Increased seismic activity suggests magmatic intrusion beneath Grjótárvatn, Iceland
Seismic activity near Grjótárvatn has increased with a tremor episode on January 2, 2025, indicating a potential deep magmatic intrusion. The IMO has reported earthquakes at depths of 15 to 20 km (9.3 to 12.4 miles) which suggests subsurface magma movement without surface deformation.
Image credit: TW/SAM, Google
- Grjótárvatn lies within the Ljósufjöll volcanic system in the Snæfellsnes Volcanic Zone.
- Ljósufjöll’s last eruption occurred in the 10th century CE and produced the Rauðhálsahraun lava field which spans 13 km2 (5 mi2).
The increase in seismic activity near Grjótárvatn has been attributed to a possible deep magmatic intrusion, according to the update by the Icelandic Meteorological Office (IMO) on January 9.
A tremor episode lasting approximately 40 minutes was recorded near Grjótárvatn on January 2 between 17:00 and 18:00 UTC. The IMO has linked this activity to a possible deep magmatic intrusion.
Two earthquakes with magnitudes of 1.5 and 1.8 were identified at depths of approximately 15 km (9.3 miles). Around 20 earthquakes were registered over the day with magnitudes ranging from 0.1 to 2 and depths between 15 to 20 km (9.3 to 12.4 miles).
Numerous small earthquakes have been detected since then, but they were mostly too minor to locate precisely.
Earthquakes at such depths are unusual in Iceland but have precedents such as at Eyjafjallajökull in 1996, Upptyppingar in 2007, and east of Bárðarbunga caldera.
Deep seismicity in volcanic regions is often linked to magmatic intrusion where rising magma increases crustal pressure which causes deformation and microseismic activity.
“High b-values of ~2, as observed here, are indicative of volcanic activity,” IMO specialists noted during a review meeting held on January 8.
No ground deformation has been detected from GNSS observations or satellite InSAR data.
The IMO has cautioned that magma accumulation at depths greater than 16 km (10 miles) may not result in detectable surface deformation until substantial volumes are involved.
The monitoring level for Ljósufjöll volcano has been increased with plans for enhanced surveillance.
An M3.2 earthquake was detected near Grjótárvatn on December 18, 2024, and was felt in nearby areas including Borgarfjörður and Akranes. It was the largest seismic event in the region since 2021.
A newly installed seismometer in Hítardalur in early November last year has improved detection capabilities for smaller earthquakes below M1.
Historical data from the South Iceland Lowland (SIL) system indicates that seismic activity at Grjótárvatn has been sporadic with events occurring in 1992 and the early 2020s. An increase in earthquakes above M1.0 was observed even before the new station was installed and showed that the rise in activity is real and not because of better monitoring.
Grjótárvatn lies within the Ljósufjöll volcanic system in the Snæfellsnes Volcanic Zone. Ljósufjöll’s last eruption occurred in the 10th century CE and produced the Rauðhálsahraun lava field which spans 13 km2 (5 mi2).
The Ljósufjöll volcanic system is located at the eastern end of the Snæfellsnes Peninsula and consists of alkali olivine basaltic cinder cones and lava flows along short fissures. The features extend approximately 90 km (56 miles) in a west-northwest to east-southeast (WNW-ESE) direction with the volcanic field widening to about 20 km (12.4 miles) at its eastern end and narrowing to 10 km (6.2 miles) in the west.
Ljósufjöll hosts some of the largest outcrops of silicic rhyolitic and trachytic rocks in the Snæfellsnes Volcanic Zone, most of which erupted during the mid to late Pleistocene.
Over the past 10 000 years, eruptions have occurred approximately every 400 years with small effusive or mildly explosive events. Potential hazards include localized lava flows, gas emissions, and tephra fallout.
Deep seismicity and high b-values at Grjótárvatn resemble patterns observed at Upptyppingar (2007 to 2008). The Gutenberg-Richter analysis for 2021 to 2024 reinforces this and shows a high proportion of small-magnitude earthquakes. Such activity suggests ongoing magmatic processes but no indications of upward magma migration.
References:
1 Magmatic intrusion at considerable depth beneath Grjótárvatn likely – IMO – January 9, 2025
2 Ljósufjöll – GVP – Accessed on Janaury 10, 2025
Rishika holds a Master’s in International Studies from Stella Maris College, Chennai, India, where she earned a gold medal, and an MCA from the University of Mysore, Karnataka, India. Previously, she served as a Research Assistant at the National Institute of Advanced Studies, Indian Institute of Science, Bengaluru, India. During her tenure, she contributed as a Junior Writer for Europe Monitor on the Global Politics website and as an Assistant Editor for The World This Week. Her work has also been published in The Hindu newspaper, showing her expertise in global affairs. Rishika is also a recipient of the Women Empowerment Award at the district level in Haryana, India, in 2022.
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