Geological and Geophysical Investigation of Grindavík, Iceland, in Response to Volcanic Activity and Fissure Movements at the Sundhnúkar Eruption Fissure

A powerful earthquake swarm related to accumulation of magma in a shallow reservoir beneath Svartsengi, on the Reykjanes Peninsula SW Iceland began in October 2023. On 10 November 2023 a large dike intrusion occurred beneath the town of Grindavík leading to the formation of a graben structure on the west side of town. Subsequently, 11 more dike intrusions have occurred along the Sundhnúkur crater row, with another graben forming on the east side of town. The maximum subsidence measured in the town is 1.5 m, and further fault movements were triggered throughout Grindavík. These events resulted in the opening of numerous fractures and caused damage to critical infrastructure. Following these events, the Icelandic Civil Protection authorities commissioned a detailed geological and geophysical investigation of the area.

A final report, alongside numerous technical memoranda, is now available, presenting the main results. One of the key outcomes of the project is a detailed fracture map of Grindavík. The map identifies seven distinct fracture zones that have been active during the ongoing unrest: Stamphólsgjá, Hópssprunga, Austurhópssprunga, Víðihlíðarsprunga, Bröttuhlíðarsprunga, Stakkavíkursprunga, and Strandhólssprunga (see:https://www.map.is/grindavik/). Stamphólsgjá is the deepest (>30 m) and widest fracture (3 m). In addition, depths greater than 20 m were measured within fractures of the Hópssprunga and Bröttuhlíðarsprunga zones. It is important to note that Stamphólsgjá and Hópssprunga are several thousand years old, and not all of the observed widening can be attributed to the current events. Historical aerial photographs show that Stamphólsgjá was already significantly open prior to the development of the town. No evidence of Austurhópssprunga, Víðihlíðarsprunga, Bröttuhlíðarsprunga, or Stakkavíkursprunga is visible on older aerial imagery, indicating that these fractures likely formed during the ongoing events. Most fractures are typically 20–60 cm wide and 1–5 m deep, while relatively few locations exhibit fractures wider than 80 cm and deeper than 8 m. It is important to consider that substantial material collapse has occurred into many fractures, and often only surface depressions and subsidence are visible, indicating the presence of open fractures beneath the surface. The investigation employed various methods, including aerial photo interpretation, LiDAR elevation measurements, ground-penetrating radar (GPR), magnetic surveys, electrical resistivity measurements, and visual inspection.

Excavations carried out in connection with road repairs provided valuable opportunities to examine several meters into the bedrock and assess its composition. These observations revealed that the upper 4–10 m of the bedrock consist of four postglacial lavas, separated by sedimentary layers and soil. No deeper hyloclastite formations from the last glacial period were observed. The youngest lava exposed at the surface is the Sundhnúkur (sh) lava (~2200 years old). Previously known fissures in Grindavík are prominent in older lava flows (>8000 years old) but are scarcely visible in Sh.

Importantly, the volcano-tectonic unrest in and around the town is ongoing, and further fracture movements may occur in the future, and existing surface fractures continue to evolve due to unconsolidated materials moving within the fractures underscoring the importance of continued monitoring.