Seismic activity in Grjótarvatn has increased in the last months
The possibility of magma accumulation at depth is considered
- The M3,2 earthquake on the evening of December 18 is the largest so far
- No ground deformation has been detected on GNSS observations
- No indication that magma is moving within the upper crust
- Small fissure eruptions are characteristic of volcanic activity within Ljósufjöll volcanic system
An earthquake with magnitude M3.2 was detected near Grjótárvatn on the evening of December 18. The IMO received reports that the earthquake had been felt around Borgarfjörður and Akranes. Seismic activity has been measured around Grjótarvatn regularly since spring 2021, but in recent months it has been increasing, as shown in the figure below. The earthquake detected on December 18 is the largest since the autumn of 2021 when two earthquakes of magnitude M3 occurred. Prior to this, significant seismic activity was last detected there in 1992, when two earthquakes of magnitude M3 occurred, one larger than M3.2, and several others above M2.0. The earthquake catalogue which we are referring to here goes back to 1991 (SIL-system).
Screenshot from the Skjálfta-Lísa system.
The IMO has increased the monitoring of the area with new installations of equipment in Hítardalur both a seismometer (at the end of September) and a GNSS station (at the beginning of November). The new seismometer has improved the monitoring system's ability to detect small earthquakes. Indeed, before it was installed, few earthquakes below M1.0 were detected because the nearest seismometer was relatively far away, at about 30 km from the active area. However, the increased level of seismicity is not an artifact of a more sensitive monitoring network. If we consider only those earthquakes above M1.0, it is evident that seismic activity in the area was increasing before the installation of the new station.
After the seismic station was installed closer to the active area, the earthquake location improved significantly. The depth of the majority of the earthquakes in the area is now well constrained to be at a depth between 15-20 km. Since the GNSS station was installed in Hítardalur, the data does not show any detectable deformation at the surface. Analysis of satellite data (InSAR) from the period 2019 to summer 2024 also shows no evidence of surface deformation.
Yesterday morning (19 December) the regular scientific meeting to assess the situation in the Reykjanes peninsula took place. Specialists from the IMO and the Earth Sciences department at the University of Iceland took the opportunity to also discuss the newest data and ongoing activity at Grjótarvatn.
It is currently not possible to determine the cause of the ongoing seismic activity, but two possibilities are considered: magma accumulation at depth or intra-plate tectonic movements.
More analysis is needed to assess with certainty which process is responsible for the ongoing activity at Grjótarvatn. However, the current monitoring data indicates that there is no evidence of magma migrating to shallow depths. The IMO will plan the deployment of additional stations (seismic and GNSS) to better constrain the ongoing phenomena and monitor its evolution.
In the event that magma starts propagating towards the surface, it is most likely that larger precursors will be detected, such as a rapid increase in seismic activity (in terms of number and magnitude of earthquakes), a migration in their depth and/or ground deformation.
Eruptive history
Grjótarvatn is within the volcanic system of Ljósufjöll in the Snæfellsnes Volcanic Zone. The last eruption of Ljósufjöll was a small effusive eruption which occurred in the 10th century CE and produced the Rauðhálsahraun lava field, of about 13 km2 (at about 15 km northwest from Grjótarvatn). On average, during the last 10,000 years, this volcanic system has erupted every 400 years. In the event of an eruption here, the most likely scenario is a small effusive eruption (< 0.1 km3), or mildly explosive, with lava fountaining and lava flows. If an eruption occurred, a small area would be expected to be affected by the main associated volcanic hazards which are: lava flows, gas pollution and very localized tephra fallout. More information on Ljósufjöll volcanicsystem is available here.
Ljósufjöll volcanic system, Holocene lava flows and scoria deposits.
