Characterization of Earthquake Clustering in Contractional Regions, based on Nearest-Neighbor distances and Network Analysis

The features of earthquake clusters in two contactional domains, located in Northeastern Italy and North-Central Iran, have been investigated. The tectonic and seismicity of the two study areas, namely the Alps-Dinarides junction and the Alborz regions, are controlled by the convergence between the African and Arabian plates and the Eurasia plate. Both regions are characterized by a rather complex structural setting, mainly including reverse and strike-slip faulting systems, and by moderate to high seismic activity.

The nearest-neighbor approach has been used for the identification of the earthquake clusters in the space-time-energy domain. This approach permits for a data-driven identification of clusters so that, within multi-event clusters, the features of secondary and higher orders dependent events can be explored. The investigation of seismicity in Northeastern Italy is based on bulletins compiled at the National Institute of Oceanography and Applied Geophysics (OGS) in 1977-2018, while in North-Central Iran the dataset was extracted from the catalog compiled by the Iranian Seismological Center (IRSC) for the period 1996-2022. According to preliminary analysis of the used earthquake catalogs, two corresponding regions have been identified, where a satisfactory completeness level is assessed for events with magnitude greater than 2.0. Robust values of the scaling parameters, namely the b-value and the fractal dimension of epicenters, have also been computed and are used to calculate the nearest-neighbor distances and to identify the earthquake clusters.

The results obtained in the two regions confirm that the complexity of clusters structure depends on the tectonic, structural, and geophysical properties of the area. Moreover, the complexity measures, borrowed from network theory (i.e. the Centralization and Outdegree indexes), consistently capture the complexity of the identified clusters. Besides, in both investigated regions, the results allowed us identifying two macro-areas, which are characterized by different clustering features, namely: high complexity indexes,, which indicate simple (burst-like) structure of clusters, and low complexity indexes, corresponding to complex multi-level (swarm-like) structure of clusters. Specifically, we found that "swarm-like" (high complexity) sequences are prevalent along the thrust faulting Alpine and Central-West Alborz systems, whereas "burst-like" (low complexity) sequences prevail along the strike-slip Dinaric and Central-East Alborz domains.