Deep Earthquakes Detected Near Grjótárvatn

M3.2 Earthquake Recorded Yesterday Morning

17 January 2025

Update January 17, 14:00 UTC

• Nearly 100 earthquakes above magnitude 1.0 have been detected so far this January.

• A magnitude 3.2 quake struck yesterday, marking the most significant seismic event since activity heightened in August 2024.

• The prevailing theory attributes this uptick in seismicity to magma movement beneath the surface.

• Currently, there are no signs suggesting that magma is migrating closer to the earth’s surface.

The seismic landscape near Grjótárvatn has been buzzing with activity this January, with nearly 100 recorded earthquakes exceeding magnitude 1.0. This figure matches the total number observed during December 2024, which set a new record for the region.

Early yesterday, on January 16, a magnitude 3.2 earthquake was registered. While no official reports to the Icelandic Meteorological Office indicated that the tremor was felt in populated areas, residents nearby may have sensed its vibrations. This quake stands out as the strongest recorded since the surge in seismic activity began in August 2024, with the previous notable quake—a 3.1 magnitude—occurring on December 18, 2024.

An illustrative representation from the IMO’s earthquake visualization tool captures seismic activities around Grjótárvatn from July 2021 to January 2025. The upper left map marks earthquake locations, the upper right graph delineates earthquake magnitudes, the lower left tracks the cumulative number of earthquakes, while the lower right outlines monthly counts.

Earlier this month, on January 2, a tremor episode lasting about an hour was noted. A subsequent tremor was recorded on January 10, also lasting just over an hour.

On January 9, an analysis was shared that emphasized the unusual b-value of the recent earthquakes, a characteristic often seen in volcanic regions and indicative of a heightened frequency of smaller quakes. Experts suspect that the causes behind this seismic activity are more aligned with deep magma intrusion rather than tectonic shifts. However, ongoing monitoring suggests no immediate threat of magma rising to the surface.

The Icelandic Meteorological Office is intensifying its surveillance efforts and developing a comprehensive monitoring plan for the region, which will involve the installation of additional instruments for better ongoing assessment and understanding of these seismic events.

Update January 9, 12:30 UTC

• A tremor episode occurred on Thursday, January 2.

• Seismic activity remains consistent, characterized mainly by deeper quakes.

• No signs of surface deformation have emerged from GNSS monitoring.

• A deep magma intrusion is considered the probable trigger for the seismicity.

• There remains no indication of magma approaching shallower depths.

• The monitoring level in Ljósufjöll has been elevated.

On the afternoon of January 2, between 5:00 PM and 6:00 PM, a tremor event centered around Grjótárvatn was recorded. This seismic occurrence, lasting approximately 40 minutes, consisted of numerous minor earthquakes, most too small to pinpoint accurately. Out of the recorded tremors, only two were located at depths around 15 kilometers, exhibiting magnitudes of 1.5 and 1.8. Overall, about 20 earthquakes were documented throughout that day, occurring at depths between 15 to 20 kilometers and magnitudes ranging typically from 0.1 to 2.0.

During the tremor on January 2, continuous signals were captured at the new seismic station in Hítardalur (indicated by the orange box). Events occurring before and after the tremor can be seen as well.

Seismic events at such depths are rare in Iceland, although historical incidents have been recorded in various volcanic systems, including Eyjafjallajökull in 1996 and Upptyppingar in 2007. Similarly, deep earthquakes east of the Bárðarbunga caldera are not uncommon.

In volcanic contexts, the deep seismicity is generally linked to increased pressure in the crust due to magma intrusion, causing sudden deformations that fracture the crust.

During a monthly meeting at the IMO on January 8, specialists reviewed recent volcanic activity across the country, particularly at Grjótárvatn. An evaluation of seismic data from 2021 to 2024 underscored that the area has been experiencing a high b-value (~2), reminiscent of the deep seismic swarms at Upptyppingar from 2007-2008. This high b-value is commonly associated with volcanic earthquakes and signifies a greater incidence of minor earthquakes.

A Gutenberg-Richter distribution graph showcasing the correlation between earthquake magnitudes and frequencies in Grjótárvatn from 2021 to 2024 illustrates a b-value of 2.

