Abstract
Fault strength spatial variability controls how earthquakes initiate, propagate, and arrest, yet remains poorly resolved in complex tectonic settings. The southeastern Alps constitute one of the most seismically hazardous regions in Central Europe, with active fault systems, a history of damaging earthquakes, and ongoing tectonic deformation. We analyze more than 9,200 small-to-moderate earthquakes (0 ≤ M(L) ≤ 4.5) recorded between 2016 and 2025 to image lateral variations in crustal stress using the Energy Index (EI), a moment-energy parameter sensitive to rupture efficiency. By extending the RAMONES framework to this region, we detect pronounced east–west contrasts in fault mechanical behavior: high EI values in the west mark zones of reduced frictional strength, whereas low EI in the east suggests mechanically stronger, segmented fault domains. These spatial patterns align with independent geophysical indicators (V(P)/V(S), Q(P)), indicating a strong link between mechanical segmentation, material properties, and permeability structure. Our results demonstrate that small earthquakes carry diagnostic signatures of fault-zone strength and segmentation, providing a scalable tool to resolve stress heterogeneity and refine seismic hazard models in structurally complex regions. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1038/s41598-026-35618-y.