The Idrija Fault is one of the most prominent active faults of the Dinaric system, cutting northwest to southeast across western Slovenia and continuing into neighbouring Italy and Croatia. It is a right-lateral strike-slip fault that accommodates part of the ongoing convergence between the Adriatic microplate and stable Europe, and it is named for the mining town of Idrija that sits along its trace.
The fault is remarkably long and geologically well defined, expressed in the landscape as valleys and offset ridges across the Julian and Dinaric ranges. Its steady tectonic activity makes western Slovenia one of the more seismically hazardous parts of the country, with the fault capable of producing strong earthquakes.
The Idrija Fault is associated with the great 1511 Idrija earthquake, an event estimated at around magnitude 6.8 that caused widespread destruction across Slovenia, Friuli and the surrounding Alpine region. It remains a benchmark for regional seismic-hazard assessment, and paleoseismic trenching continues to refine estimates of its slip rate and recurrence.