Analysis of satellite data spanning from 2019 to summer 2024 reveals no measurable surface deformation, and current InSAR observations are limited by snow cover in the area.

In November 2024, a GNSS station was installed in Hítardalur, roughly 4 km northwest of Grjótárvatn. Since its installation, surface deformation has not been detected, yet this does not discount the potential for deep magma activity. If magma is accumulating at depths greater than 16 kilometers, geodetic modeling predicts that surface deformation may not become apparent until a significant volume of magma has intruded.

Both the depth of recent earthquakes and their b-values, coupled with the continued tremors, strongly suggest that magma intrusion is the likely cause of this seismicity. Nonetheless, data currently indicate no magma movement toward the surface.

As long as seismic activity persists at these depths, aftershocks near magnitude 3 are to be anticipated, while the probability of larger quakes surpassing magnitude 4 remains low.

In light of the heightened seismic activity and potential magma intrusion, the Icelandic Meteorological Office has escalated the monitoring level for Ljósufjöll and is actively pursuing an expansive monitoring strategy for the area.

Update December 20, 2024

• The M3.2 earthquake reported on December 18 is the largest occurrence to date.

• No ground deformation has been detected through GNSS observations.

• No indication exists suggesting magma migration within the upper crust.

• Small fissure eruptions are typical of volcanic activity in the Ljósufjöll system.

On December 18, an M3.2 earthquake was registered near Grjótárvatn and was reportedly felt in regions like Borgarfjörður and Akranes. Since early 2021, seismic events in this vicinity have been monitored consistently, and recent months have indicated a significant uptick in seismicity.

This magnitude 3.2 quake is the largest since autumn 2021, when two seismic events of similar magnitude occurred. Before this, the last notable activity was observed in 1992.

A snapshot from the Skjálfta-Lísa system.

The IMO has enhanced monitoring in the area, installing new equipment in Hítardalur, including both a seismometer in late September and a GNSS station in early November. The newly installed seismometer has bolstered the network’s ability to detect minor earthquakes, significantly improving data collection near the active area.

This surge in seismicity cannot be attributed solely to improved monitoring technology; in fact, earthquake data above magnitude 1.0 suggested rising activity prior to the installation of the newer station.

Post-installation, earthquake location accuracy has seen significant improvement, establishing that most seismic events have occurred at depths between 15 and 20 kilometers. Since the installation of the GNSS station in Hítardalur, no detectable surface deformation has been recorded. Similarly, satellite data analysis from 2019 until summer 2024 shows no evidence of surface changes.

During a scientific review on December 19 regarding the situation on the Reykjanes peninsula, experts from the IMO and the University of Iceland took the opportunity to discuss the latest findings and activity at Grjótárvatn.

Currently, the exact cause of the ongoing seismic activity remains uncertain, though research points toward two main possibilities: magma accumulation at depth or intraplate tectonic movements. Further analysis is essential to clarify the driving force behind this recent activity. Nevertheless, current data does not suggest any magma is ascending to shallower depths.

The IMO plans to deploy additional seismic and GNSS stations to gain further insight into the ongoing phenomena and monitor its evolution effectively.

If magma does begin to migrate toward the surface, experts anticipate larger precursors, such as a swift rise in seismic activity (in terms of both quantity and magnitude), shifts in depth, and/or surface deformation.

Eruptive History

The Grjótarvatn area lies within the Ljósufjöll volcanic system, situated in the Snæfellsnes Volcanic Zone. The last known eruption of Ljósufjöll occurred in the 10th century CE, which produced the Rauðhálsahraun lava field, spanning roughly 13 square kilometers northwest of Grjótárvatn. Over the past 10,000 years, this volcanic system has averaged an eruption every 400 years. In the event of a future eruption, the most plausible scenario would involve either a small effusive event (under 0.1 km³) or a mildly explosive incident featuring lava fountaining and flow. Such an eruption would likely have localized impacts, mainly through lava flows, gas emissions, and limited tephra fallout. For further information on the Ljósufjöll volcanic system, please refer to the provided sources.

The Ljósufjöll volcanic system remains a fascinating subject for ongoing research, showcasing Holocene lava flows and scoria deposits.