Paleosismological evidence of the 1511 Idrija earthquake along the Colle Villano blind thrust (Friuli, NE Italy).

New geological, geomorphological, geophysical and paleosismological studies were performed in the eastern corner of the NE Italy (Racchiusana valley, Julian Prealps), where the S-verging polyphase fold and thrust belt of the Eastern Southern Alps joints with the NW-SE trending dextral strike-slip Idrija fault system. The area was hit by destructive earthquakes both historical and instrumental: the 1348 Carinzia (Mw= 7.02), the 1511 Idrija (Mw=6.98) and the 1976 Friuli earthquakes on May (Mw=6.46) and September (Mw=5.98) (Locati et al., 2011). The structural framework of the study area is characterized by a series of WNW-ESE trending, SSW-verging thrusts bordering the hills overlooking the plain or buried under the Upper Pleistocene-Holocene alluvial deposits and probably linked to the NW-SE Borgo Faris dextral strike slip fault that runs at the base of the inner Prealps (Zanferrari et al., 2008). We studied deformational effects of the buried Colle Villano thrust near Magredis village: here surficial geomorphic evidence (drainage anomalies, gentle scarps and back tilted surfaces) were highlighted. The analysis of the seismic industrial lines confirms that the Upper Pleistocene–Holocene deposits are involved in the deformation. In order to characterize the Late Pleistocene-Holocene activity of Colle Villano thrust, paleoseismological investigations were performed. We dug three trenches 1 km to the north of the Magredis village. The analysis of the trench walls allowed identifying deformation events induced by the fault activity. Two subsequent episodes of deformation were distinguished by localised warping (few metres in wave length) of the sedimentary sequences exposed by the excavations and secondary extrados faulting. One event occurred between 544-646 AD (14C 2σ) and 526-624 AD, the other – probably the last one – occurred close to 1485-1604 AD. The last displacement event is chronologically consistent with the 1511 earthquake. This seismic event has been attributed to the activation of Idrija fault by Fitzko et al. (2005). However, no paleoseismological evidence of this has been provided to date. The damage distribution of this event suggests its seismogenic source to be located at the easternmost portion of the Julian Prealps, and our investigations may indicate that the 1511 earthquake could have triggered not only the Idrija fault, but also some segments of the others NW-SE dextral strike slip-systems of the Italian-Slovenian border area