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.

In this study, we show how the Global Navigation Satellite System (GNSS)-derived vertical velocities contribute to the correction of tide gauge (TG) measurements used for the sea level rise estimation in Greece. Twelve sites with records of local sea level heights are processed in order to estimate their trend. Certain error sources related to TGs, e.g. equipment changes, data noise, may lead to biased or erroneous estimations of the sea level height. Therefore, it would be preferred to follow a robust estimation technique in order to detect and reduce outlier effects. The geocentric sea level rise is estimated by taking into account the land vertical motion of co-located GNSS permanent stations at the Hellenic area. TGs measure the height of the water relative to a monitored geodetic benchmark on land. On the other hand, using GNSS-based methods the vertical land motion can be derived. By means of extended models fitted to the GNSS time-series position, obtained from seven years of continuous data analysis, periodic signals are well described. The synergy of the two co-located techniques results in the correction of TG relative sea level heights taking into account the GNSS vertical velocities and consequently obtaining the conversion to absolute (geocentric) sea level trend.

The Northern Thessaly Basin in central Greece ranks amongst the most well pronounced extensional (graben) basins in the back-arc Aegean Sea region, with well–mapped faults having an ∼E–W orientation, compatible with the ongoing predominant ∼N–S extension. The southern margin of the basin is bounded by major faults associated with strong (M6 to M7) earthquakes, whereas along its northern margin, strong events are more scarce, in the documented catalogues. Along this northern margin, a weak, albeit persisting foreshock activity, culminated within three days, to an M_w6.3 earthquake on 3 March 2021 associated with a 15 km–long NE dipping fault segment. It was followed the next day, by the second M_w6.0 main shock associated with a 13 km–long NE dipping fault segment and nine days later by an M_w5.5 earthquake associated with an 8 km–long SW dipping fault segment, with its aligned epicenters, showcasing the cascade type activation of adjacent fault segments. The sequence, evolved to be very productive, with aftershocks extending ∼50 km along a ∼NW–SE trending narrow seismic zone. All events indicate pure normal faulting, with an NNE–SSW oriented extensional axis, oblique to our previous consensus of the prevalence of ∼N–S extension. This observation documents that inherited fault fabric can be reactivated within the modern tectonic stress field. We use high–quality seismological data, alongside InSAR (Interferometric Synthetic Aperture Radar) methodology and GNSS (Global Navigation Satellite System) data, to study the temporal and spatial evolution of the sequence, and to provide inferred kinematic models that describe the complexity of the seismic process, in terms of heterogeneous slip distribution, activated fault planes, fault geometry, and displacement field. Cross–sections show that the activity defines the crustal seismogenic layer at depths between 5 and 10 km, associated with low–angle fault segments dipping to the NE. Other faults, both antithetic and secondary ones, appear active and accommodated aftershocks clusters. Using our preferred finite fault source model, we calculated the changes of Coulomb failure stress on the neighboring faults.

In this study, an updated crustal velocity field for the Greece area was estimated using a time series analysis that covers a duration of 16 years (2001–2016) from 227 Global Navigation Satellite System (GNSS) permanent stations. The GPS processing was carried out using GNSS Analysis software at MIT (GAMIT) and the velocity field expressed with respect to Eurasian plate. For the time series analysis, we applied a robust Median Interannual Difference Adjusted for Skewness trend estimator to mitigate the effects of discontinuities due to geophysical phenomena on the estimation of geodetic velocities and their uncertainties. The main earthquake events that occurred in the GPS time series analysis in the study area are analyzed, providing the co-seismic displacements in the permanent GNSS stations. We also compare our geodetic velocities with five previous publications, where we found consistency at the mm/year level, leading to reliable results for the geodynamic behavior of the Greek area, providing a dense velocity field.