The 2020–2021 Petrinjia (Croatia) earthquake sequence in terms of Tsallis entropy

The Petrinja earthquake sequence began on December 28, 2020, with a devastating ML 6.2 mainshock occurring in the area, preceded by a ML 5.08 foreshock, following a long period of relative seismic inactivity. Over the first six months of the Petrinja earthquake sequence, nearly 14,000 seismic events were recorded. In this work, we applied a density-based clustering algorithm, DBSCAN, to separate these seismic events based on their spatial concentration. Four main clusters were identified, and we analyzed their spatiotemporal properties using the framework of Non-Extensive Statistical Physics (NESP), which appears suitable for studying complex systems. The behavior of complex systems where multifractality and strong correlations among the elements of the system exist can adequately be described by Tsallis entropy (Sq), introducing the q-exponential function and the entropic parameter q that expresses the degree of non-additivity of the system. Hence, for each cluster we investigated the inter-event time (T) and distance (D) distributions, providing the qT and qD entropic parameters, respectively. Additionally, we studied the frequency–magnitude distributions in terms of the fragment–asperity model, leading to the determination of the non-extensive parameter qM. Our findings indicate that the statistical properties of the Petrinja earthquake sequence can be effectively reproduced utilizing NESP. Lastly, we estimated the coseismic static Coulomb stress changes, suggesting that the clusters’ seismic events may have been triggered by a complex fault system's (re)activation.