###### Abstract
The second major earthquake occurred one day later, on 4 March 2021 (18:38:19.1, UTC), with M\({}_{\rm w}\) 6.2. The epicenter of this event was calculated at 39.7916\({}^{\circ}\) N, 22.1274\({}^{\circ}\) E, and at a depth of 8 km. Eight days later (12 March 2021, 12:57:50.6, UTC), a significant event, the third chronologically, eight days after the main one, with a magnitude M\({}_{\rm w}\) = 5.6, occurred in a different hypocentral area than the other two. A rich aftershock sequence followed these earthquakes, most felt by the area's residents mainly due to their small focal depth. It is noted that in the first 24 h only, 381 events occurred, ranging from M\({}_{\rm L}\) 1.2 to 5.2. The intense aftershock sequence activity with M\({}_{\rm L}\) > 4.0 aftershocks continued for a few days, with a smaller magnitude but high aftershock occurrence rate. These two significant earthquakes have been the strongest in the region since 1941 when an earthquake of the magnitude M\({}_{\rm L}\) 6.3 occurred very close to the city of Larissa. The area of Thessaly, Central Greece, has been well known for its land subsidence phenomena since the 1980s [6, 7, 8, 9, 10, 11]. Several institutions providing seismic moment tensors consistently determined an almost normal fault on the east-south and northwest plane for the first and second mainshocks. Moving in the fault, the two earthquakes had a similar focal mechanism. This means that they probably came from the same fault but different parts. From these data, we probably have the phenomenon of a seismic pair, which is extremely rare. Earthquake doublets commonly occur in subduction zones and are mainly characterized by having similar mechanisms, spatially and temporally [12, 13, 14], correlated with several tens of kilometers and hours to days, respectively [15, 16]. They can mainly be explained by Coulomb stress triggering [17]. Intraplate earthquake doublets are rare and less studied. Doublet earthquakes--and, more generally, multiple earthquakes--were initially identified as numerous earthquakes with nearly identical waveforms originating from the exact location [18]. They are now characterized as single earthquakes having two (or more) mainshocks of similar magnitude, sometimes occurring within tens of seconds but sometimes separated by years [19]. The magnitude similarity--often within four-tenths of a unit of magnitude--distinguishes multiple events from aftershocks [20], which start at approximately 1.2 magnitudes more minor than the parent shock [21, 22] and decrease in magnitude and frequency according to known laws. Doublet events also have nearly identical seismic waveforms, as they come from the same rupture zone and stress field, whereas aftershocks, peripheral to the main rupture, typically reflect more diverse circumstances of origin [18]. Multiple events overlap in their focal areas (i.e., rupture zones), up to 100 km across for magnitude 7.5 earthquakes [23; 24; 25]. Doublets have been distinguished from triggered earthquakes, where the seismic waves' energy triggers a distant earthquake with a different rupture zone, although it has been suggested that such a distinction reflects \"imprecise taxonomy\" more than any physical reality. Figure 1 represents a general structure map showing the prefectures of the broader region of Central Greece. For the study area, the portable seismological station sites' selection criterion was based on the existence and operation of permanent seismological and geodetic stations that were already installed. In addition, faults were collected from the international literature and digitized in QGIS. The contribution of the present study is the localization of the significant events, i.e., epicenter (i.e., latitude and longitude), depth (d), and local magnitude (ML), as well as the calculation of the source parameters (i.e., strike, dip, rake, depth, and moment magnitude) for the strong events and all of the significant aftershocks using moment tensor inversion.