Investigation of variations of Seismic stress, Seismic quiescence, and Seismic velocities Before and after the occurrence of MN≥5.0 Earthquakes within the Azarbayejan-Alborz seismotectonic province

In the present study, we investigate some statistical features of earthquake precursors, namely the variations of the b-value, Z-value, and  parameters in time and space, before and after the occurrence of MN≥5.0 earthquakes in the Azarbayejan-Alborz seismotectonic province. Based on the literature, the b-value of the Gutenberg–Richter distribution is connected to the field of seismic stress, so that it decreases linearly with increasing differential stress, and vice versa. Still, a high differential stress at later stages of an earthquake cycle causes failure of large patches. The Z-value parameter is also associated with seismic quiescence periods, implying the regional preparedness for the occurrence of strong earthquakes.
It is worth mentioning that our study covers seven MN≥5.0 target earthquakes that occurred between 2010 and 2022. However, due to the large number of target earthquakes, the text body is focused on one of the events accompanied with all investigated anomalies, namely the 2017 Malard MN 5.2 earthquake.
In order to study the anomalies of seismic parameters, an earthquake catalog, reporting post-2006 MN≥0.4 events, was extracted from the Iranian Seismological Center. As the raw catalog includes small size events, it was preprocessed via removing possible non-tectonic events. These events, being mostly quarry blasts, have been statistically recognized by an unrealistic increase in the number of events during working hours (i.e. 6.00 to 16:00 O’clock). Accordingly, the Rq method, implemented in the ZMAP software package, has been used for removing possible quarry blasts. The estimation of seismic parameters has been done by assuming the Poisson distribution of earthquake occurrences. Therefore, dependent events (i.e. aftershocks and foreshocks) were also eliminated from the used catalog, using a declustering procedure proposed by Reasenberg (1985). However, in the case of the 2017 Malard earthquake, the de-quarrying and declustering procedures remove 11900 events out of 26836. Having estimated the magnitude of completeness (Mc) for the region of the Malard earthquake, we removed earthquakes with magnitudes less than Mc=2.2. Furthermore, as earthquake catalogs commonly involve in a sort of temporal fluctuation of Mc which is mostly due to increase in the number of seismographs over time, the stability of Mc=2.2 for the region has also been checked. In total, the residual catalog includes 4919 MN≥2.2 earthquakes over the Malard region.
In total, our results indicate that at least one of the investigated anomalies can retrospectively be observed prior to all target earthquakes. Still, in some cases both anomalies can simultaneously be detected in the space and time domains. Moreover, the results show that regions characterized by both b-value≤1.0 (after experiencing a temporally decreasing trend of the b-value parameter) and Z-value≥3.0 are most prone areas for future MN≥5.0 earthquakes. Moreover, the ratio like the other mentioned seismic parameters is connected to the field of seismic stress, so that it decreases proportionally with increasing the differential stress. In total, our results show that the spatio-temporal variations of the above-mentioned seismicity parameters and the ratio can be interpreted as the effective indicators for areas prone to forthcoming significant earthquakes.