## 3 Methods and Processing  
_Synthetics Seismograms at Regional Distances_  
The first deviations of P-waves are the most commonly used method to determine the focal mechanism of an earthquake. In this method [44], the information is provided by the arrival of the first P-waves and the fact that the first deviations depart as compressions at the two vertical angles formed by the plane of the fault and the auxiliary plane and as dilutions at the other two vertical angles is utilized. The above method provides reliable results for short but focused distances (<100 km) when azimuth coverage and clarity record the first arrival. This is how the focal mechanism of an earthquake is calculated. However, in most cases, this method is considered insufficient [44] for determining the focal mechanism, whether since the stations have a poor azimuth distribution or that these stations did not  
Figure 2: Geographical distribution of the Hellenic Unified Seismological Network’s stations in the region of Thessaly. Green triangles represent the seismological stations belonging to the University of Patras, purple triangles denote the portable network installed by the Aristotle University of Thessaloniki immediately after the earthquake, and the red triangles are the stations of the National Observatory of Athens. The light blue color represents the cities near the epicenter. record the first movement. The above is evidence that the method is uncertain. Therefore, there is no reliability for the focal mechanism. To determine the source parameters, it is required to create synthetic seismograms for each station and to compare them with corresponding observations. It is mathematically possible [45; 46; 47] to model and develops a process so that it is possible to predict the character of the seismogram, taking into account a realistic model of seismic wave propagation. This modeling is called a synthetic seismogram. As mentioned in previous studies [26; 27], the proposed methodology calculates the Green functions to determine the Earth's response. Then, the corresponding synthetic seismograms are compared with those observed to minimize their difference. For example, [PERSON]'s theory is used to create the Green functions [45; 46]. The geometry of seismic rays is determined by the principle of Fermat, according to which the wave recorded at a certain point by a specific source follows the shortest path from those that can follow; that is, it follows the one that requires the least time. The above is practically equivalent to the researcher considering the focal length. This is why it is necessary to distinguish, when studying an event, at what distances the data used are located. According to the above, there are two major categories: the first includes data at distances less than \(30^{\circ}\), and the second is for those stations with an epicentral distance of \(30^{\circ}<D<90^{\circ}\)[47]. Limiting the epicentral distance is considered necessary because stations with epicentral distances less than \(30^{\circ}\) have radii with departure angles dependent on the structure of the upper mantle. In this case, combining the knowledge of the Earth's response, a velocity model, and an appropriate methodology results in the calculation of Green functions. Three different methodologies are used to calculate synthetic seismograms for earthquakes where near-regional data are used [48; 49; 50; 51]. In the present article, Green functions were computed using the method in [48]. This method was initially presented in [52; 53]. The basic idea of this method is that Green functions for an elastic layered structure can be calculated as a double integral on the frequency and horizontal waveforms [54]. Discretization was performed for each elementary point source. According to this theory, an infinite number of sources are gathered around it and distributed in equal intervals. The choice of distance depends on the time required for the point source to respond and identify the discrete number from the horizontal waveforms [55].