Low hazard still means hazard - the case of Florina, Greece

Published: 01/10/2022

A strong earthquake hit the Greek region with the lowest seismic hazard last night. A warning for the future.

For many people in northern Greece and the neighboring areas of Northern Macedonia and Albania, the night to Monday (January 10th) turned out to be very uncomfortable: The strongest earthquake in this region for decades shook the cross-border Florina-Bitola basin, caused damage in both cities and left thousands of people terrified. Terrified, because Florina is considered to be one of the few places in Greece without a significant earthquake history, one of the regions of Greece with the lowest earthquake hazard. The quake of the night is therefore also a warning that the quiet past and the supposedly low risk are not a promise for the future.

Magnitude 5.3 at a shallow depth on the outskirts of a small town: Last night's earthquake caused some building damage in Greece and Northern Macedonia (more details about this event). In Florina and Bitola, cracks formed in houses, facades and balconies collapsed in some cases, and the power supply of some villages was interrupted. Even Albania, about 100 km to the west, was still affected with some small cracks in buildings of the town Bilisht. Fortunately, no one was injured except for a few people who needed treatment for panic attacks. Also the damage does not seem to be serious at all, although the evaluation is still ongoing.

If you look into the past for examples of more serious earthquakes, most regions of Greece would provide historic cases of devastating shocks. The entire country is considered tectonically very active with many large and small fault zones. Hardly any region has not been hit by a catastrophic earthquake at least once in the past - although "catastrophic" at least on local level might already result from magnitude 6 earthquakes due to the geological and architectonic conditions. Nevertheless, there are regional differences. One of the few missing pieces in the tectonic puzzle is the immediate vicinity of Florina.

There has been no major damage quake in historical times in this region. And even in times of instrumental recordings, there were no earthquakes above magnitude 5 until 2022. As a result, Florina and neighboring parts of Western Macedonia region belong to Earthquake Hazard Zone 1 in Greece. Only a few other regions such as the central Aegean Islands and the Northeast of Greece close to the Turkish border are situated in the same Hazard zone. According to this hazard definition, the earthquake of last night would have an average recurrence time of about 500 years.

The fact that a low earthquake hazard zone does not mean that there are no strong earthquakes at all can be seen in the immediate vicinity: At Bitola in Northern Macedonia a magnitude 5.2 earthquake occurred in 1994, leaving damage to buildings and at least 10 people injured. An even stronger quake of magnitude 6.6 happened one year later and around 100 kilometers south of Florina. Fortunately, it took place in a sparsely populated region, but it also lead to injuries and damage in nearby towns and villages. Both earthquakes also affected the area around Florina, which is therefore not totally unprone to seismicity. But, last night's quake can definitely be seen as an exception in the recent history of the town. However, studies of the past few decades have shown that larger quakes are likely to happen in the future.

Immediately south of Florina, the so-called Amyndeo fault system strikes in a southwest-northeast direction. This tectonic fault is the largest known in the Florina region and source of most of the earthquake hazard. Although no major historical earthquakes are known, there are indicators for tectonic activity in recent times and likely ongoing activity. Earthquakes up to magnitude 6.9 are possible.

Last night's earthquake is also associated with this fault system. It shows a very similar rupture orientation but appears to be on a previously unknown branch of this fault, which runs directly beneath Florina. Future studies will have to show whether this may even mean a greater earthquake hazard than previously known.

In any case, the recent quake should be a reminder that a low earthquake hazard does not exclude large earthquakes in the future. In some cases, the interseismic event time between two large earthquakes can be as long as several thousand years. The reference period for earthquake hazard evaluation is based on a few hundred years of records, if at all. If, as in this example, this reference period is taken completely from the interseismic period between two large earthquakes, a false impression may be created.

Although one cannot draw any conclusions for the immediate future based on the current earthquake, it should be seen as a warning that the impression of the past might not reflect the actual state.

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