March 2021 Thessaly, central Greece, seismic sequence: domino effect of a complex normal fault system

The Thessaly seismic sequence (TSS) in Central Greece, started on 3 March 2021 with a Mw 6.3 event that struck an area located about 25 km WNW of the Larissa town. In the following days, TSS was affected by other two major events: An Mw 6.0 on March 4, localized about 7 km to the northwest of the first one, and a Mw 5.6 on March 12, located 12 km further towards the northwest of the second one. A large number of smaller events have been also recorded until mid-April when the sequence decreased in frequency and magnitude. The TSS represents the largest seismic sequence affecting a continental extensional domain in Greece that has been monitored by modern geodetic techniques. Thanks to the short satellite revisit time, InSAR measurements made it possible to isolate each contribution of the three major earthquakes of the sequence, thus allowing the study of their interactions. In addition, available geological data indicate that the northern sector of Thessaly represents a large seismic gap. This may be a direct consequence of the limited size of the faults (less than 20 km) and their intrinsic capability to originate earthquakes of small-to-moderate magnitude only. TSS, which finally filled the gap, confirmed this hypothesis.

We modelled the available InSAR deformation maps to retrieve the parameters characterizing some finite dislocation sources, which were used to perform a Coulomb stress transfer in order to investigate possible faults interactions. To constrain the geometry and location of the main fault structures involved during the TSS, we considered 1853 earthquakes occurred in the area from 28 February 2021 to 26 April 2021 with magnitude ranging between 0.2 and 6.3. Our model shows that the TSS has nucleated at shallow depths (<12 km) and is related to the activation of several blind, previously unknown, faults; moreover, the seismic sequence developed in a sort of domino effect involving a complex interaction among the normal faults within the activated crustal volume. As for the temporal evolution of the sequence, the delayed triggering of the Mw 6.0 earthquake can be explained by the distribution of the events occurred earlier, which encircle the asperities that will fail in the subsequent event together with a fluid diffusion in the seismogenic volume.

Finally, we highlight the key role played by the configuration of the Thessaly Basin characterized by blind faults interconnected at depth, particularly interesting from the neotectonics point of view. The used approach can help improving our knowledge on the seismic potential of the Thessaly region and refine the associated seismic hazard.