Update on the seismic swarm in Greece

A look at earthquake patterns beneath the Aegean Sea

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The seismic swarm below the Aegean Sea continues. We wrote a summary of the situation and the tectonic setting on February 3rd. Since then, the maximum earthquake magnitudes have continued to grow: the largest earthquake recorded so far was a M5.2, which occurred at 3:04 PM local time yesterday (February 4). News reports indicate that many people have voluntarily evacuated the nearby island of Santorini.

In a lucky coincidence, it appears that a scientific team had already planned to deploy four underwater seismographs nearby as part of a research effort; the installation is currently underway. The recordings should eventually be useful for improving the locations of the earthquakes, and might also be able to listen for more subtle signals that cannot be seen at stations farther away on land.

However, even without those local stations, we can still learn a lot from existing seismic catalogs.

First, let’s take a look at the sequence as a whole. Below, we have plotted earthquakes reported by the EMSC, together with focal mechanisms from a variety of sources, since January 1. (Focal mechanisms are more detailed representations of the ruptures that cause the earthquakes, but are usually available only for larger events.) Below the map are two timelines: one showing events over time, and the other a histogram of event counts over time.

Figure 1. A map and timelines of the 2025 earthquake swarm northeast of Santorini, Greece.

First, it is important to remember that this is a seismically active area: even before the swarm, the timeline shows persistent, sporadic small events. The swarm ramped up starting around January 27th. Between January 27 and February 1st, we can see an increase in both the maximum magnitudes of events, and earthquake rate. However, since February 2nd, the rate of seismicity seems to have been pretty steady, with up to 10 M4+ earthquakes every six hours.

We also tried coloring the earthquakes by time. The following map includes earthquakes since June 2024; with events prior to January 29 in black. The earthquakes are projected to the right onto a timeline showing the swarm.

Figure 2. A timeline of seismicity for the earthquake swarm, beginning in June 2024. Black dots are earthquakes recorded before January 29, 2025. Blue/green symbols are earlier events, and orange/red symbols are more recent.

What can we see from this plot?

• While some small events have indeed been recorded below Santorini, they are pretty sparse, and they mostly predate this swarm. Recall that the map includes events back to June 2024.

• The timeline shows progressive movement of the swarm. The swarm began with small earthquakes. Starting on February 1, the seismicity started to intensify, broaden, and move slightly towards the northeast.

• Focal mechanisms are largely normal-type, striking NE-SW. It is not immediately obvious which nodal plane represents the fault plane orientation; indeed, it is possible that faults are being activated in both dip directions. There are also a handful of small strike-slip earthquakes, which are still consistent with overall northwest-southeast stretching of the crust.

The map above also shows the locations of two cross-sections, one (A-A’) aligned with the fault zone strike, and the other (B-B’) crossing the fault zone perpendicularly. Let’s look at those now.

We have made the along-strike profile long enough to capture both Santorini and the swarm. In this profile, we can see that there has indeed been some microseismicity below Santorini, but most of the circles are black — indicating events that predate this swarm.

Again, we can see the change in color of the swarm from southwest to northeast, indicating that seismicity is migrating over time. The cross-section also shows that the maximum depth changes, with deeper events in the southwest. So, it seems that the seismicity may be shallowing as it migrates northeastward. This kind of migration would be consistent with the movement of fluids through the crust, which remains the most likely explanation for what is going on.

Figure 3. An along-strike cross section reaching from Santorini at left to Amorgos at right.

Here is the along-dip profile. Mostly, the earthquakes seem to form a cloud. This could largely be due to location uncertainties, but could also reflect distributed faulting of the crust. There isn’t a strong preference for dip direction, but if we had to choose one, it looks like the events in the southeast are a bit deeper than those in the northwest. This would align with the overall direction of the Amorgos Fault, thought to be responsible for the 1956 M7.8 earthquake.

Figure 4. An across-strike (along-dip) cross section across the earthquake swarm.

As we noted in our previous post, seismic swarms can sometimes, but often do not, trigger larger earthquakes. It remains unclear how this swarm will evolve. Our plots can provide some idea of which faults are activating, and why, but we cannot provide an estimate of the likelihood that the swarm could trigger a large earthquake. The most likely situation is that the swarm will settle down without triggering a large earthquake. However, the risk is certainly elevated above its normal level.

References

J. Hubbard and K. Bradley, Earthquake swarm beneath the Aegean Sea, Earthquake Insights, https://earthquakeinsights.substack.com/p/earthquake-swarm-beneath-the-aegean