The northern Thessaly strong earthquakes of March 3 and 4, 2021, and their neotectonic setting

Abstract

A sequence of earthquakes occurred on March 3rd and 4th in Northern Thessaly, northern Greece, associated with previously unknown, blind normal faults within the crystalline Palaeozoic basement of the Pelagonian geotectonic zone. Surficial ground deformation, such as liquefaction phenomena in fluvial plains, as well as soil fissures and rock falls, have been mapped. Geological indications of the unmapped seismic fault, i.e., reactivated shear zones, open cracks, etc., have been identified within the bedrock. Based on geological indications, the main fault projection to the surface could be considered a 15 km NW-SE trending structure and average dip of 45o to the NE. The seismic source of the main shock was modelled, and the Coulomb static stress changes calculated for receiver faults similar to the source. The determination of the active tectonic regime of the region by geodetic data and the well-known faults of NE Thessaly plain are also presented, as well as the revised historical and instrumental seismicity. This earthquake raises new concerns and challenges, revising some established views, such as the status of main stress orientations, the orientation of active tectonic structures, the occurrence of a seismogenic fault in a mountainous massif of crystalline rocks without typical geomorphological expression and the role of blind faults in Seismic Hazard Assessment.

The Upper Crustal Deformation Field of Greece Inferred From GPS Data and Its Correlation With Earthquake Occurrence

“…Interestingly, in the vicinity of the broader epicentral area of the 2021 Tyrnavos seismic sequence the extensional axes that we calculated follow a NE‐SW direction which deviates from the general N‐S orientation of extension in Thessaly that has been estimated from geological data (Caputo & Pavlides, 1993). However, this NE‐SW direction is compatible with the NW‐SE strike of the coseismic ground ruptures that were revealed by post‐earthquake field observations and InSAR data (Chatzipetros et al., 2021; Kontoes et al., 2022; Koukouvelas et al., 2021; Mouslopoulou et al., 2022; Sboras et al., 2022) and is in agreement with the NE‐SW extensional stress field obtained from the focal mechanisms of the 2021 Tyrnavos seismic sequence (Kassaras et al., 2022) and with geomorphic indicators which suggest repeated past ruptures on the 2021 causative faults (Mouslopoulou et al., 2022). In this context, it turns out that the orientation of the faults that ruptured during the 2021 sequence was not incompatible with the present‐day crustal strain orientation as it was initially believed (Lazos et al., 2021; Sboras et al., 2022) and it appears that the occurrence of this earthquake sequence is well‐justified by the active strain field.…”

A Deeper Look Into the 2021 Tyrnavos Earthquake Sequence (TES) Reveals Coseismic Breaching of an Unrecognized Large‐Scale Fault Relay Zone in Continental Greece

“…Specifically, here we find that the only coseismic displacement recorded by InSAR to have ruptured the ground surface during the TES (see solid blue line in Figure 6 and in the wrapped interferogram of Figures S14a, S14c, S14d in Supporting Information S1), extends along a section of the Zarkos FS that has clear geomorphic signature (Figure 4a). Thus, the fault on which the mainshock took place is not blind-as many have suggested (e.g., Chatzipetros et al, 2021;De Novellis et al, 2021;Ganas et al, 2021;Papadopoulos et al, 2021; but, instead, includes traces with a clear history of preceding surface ruptures prior to the March 3rd event (Figure 4a).…”

“…Interestingly, the 2021 TES was not accommodated on any of the previously known active normal faults (Figure S1 in Supporting Information S1), which are mostly characterized by spectacular scarps of Holocene age (Caputo & Helly, 2005;Caputo et al, 2004;Tsodoulos et al, 2016) (Figure 3). Instead, all three TES mainshocks (M w 6.3 Event 1 on 3 March, M w 6 Event 2 on 4 March and M w 5.7 Event 3 on 12 March; Figure 2) ruptured previously unrecognized faults that extend beneath hilly country (e.g., Chatzipetros et al, 2021;Ganas et al, 2021;Koukouvelas et al, 2021;Papadopoulos et al, 2021;.…”

“…As the fault that accommodated the M w 6.3 event was characterized "blind" (e.g., Chatzipetros et al, 2021;Ganas et al, 2021;Papadopoulos et al, 2021;, field studies (i.e., Galanakis et al, 1b; see Section 3.3 for details). The majority of the published studies also seem to agree that the TES ruptured overall immature faults (e.g., Karakostas et al, 2021;Kassaras et al, 2022;Koukouvelas et al, 2021) due to a domino effect of stress-transfer (De Novellis et al, 2021;Ganas et al, 2021;Kassaras et al, 2022) that underpins the westward propagation of the faults of the Larissa Basin (Chatzipetros et al, 2021;Koukouvelas et al, 2021).…”

“…This normal faulting event was unambiguously modeled by all above studies to have ruptured a shallow (∼40°) NE dipping plane of NW‐SE strike for a length of ∼15 km. As the fault that accommodated the M w 6.3 event was characterized “blind” (e.g., Chatzipetros et al., 2021; Ganas et al., 2021; Papadopoulos et al., 2021; Pavlides & Sboras, 2021), field studies (i.e., Galanakis et al., 2021; Koukouvelas et al., 2021) focused mostly on reporting secondary structures and associated displacements (Figure 1b; see Section 3.3 for details). The majority of the published studies also seem to agree that the TES ruptured overall immature faults (e.g., Karakostas et al., 2021; Kassaras et al., 2022; Koukouvelas et al., 2021) due to a domino effect of stress‐transfer (De Novellis et al., 2021; Ganas et al., 2021; Kassaras et al., 2022) that underpins the westward propagation of the faults of the Larissa Basin (Chatzipetros et al., 2021; Koukouvelas et al., 2021).…”