Hidden fractures and cavities under Grindavík mapped using magnetic surveys

Researchers at the University of Iceland and partner institutions have conducted magnetic surveys to map hidden fractures and cavities under Grindavík. In several places, where little or no surface-level evidence was visible, the measurements revealed previously unknown fractures and cavities, which were later confirmed through excavation.

“We took the measurements on foot and with drones, focusing on traces of the earthquakes and volcanic activity that occurred in the area between 2023 and 2025. Our goal was to get a clearer picture of what had happened under the town when the fractures opened during the earthquakes, magma intrusions and eruptions at Svartsengi, and to assess the extent of the damage,” says Elisa Johanna Piispa, senior research lecturer at the UI Institute of Earth Sciences and first author of the article on this topic recently published in Geophysical Research Letters.

Fractures not always visible at the surface

The surveys were conducted as part of a study looking at the significant crustal movements that have accompanied magma intrusions and eruptions in the Svartsengi system since the autumn of 2023. On 10 November 2023, a large dike formed under Grindavík. This was accompanied by an intense earthquake swarm and significant deformation of the Earth’s crust. Grindavík was evacuated, and fractures opened in and around the town. More magma intrusions followed, and the fractures moved again, forming cavities and sinkholes.

The results of these magnetic surveys could be highly significant for risk assessments and planning in areas exposed to natural hazards. “A fracture can open in the bedrock with no obvious signs on the surface. It may be concealed by soil, streets or other man-made structures. Hidden fractures can pose a risk to people and infrastructure. A better understanding of what is happening under the surface can reduce that risk, prevent damage and minimise the costs of developing infrastructure,” says Elisa.

Around 320 kilometres surveyed

The magnetic field over Grindavík was surveyed with drones flown along dense grid lines over the town, covering a total of 220 km. The scientists also surveyed around 100 km on foot with magnetometers.

“Our drones enabled us to survey large, challenging and even dangerous areas quickly, while ground measurements provided a more precise picture of individual fractures,” explains Elisa.

“We measured fractures visible from the surface and where we believed they continued underground. Then we compared those findings to other measurements and observations. Most importantly, we excavated selected sites to confirm what anomalies in the magnetic field meant,” says Elisa.

In some places, vertical displacement reached up to 1.3 metres. The cumulative crustal extension within the town has been estimated at up to five metres. The largest documented fracture is around three metres wide and at least thirty metres deep.

The magnetic surveys were compared with field observations, LiDAR measurements, photographs and older aerial imagery. Excavations at selected sites then confirmed the fractures and cavities indicated by the magnetic data. The lava beneath Grindavík is basalt with strong magnetic properties. When the rock breaks, forming fractures and cavities, the magnetic field changes. This means that the scientists were able to identify anomalies which indicated fractures, even where nothing could be seen at the surface.

Large fracture under Túngata

One of the clearest examples in the study is from the street Túngata in Grindavík. No mapped fractures were visible on the surface, but the survey revealed clear signs of a lowered magnetic field. Excavations revealed a large fracture under the street, around two to three metres wide. It was hidden beneath younger layers of soil and lava, so there was no sign of it at the surface.

Similar information came to light at the football pitch in Grindavík, where magnetic surveys indicated significant fractures and cavities, later confirmed by excavation. Comparisons with aerial photographs taken in 1954 also showed that some magnetic anomalies follow old fractures that later disappeared beneath streets, houses, and other structures. The findings indicate that the volcanic activity from 2023 not only created new fractures but also reopened old ones.

Computer models suggest that, in favourable conditions, it is possible to identify fractures and cavities to a depth of around 10-20 metres. The method is, however, not equally effective everywhere, and magnetic surveys do not replace other research methods, but rather complement them.

Results were available within a few days

“This is the first time that magnetic surveys have been used to map hidden fractures in a built-up area during active volcanic and seismic activity. Information was available almost in real time and could be used directly to assess imminent hazards,” says Elisa. The data was processed quickly, and information on potential hazards was passed on to those responsible for risk assessments and emergency response.

Magnetic surveys could also be used in other volcanic areas, where earthquakes, subsidence or landslides can cause fractures and cavities beneath the surface. It is important to survey hazardous areas before natural disasters occur, to make it easier to later identify changes.

Extensive collaboration

The study involved researchers from the University of Iceland, Iceland GeoSurvey (ÍSOR), the Icelandic Institute of Natural History, EFLA Engineers and the Icelandic Meteorological Office. Other contributions came from the Department of Civil Protection, the Municipality of Grindavík, the police in Grindavík, Sigmenn, Verkís and many others. In addition to Elisa, the authors of the article from UI are Catherine Rachael Gallagher, postdoctoral researcher in geology and natural hazards; Sindri Bernholt, an MS student who wrote his thesis on this project; Magnús Tumi Guðmundsson, professor of geophysics; and Gunnlaugur Björnsson, astrophysicist and research scientist.

Authors from Ísor are Gunnlaugur Magnús Einarsson, geographer, and the geologists Ögmundur Erlendsson, Katrín Ásta Karlsdóttir and Magnús Á. Sigurgeirsson. From the Icelandic Institute of Natural History: geologists Robert A. Askew and Birgir Vilhelm Óskarsson, and Sydney R. Gunnarsson, remote sensing specialist. Also among the authors is Daniel Ben-Yehoshua, specialist in natural hazards and surveying at EFLA (now at the Icelandic Meteorological Office).