The Periadriatic Fault, also called the Insubric Line, is a major fault system stretching more than 700 kilometres across the eastern and central Alps from Italy through Austria and Slovenia. It marks the deep boundary between the Adriatic microplate to the south and the European plate to the north, formed during the Alpine collision.
Its most active motion occurred during the Oligocene and Miocene as a dextral strike-slip zone, exhuming granitic intrusions along its trace and helping to shape the arc of the Alps. Today it is far less active than the fast Mediterranean faults, but its splays still localise moderate seismicity in the surrounding valleys.
The fault system frames some of the Alps' most seismically active margins, including the Friuli region of northeast Italy where the 1976 earthquake (magnitude ~6.5) killed nearly a thousand people. Geologists study the Periadriatic Line as a textbook example of how continental collision partitions strain along a lithospheric suture.