcanic Seismic Unrest

 

The plot showing the continuous tremor at the Hítardalur seismic station between 05:00 PM and 06:00 PM on 2 January (image: Icelandic Meteorological Office)

On Thursday, January 2nd, a tremor event was recorded between 5:00 PM and 6:00 PM, originating near Grjótárvatn. The tremor, lasting approximately 40 minutes, consisted of a series of continuous small earthquakes, most of which were too small to be precisely located. Only two earthquakes within the tremor sequence were located, occurring at a depth of about 15 km, with magnitudes of M1.5 and M1.8. In total, around 20 earthquakes were detected that day, all at depths of 15–20 km, with magnitudes ranging from M0.1 to M2.0.

Yesterday morning, January 8th, the Icelandic Meteorological Office (IMO) held its monthly meeting to review recent activity across the country's volcanic systems. During the meeting, the increased seismic activity near Grjótárvatn was analyzed and possible explanations discussed. A recent analysis of seismic data from 2021 to 2024 in the Grjótárvatn area revealed a high b-value (~2) associated with the earthquakes. This is similar to the deep seismic swarm observed at Upptyppingar in 2007–2008. High b-values, often seen in volcanic regions, suggest an unusually high proportion of small earthquakes.

Satellite data from 2019 to mid-2024 does not indicate measurable surface deformation, and current InSAR observations are unusable due to snow cover. However, a GNSS station installed in Hítardalur, about 4 km northwest of Grjótárvatn, in early November 2023, has not detected any surface deformation. Nonetheless, this does not rule out the presence of magma at depth. If magma is accumulating deeper than 16 km, geodetic modeling suggests that surface deformation would only be detectable once the intruded magma volume becomes significant.

The depth of the earthquakes, their high b-value, the tremor pulses, and the duration of the activity strongly suggest that this seismicity is caused by a magmatic intrusion at depth. Despite this, current monitoring data provide no evidence that magma is migrating toward the surface.

As long as the seismic activity remains at these depths, additional earthquakes of up to magnitude 3 are expected. However, the likelihood of larger earthquakes exceeding magnitude 4 is low at this depth.

Given the increased activity and the probable deep magma intrusion, the Icelandic Meteorological Office has raised the monitoring level for the Ljósufjöll volcanic system. Efforts are underway to develop an enhanced monitoring plan for the area.

Earthquakes at such depths are rare in Iceland but have been previously observed in volcanic systems such as Eyjafjallajökull in 1996, Upptyppingar in 2007, and east of the Bárðarbunga caldera, where deep seismic activity occurs regularly. In volcanic settings, this type of deep seismicity is typically caused by magmatic intrusion, which increases pressure in the crust, resulting in sudden deformation and fracturing.

Source: Icelandic Meteorological Office volcano activity update 10 January 2025