Revealing seismic sequence characteristics in the south-eastern Alps and the Western Dinarides by clustering analysis and refined location

Abstract The characterization of seismic sequences provides key constraints on fault geometry, rupture processes, and seismic hazard. In this study we investigate seismicity recorded between 2015 and 2024 in the south-eastern Alps and Western Dinarides (SEAWD), one of the most tectonically complex regions of the Alpine–Adriatic domain. Starting from the bulletin seismic catalogue of the Northeastern Italy Seismometer Network, we implement a multiscale relocation workflow. The catalogue is first relocated at regional scale using a multi-iterative NonLinLoc Source-Specific Station Term (NLL-SSST) approach, progressively refining residual grids to account for velocity-model uncertainties. Seismic sequences are then identified through nearest-neighbour clustering. Selected sequences are finally relocated at local scale using waveform-coherence–based NonLinLoc (NLL-coherence), enforcing spatial consistency among similar events. This workflow yields locations with ~hundreds-of-meters resolution and resolves fine-scale fault structures. We identify 71 sequences, mainly foreshock–mainshock–aftershock and mainshock–aftershock types, with limited swarm-like activity. Principal component analysis quantifies sequence volume and orientation of seismicity distributions. Most clusters define tabular structures: larger volumes correspond to planar fault zones, whereas compact clusters involve low-magnitude and low seismic activity. These patterns highlight the structural complexity of the Alpine–Dinaric transition zone and provide new constraints on active seismogenic structures and seismic hazard in northeastern Italy and adjacent regions.