Morphotectonic investigation of the active faults in Boeotia, central Greece, before the 2020-2023 seismic crisis

In November 2020, an earthquake sequence started to evolve ca. 10 km east of Thebes (Boeotia, central Greece) with a maximum magnitude of M_L4.4 (02/12/2020). In July 2021, the seismic activity migrated westwards, under the town of Thebes. Before the activity faded away completely, a new outburst started in March 2022. The whole activity sparsely continued until the first quarter of 2023. In total, 5 earthquakes of 4.0≤M_L≤4.4 occurred during the seismic crisis. Published focal mechanisms of the strongest events revealed normal faulting on E-W- to WNW-ESE-striking nodal planes.

A few years before this episodic seismic activity near Thebes, a morphotectonic field mapping and analysis were carried out in the broader area. In more particular, within the epicentral area, two fault groups were detected: i) the ‘Kallithea’ fault zone, i.e. a series of two aligned SSW-dipping normal fault segments, and ii) the ‘Thebes’ fault system of parallel to subparallel, occasionally imbricated, roughly E-W-striking oblique-normal faults dipping to the South.

The Kallithea fault segments separate the two rather elongated hills of alpine carbonates from the Neogene basinal deposits (including Holocene slope debris). Slickenlines preserved on mildly eroded limestone free-faces show quasi-pure normal faulting in a NNE-SSW trending extensional stress field. Morphometric analysis along the fault zone suggests that the two segments are not yet linked to each other, reducing significantly the earthquake potential of the fault zone.

The Thebes fault system has formed a subdued topographic relief on Plio-Pleistocene and Pliocene deposits demonstrating variable throws on the overstepping faults array at the order of 1-2 m, producing a total stepwise downthrow of the hanging-wall of about 15-20 m. Although the lithology does not allow a good preservation of free-faces, few exposures bear oblique slickenlines revealing a ca. N-S direction of extension. This fault system is probably the surficial manifestation of a single deeper structure.

Both fault groups are considered responsible for the 2020-2023 seismic crisis, transferring stresses from one to the other as also suggested by the episodic activity and the horizontal migration of the epicentres. In fact, the northwestern segment of the Kallithea fault zone is probably associated to the eastern cluster of the seismic activity, also suggesting a further northwestward continuation of the fault segment. The surficial length of the specific faults suggests moderate expected magnitudes, although strong historic and early instrumental earthquakes have been recorded in the surrounding area.