The Geological Structure and Tectonic Complexity of Northern Thessaly That Hosted the March 2021 Seismic Crisis

Knowing the rich presence of active faults in northern Thessaly and lack any significant seismic activity since at least mid-1940s, 2021 sequence did not surprise us. What us was fact that (i) despite great knowledge neotectonic area, causative were unknown, or almost unknown; (ii) direction faulting different than expected, given focal mechanisms showed pure normal dip-slip motion, extensional main axis also one we thought knew for this area; (iii) besides co-seismic ruptures occurred within Domeniko-Amouri basin along Titarissios River valley, there is evidence rupturing alpine basement Zarkos mountains. After thoroughly reviewing both structural setting all available literature concerning seismotectonic data interpretations sequence, including investigations our own, end up a complex tectonic with older structures now operating as inherited faults, suggest possible occurrence roughly N-dipping, low-angle, detachment-type fault. This fault runs below Mt Zarkos, reaching Elassona Basin, splay bifurcating upwards from zone. Following complexity, rupture first mainshock must have chosen split route surface through gneiss rocks (?) basinal sediments local depressions. perfect case study to shed some light on processes context geodynamics hazard assessment.