## 6 Aftershock Sequence Analysis in Relation with Temporal Variations of \(b\) Value  
The evolution of an aftershock sequence is among the key factors for operational forecasting, given that the areas already hit by the main shock are continuously under the threat of a strong aftershock or even a higher magnitude earthquake. Monitoring the seismicity is the means to evaluate the imminent seismic hazard and several powerful tools have been developed in this respect. The occurrence of a strong aftershock, or of a larger magnitude earthquake that is afterwards considered as the main shock, along with the occurrence frequency of aftershocks depend upon the stress residuals and stress redistribution before and after the target earthquake. The direct manifestation of the stress state is the seismicity and one approach to detect fluctuations in its temporal behavior might be through the detection of the temporal variations in the \(b\) value. The disadvantages of this procedure are frequently connected with the lack of adequate data before the main shock that could prevent us from a robust estimation of the \(b\) value, and the overlapping of the coda-waves in the first hours after the main shock that can limit the early aftershocks detection capability [30]. However, the duration of our relocated catalog starting one year before the main shock and careful inspection and manual phase picking for aftershocks augmented the data sample and diminished the possible uncertainties in the \(b\) value estimations. The approach that we apply is described in the next steps and follows the procedure of [31]. A catalog encompassing 662 earthquakes that occurred from 4 February 2018 up to 10 October 2019 consists of our data sample for investigating the temporal behavior of the \(b\) value before and after the main shock that occurred on 5 February 2019, one year after the incipience of the catalog. Firstly, the completeness magnitude is detected through the maximum curvature (MAXC) method [32], which is insensitive to sample sizes and shows great stability in estimating M\({}_{\rm C}\)[33]. The magnitude bin with the highest frequency of events corresponds to the threshold value, which in our case is found  
Figure 6: (**a**) Coulomb stress changes caused by the coseismic slip of the main shock, calculated at a depth of 16.5 km, according to the faulting type of the main shock. Thin black line is the fault trace at the depth of the top of the seismogenic layer, (**b**) same as in (**a**) for the faulting type of the strongest M4.0 aftershock at a depth of 11.5 km. to be \(M_{c}=1.3\) (Figure 7). The \(b\) value of the GR law is computed through the maximum likelihood estimation (MLE) method [34]:  
\[b=\frac{1}{\ln(10)\Big{(}\overline{M}-M_{c}+\Delta M/2\Big{)}}, \tag{5}\]  
where \(\overline{M}\) is the mean value of the magnitudes and \(\Delta M\) is the magnitude bin, usually chosen to be equal to 0.1. This relation evidences the dependence of the \(b\) value on the choice of \(M_{C}\). Figure 7b reveals a lack of earthquakes with magnitudes M \(\geq\) 3.2 compared with expected ones from the FMD fit. Similar breaks in FMD, also known as the nonlinearity problem, are attributed to aseismic stress release [35] as well as to a lack of faults in a particular area capable to fail in specific magnitude earthquakes [36]. In our case, the observed deficit could be related either to the seismotectonic characteristics of the study area or to the short time period of our dataset, or perhaps to both reasons. Nevertheless, the investigation of the physical mechanism responsible for the nonlinearity in the FMD remains an open issue for investigation and we believe that the data adequately support the investigation of the temporal variations in the \(b\) value for the specific seismic sequence. For the assessment of the temporal variations in the \(b\) value before and after the main shock, firstly, we divided the catalog into two sub-catalogs, before and after the main shock with the \(N_{pre}\) and \(N_{after}\) sample sizes, respectively.