Seismic clusters analysis in Northeastern Italy by the nearest-neighbor approach

The main features of earthquake clusters in Northeastern Italy are explored, with the aim to get new insights onlocal scale patterns of seismicity in the area. The study is based on a systematic analysis of robustly and uniformlydetected seismic clusters, which are identified by a statistical method, based on nearest-neighbor distancesof events in the space-time-energy domain. The method permits us to highlight and investigate the internalstructure of earthquake sequences, and to differentiate the spatial properties of seismicity according to thedifferent topological features of the clusters structure.To analyze seismicity of Northeastern Italy, we use information from local OGS bulletins, compiled at theNational Institute of Oceanography and Experimental Geophysics since 1977. A preliminary reappraisal of theearthquake bulletins is carried out and the area of sufficient completeness is outlined. Various techniques areconsidered to estimate the scaling parameters that characterize earthquakes occurrence in the region, namely theb-value and the fractal dimension of epicenters distribution, required for the application of the nearest-neighbortechnique. Specifically, average robust estimates of the parameters of the Unified Scaling Law for Earthquakes,USLE, are assessed for the whole outlined region and are used to compute the nearest-neighbor distances.Clusters identification by the nearest-neighbor method turn out quite reliable and robust with respect to theminimum magnitude cutoff of the input catalog; the identified clusters are well consistent with those obtainedfrom manual aftershocks identification of selected sequences. We demonstrate that the earthquake clusters havedistinct preferred geographic locations, and we identify two areas that differ substantially in the examinedclustering properties. Specifically, burst-like sequences are associated with the north-western part and swarm-likesequences with the south-eastern part of the study region. The territorial heterogeneity of earthquakes clusteringis in good agreement with spatial variability of scaling parameters identified by the USLE. In particular, thefractal dimension is higher to the west (about 1.2–1.4), suggesting a spatially more distributed seismicity,compared to the eastern parte of the investigated territory, where fractal dimension is very low (about 0.8–1.0).