When over 1,500 earthquakes strike Iceland in less than a week, near a volcano, it's natural to ask questions. The IMO has answered them — thoroughly, and with an important historical footnote that puts the South Iceland Seismic Zone in perspective.
What Happened
The swarm began in the early hours of 31 May 2026, in an area west of Hengill — one of Iceland's largest and most active volcanic systems, and home to the Hellisheiðarvirkjun geothermal power station. By 4 June, over 1,500 earthquakes had been measured. The largest, an M4.5 on 1 June, was felt across much of southwest Iceland — around 150 reports came in to the IMO from as far afield as Akranes and the Eyjafjöll area.
Three additional earthquakes exceeded M3. Activity peaked on the afternoon of 1 June and has been declining steadily since. As of the 4 June bulletin, the swarm is winding down.
IMO verdict: The swarm is caused by tectonic stress release on north–south trending strike-slip faults in the South Iceland Seismic Zone. This type of activity is common in this region. The swarm is not caused by magma movement, ground deformation, or geothermal operations.
Why It's Not Volcanic
Three independent lines of evidence rule out a volcanic or magmatic origin, and the IMO is explicit about each of them.
First, GPS stations near Húsmúli show no ground deformation — the sensors that would detect crustal swelling from magma accumulation show no meaningful change in the horizontal or vertical measurements recorded over the past year.
Second, InSAR satellite data covering 31 May to 3 June shows no sign of ground uplift or subsidence that would suggest magma movement beneath the surface. If magma were involved, deformation signals would typically appear in the days before a swarm begins.
Third, the seismicity is shallow — consistent with stress release in the upper crust rather than the deeper, pressurised pathways associated with magmatic intrusion. The earthquake focal mechanisms also match north–south oriented strike-slip faults characteristic of the South Iceland Seismic Zone, not the ring or radial fault patterns associated with volcanic unrest.
What about Hellisheiðarvirkjun? The IMO has also examined whether the swarm could be linked to the injection or extraction of geothermal fluids at the Hellisheiðarvirkjun plant. Based on the location of the earthquakes and their nature, there is no evidence of a connection. The faults involved are part of the regional tectonic system, not the geothermal field.
The Bigger Picture: What the SISZ Is Capable Of
This is where the IMO's bulletin becomes genuinely important reading for anyone monitoring Iceland. A stress-release swarm fading after a week is not the story. The story is what it reminds us about the South Iceland Seismic Zone.
The SISZ and Reykjanes Peninsula sit on a plate boundary — one of the most seismically active regions in Europe. Swarms of smaller earthquakes are common and expected. But the same fault system is capable of producing events an order of magnitude larger, and the IMO is clear about this.
- Brennisteinsfjöll — capable of earthquakes up to M6–6.5 that would be felt across the entire capital region. The last significant events here were M6.4 and M6.1 in 1929 and 1968
- South Iceland (Holt and Hestfjall) — M6.5 and M6.4 in June 2000, followed by a major aftershock sequence
- Ölfus — M6.3 in 2008, on the same north–south fault system
- Major South Iceland earthquakes occur on approximately a 100-year cycle — the IMO notes that significant stress was released in the 2000 and 2008 events, meaning a long period of rebuild typically follows
- The timing of future large earthquakes cannot be predicted
The IMO closes its bulletin with a reminder: this swarm is a useful prompt to review earthquake preparedness. Iceland's building stock has improved significantly since 2008, but the Reykjanes Peninsula and South Iceland remain in a seismically active zone regardless of volcanic status.
What to Watch For
The IMO's 24-hour watch continues. The current bulletin will be updated immediately if the character of the seismicity changes — for example, if the earthquakes deepen, if deformation is detected, or if a pattern emerges that could suggest volcanic involvement. As it stands, none of those signals are present.
For our regular Iceland monitoring streams, this swarm falls outside the Svartsengi/Sundhnúkur picture we usually track — it's a separate system entirely, and the two are unrelated. Svartsengi monitoring continues unchanged.