In early March 2021, three shallow earthquakes, two mainshocks with M6.3 and M6.0 and one major aftershock with M5.6 impacted both the mountainous Damasi-Tyrnavos region (northern Thessaly, Greece) and the adjacent Plio-Quaternary basin. Each major event was followed by rich aftershock activity recorded by local and regional seismographs and accelerographs. Herein, we present a comprehensive analysis of the seismic sequence, from its foreshock activity starting on 28 February, 2021 and for a period of two months using new high-resolution catalogues of relocated earthquakes and hundreds of focal mechanisms. The results indicate that the aftershocks form a zone that spans ~50 km NW-SE, while focal depths range between 5 and 15 km. More than 400 focal mechanisms, computed for events with M≥ 2.5, mainly exhibit normal faulting in a NW-SE direction, while WNW-ESE to E-W normal faulting is also evidenced, in particular after the occurrence of the last major event on 12 March. The stress-field was reconstructed on a local and broader scale by inverting focal mechanism data, revealing a rotation of the σ3 axis trend from NNE-SSW, in the Damasi-broader region, to NW-SE northwards, to the region of Kozani-Grevena that hosted an Mw = 6.6 shallow mainshock in 1995. Subcrustal seismicity, present beneath those areas, implies that large-scale tectonics and plate dynamics are likely involved in the deformation of the upper crust. Coulomb stress transfer after the 3 major events of the 2021 Damasi-Tyrnavos sequence reveals that stress-loaded areas include those where most aftershocks were triggered. The analysis provides implications to the seismic hazard of the activated area, as a major NW-SE active normal fault close to Larissa city became stress-loaded, constituting a possible candidate source for significant future earthquakes.

We investigate an earthquake sequence involving an Mw = 4.6 mainshock on 2 December 2020, followed by a seismic swarm in July–October 2021 near Thiva, Central Greece, to identify the activated structures and understand its triggering mechanisms. For this purpose, we employ double-difference relocation to construct a high-resolution earthquake catalogue and examine in detail the distribution of hypocenters and the spatiotemporal evolution of the sequence. Furthermore, we apply instrumental and imaging geodesy to map the local deformation and identify long-term trends or anomalies that could have contributed to stress loading. The 2021 seismic swarm was hosted on a system of conjugate normal faults, including the eastward extension of the Yliki fault, with the main activated structures trending WNW–ESE and dipping south. No pre- or coseismic deformation could be associated with the 2021 swarm, while Coulomb stress transfer due to the Mw = 4.6 mainshock of December 2020 was found to be insufficient to trigger its nucleation. However, the evolution of the swarm is related to stress triggering by its major events and facilitated by pore-fluid pressure diffusion. The re-evaluated seismic history of the area reveals its potential to generate destructive Mw = 6.0 earthquakes; therefore, the continued monitoring of its microseismicity is considered important.