Magnitude 4.4 earthquake shakes Central Greece
Magnitude 4.4 earthquake shakes Central Greece
Normal faulting event close to an unusual earthquake swarm in 2020-2021
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Citation: Bradley, K., 2024. Magnitude 4.4 earthquake shakes Central Greece. Earthquake Insights, https://doi.org/10.62481/4f297ba2
On October 7 at 9:18 AM local time, a magnitude 4.4 earthquake struck several kilometers to the southwest of the historic city of Thebes (Θήβα, or Thiva). More than 70 testimonials submitted to EMSC indicate only weak shaking within ~100 km of the epicenter (including the city of Athens).
A focal mechanism for the earthquake indicates normal faulting, which is unsurprising for this area. Central Greece contains abundant active normal faults due to the presence of the Central Hellenic Shear Zone, which we have previously written about at some length. The integrated activity of these normal faults over geologic time has produced the beautiful and rugged basin-and-range terrain in this region.
In late 2020 and through much of 2021, there was an unusual earthquake sequence just to the northeast of today’s event, also due to activation of normal faults. Starting with a magnitude 4.6 mainshock, a long-live seismic swarm occurred that activated several different normal faults, at depths between about 5 and 10 kilometers in the crust.
George Kaviris and a long list of authors wrote a very interesting paper on this swarm (Kaviris et al., 2022 — the paper is open access). By carefully determining the 3D locations of the earthquakes, they tried to figure out whether the swarm was caused solely by stress changes from the mainshock, or required a different mechanism like fluid movements in the crust.
This 3D model from their supplementary information shows the earthquakes, classified into 12 groups based on their location, along with some possible fault planes:
Another fascinating diagram from the main text shows the evolution of the swarm over space and time. The vertical axis is the distance from a fixed point, and the horizontal axis is time. The swarm migrated eastward (upward on the vertical axis) over a period of about 4 months. This slow movement of the micro-earthquake locations over time might have been caused by movement of pressurized fluids along the fault system, driven by stresses from the mainshock.
These kinds of earthquake swarms are interesting because they provide a window into the condition of the deeply buried faults. A swarm can follow a mainshock (as in the 2020-2021 swarm near Thiva, and after the April 3, 2024 earthquake in eastern Taiwan). Earthquake swarms can also happen before mainshocks (as for the January 1, 2024 earthquake in Noto, Japan, and offshore Vancouver Island in July 2024). They can also happen without any large earthquake occurring as part of the sequence.
Today’s earthquake was a bit smaller than the M4.6 event in 2020 that kicked off the 2021 swarm. It will certainly be interesting to see whether a renewal of the Thiva seismic swarm happens over the coming months — or not!
Central Greece is highly seismically active, and earthquakes larger than magnitude 6 can certainly occur here. In 1981, over a period of less than two weeks, three magnitude 6+ earthquakes struck along the eastern Gulf of Corinth (technically, the Gulf of Alkyonides). Surface ruptures were mapped on both the north and south sides of the gulf.
Like today’s earthquake, those larger earthquakes all arose from rupture of normal faults that are accommodating the rapid north-south stretching of the crust within the Central Hellenic Shear Zone.
Damage from the 1981 earthquakes was locally severe, especially along the coasts, and the mainshocks were followed by a period of intense aftershocks. An early on-the-ground report from the Earthquake Engineering Research Institute provides a lot of interesting information about the effects of those earthquakes.
Like many earthquake-prone countries, Greece has progressively improved its building codes in response to significant earthquakes. However, the stock of old and non-retrofitted buildings is still large, and most areas have not yet been challenged by the most intense possible shaking. At present, it is not possible to say whether swarm-like behavior seen at Thiva has any meaningful implications for the potential for a larger earthquake in that area. In any case, it is always a good idea to use M4.4 earthquakes — and their noticeable but not terrifying shaking — as a reminder to revisit plans for the large earthquakes that must eventually happen.
References
Bradley, K., 2023. Magnitude 5.2 earthquake shakes central Greece. Earthquake Insights, https://doi.org/10.62481/e6df8a3a
Hubbard, J. and Bradley, K., 2024. Seismic swarm near southern tip of April 3 M7.4 in Taiwan. Earthquake Insights, https://doi.org/10.62481/b28eb629
Hubbard, J. and Bradley, K., 2024. Seismicity patterns around the Jan 1 earthquake in Japan. Earthquake Insights, https://doi.org/10.62481/72ea1b55
Hubbard, J. and Bradley, K., 2024. Swarm offshore Vancouver Island punctuated by M6.4 earthquake. Earthquake Insights, https://doi.org/10.62481/a23a9f0a
Kaviris G, Kapetanidis V, Spingos I, Sakellariou N, Karakonstantis A, Kouskouna V, Elias P, Karavias A, Sakkas V, Gatsios T, et al. Investigation of the Thiva 2020–2021 Earthquake Sequence Using Seismological Data and Space Techniques. Applied Sciences. 2022; 12(5):2630. https://doi.org/10.3390/app12052630