Earthquake Geology Research Team
12 March 2021
As far as the insofar unknown and unmapped seismic fault is concerned, fieldwork showed that there are characteristic geological indications in the Pelagonian bedrock, consisting of Paleozoic mica schist and gneiss (e.g. Kilias and Mountrakis, 1987; Kilias et al., 1991), indicating that a low angle normal fault has acted as a hidden or blind fault during the earthquake. It is associated with the bedrock schistosity, as well as with small high angle reverse faults of the Pelagonian anticline. The presumed seismic fault extends in the broader area between the villages of Zarko and Megalo Eleftherochori, as an inherited shear zone (Fig. 5). Geologic indications include outcrops of the post Alpine shear zone (Fig. 5a,b,c), located along the boundary between interferometrically-indicated uplift and subsidence terrains (fig. 4, that is 0 line of displacement), the existence of cataclasite and fault gouge in the shear zone (Fig. 5a-e), which indicates reactivation of the fault in brittle conditions during the neotectonic period (inverse tectonics) and slickenlines compatible with the active stress field (Fig. 5a,c,d). Fault surfaces strike at N160oE and dip at 50o on average which is in good agreement to the published focal mechanisms by Greek (NOA and AUTh) and other international Institutes. Coseismic indicators include small, ruptured fault surfaces with detached rock slabs and pieces, as well as small-scale soil fractures following the trace of the mapped fault (Fig. 5d,g), with negligible vertical displacement and small heave (up to 2 cm).
In conclusion, this earthquake raises new questions and concerns, while revising some established views, such as:
- The status of active stress trends (σ3 extension),
- The direction of active tectonic structures,
- The existence of a seismogenic fault in a mountainous volume of crystalline rocks without typical geomorphological expression,
- The role of blind faults to Seismic Hazard Assessment.
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