###### Abstract  
On 3 March 2021 (10:16, UTC), a strong earthquake, M\({}_{\rm w}\) 6.3, occurred in Elassona, Central Greece. The epicenter was reported 10 km west of Tyranvos. Another major earthquake followed this event on the same day at M\({}_{\rm w}\) 5.8 (3 March 2021, 11:45, UTC). The next day, 4 March 2021 (18:38, UTC), there was a second event with a similar magnitude as the first, M\({}_{\rm w}\) 6.2. Both events were 8.5 km apart. The following analysis shows that the previous events and the most significant aftershocks were superficial. However, historical and modern seismicity has been sparse in this area. Spatially, the region represents a transitional zone between different tectonic domains; the right-lateral slip along the western end of the North Anatolian Fault Zone (NAFZ) in the north Aegean Sea plate-boundary structure ends, and crustal extension prevails in mainland Greece. These earthquakes were followed by rich seismic activity recorded by peripheral seismographs and accelerometers. The installation of a dense, portable network from the Aristotle University of Thessaloniki team also helped this effort, installed three days after the seismic excitation, as seismological stations did not azimuthally enclose the area. In the present work, a detailed analysis was performed using seismological data. A seismological catalogue of 3.787 events was used, which was processed with modern methods to calculate 34 focal mechanisms (M\({}_{\rm w}\) > 4.0) and to recalculate the parameters of the largest earthquakes that occurred in the first two days. Big data; source observations; seismological data; data management; synthetics seismograms; regional data 2021 22 20  
Regarding the tectonic setting, Thessaly is located at the western end of the North Anatolian Fault Zone (NAFZ) in the north Aegean Sea, where the right-lateral slip along this plate-boundary structure ends and crustal extension prevails in mainland Greece [1,2]. The most prominent structural and geomorphic features strike NW-SE, such as the coastal and interior mountain ranges, bounding faults, and the late Tertiary sedimentary basins [3]. The seismicity of the Thessaly Basin is detected mainly along two fault zones relative to the maximum magnitude an earthquake can provide. The first is the northern fault zone, associated with earthquakes with magnitudes up to 6.5, and the second is the southern fault zone, related to earthquakes with a magnitude of 7.0. In early March 2021, a doublet earthquake (M\({}_{\rm w}\) 6.3 and 6.2) occurred in the northern Thessalian region. The spatial distance of the two earthquakes was determined at only 8.5 km, while their time difference at 32 h. The first earthquake, magnitude 6.3, occurred on 3 March 2021 (10:16:08.3, UTC). The epicenter, as calculated and presented in the next paragraph, was found at \(\varphi=39.7453^{\circ}\) N, \(\lambda=22.2340^{\circ}\) E at a depth of 12 km. The earthquake was felt in Larissa, Trikala, and Kozani. This fact is due to the epicenter being a short distance from large cities. The epicenter was calculated 19 km northwest of the town of Larissa, 46 km northwest of the city of Trikala, and 72 km south of Kozani. According to the above, the mainshock caused extensive damage due to its short distance in a populated urban area, according to the European Mediterranean Seismological Center [4]; [[https://www.emsc-csem.org/#2](https://www.emsc-csem.org/#2)]([https://www.emsc-csem.org/#2](https://www.emsc-csem.org/#2)) (accessed on 8 June 2022). Several buildings collapsed after this earthquake. One person died from the earthquake in the worst-hit village of Mesohori, near Eliassona, where the local church and some old houses were severely damaged. Of the 3.675 homes, 1575 were deemed uninhabitable in the earthquake-stricken areas of Larissa, Eliassona, Tyranvos, and Farkadona. The earthquake caused much damage in the surrounding regions, and the Damasi primary school, built in 1938, suffered significant damage, resulting in its demolition eight days after the earthquake [5].