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

Abstract

shows that such events rupture the ground surface through complex slip patterns that often reveal coseismic slip transfer between numerous synthetic and/or antithetic faults that intersect at depth (Figure 1a). There are also documented cases in which the mainshock triggered, in addition to coseismic, also syn-seismic (or sympathetic) slip on neighboring faults that do not appear to be connected to the seismogenic zone (Nurminen et al., 2020), with an outstanding example being the 1987 Edgecumbe Earthquake in New Zealand (Beanland et al., 1989;Delano et al., 2022) (Figure 1b). The spatial complexity of normal fault earthquakes is notably matched by their temporally variable occurrence (Figure 1c). Indeed, large-magnitude normal fault earthquakes often form clusters that span hours (i.e.,

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.…”

“…This pattern remarkably reproduces, in terms of strain rates, the westward propagation of the NAF across the north Aegean which separates the central/southern Aegean province from Eurasia and allows its trenchward motion in a quasi-rigid manner. An interesting pattern that arises from a closer examination of Figure 3a relates to a narrow NW-SE trending zone in central Greece, immediately north of Pagasitikos Gulf, that suggests NE-SW extension (of 50-80 nstrain/yr) along the newly identified (Mouslopoulou et al, 2022) ∼100 km long relay fault zone that accommodated the three normal fault earthquakes during the 2021 Tyrnavos seismic sequence.…”