A state of emergency was declared after more than 800 quakes were recorded in 14 hours – described by Iceland's meteorological service as “a dense swarm” – about 40km from the capital Reykjavik. The most powerful tremor had a magnitude of 5.2.
“The national police chief … declares a state of emergency for civil defence due to the intense earthquake [activity] at Sundhnjukagigar, north of Grindavik,” the Department of Civil Protection and Emergency Management said.
“Earthquakes can become larger than those that have occurred and this series of events could lead to an eruption.”
Grindavik, home to 4,000 people, is 3km south-west of the area where the earthquake swarm was registered.
On Saturday police evacuated the town amid the looming danger of an eruption.
Authorities have been on alert after weeks of tremors indicated magma is accumulating about 5km underground near Grindavik, on the Reykjanes peninsula. Should it start moving towards the surface, it could lead to a volcanic eruption.
The fishing town is not far from the Blue Lagoon, Iceland’s biggest tourist attraction, and the Svartsengi geothermal power plant which supplies electricity and water to about 30,000 inhabitants of the peninsula.
“There is a likelihood that a magma intrusion has extended beneath Grindavik,” Iceland’s Met Office said in a statement after activity in the town intensified to near-constant tremors.
At about 5.30pm GMT on Friday, two strong earthquakes were felt as far away as the capital and along much of the country's southern coast, rattling windows and household objects.
On top of the 800 quakes recorded on Friday, there have been about 24,000 tremors on the Reykjanes peninsula since late October.
The Met Office has said an eruption could take place “in several days” rather than hours.
On Friday it said: “If a fissure were to appear where the seismic activity is at its highest now, lava would flow to the south-east and to the west, but not towards Grindavik.”
However, the Department of Civil Protection said it was sending the patrol vessel Thor to Grindavik “for security purposes”.
Emergency shelters and help centres were being opened in Grindavik and three other locations in southern Iceland.
On Thursday, the Blue Lagoon, famed for its geothermal spas and luxury hotels, closed as a precaution following another earthquake swarm.
The Svartsengi plant has contingency plans in place to protect the plant and its workers in the event of an eruption.
Since 2021, three eruptions have taken place on the Reykjanes peninsula, in March 2021, August 2022 and July 2023.