Ground-Motion Variability from Small Earthquakes Tracks Fault Mechanical Evolution in the Southeastern Alps

Understanding how fault strength evolves through time remains a central challenge in seismology, particularly in complex plate-boundary regions characterized by structural segmentation and heterogeneous coupling. We investigate whether ground-motion variability from small earthquakes can track mechanical evolution of active faults in the southeastern Alps (northeast Italy—W Slovenia)—a region that hosted destructive earthquakes such as the 1511 (∼Mw 6.8) and the 1976 Mw 6.5 Friuli events. We analyze ∼115,000 near-fault peak ground acceleration (PGA) recordings from 6,219 earthquakes (0.5≤Mw≤4.2; 2016–2025) using a mixed-effects ground-motion model that isolates the between-event term (δBe), representing source-related variability independent of magnitude, distance, and site effects. δBe exhibits clear spatial clustering and correlates positively with independently estimated stress-drop residuals, confirming that ground-motion variability reflects rupture efficiency and relative fault strength. Mapping the temporal evolution of a normalized cumulative stress index derived from δBe reveals persistent east–west mechanical segmentation and multi-year trends. Notably, the epicentral area of the 1976 Friuli earthquake exhibits a progressive post-2022 increase in relative stress drop. Our results demonstrate that small-earthquake ground-motion variability provides a model-independent proxy for tracking spatial and temporal changes in fault mechanics. In the southeastern Alps, mechanically distinct fault domains evolve differently through time, suggesting heterogeneous stress loading and redistribution across adjacent segments of the plate boundary.