SUMMARY New geomorphological and structural data permitted to define the active faulting framework of the eastern Southern Alps (NE Italy). All the active faults detected in the investigated area are thrust segments of the complex thrust system, which has been responsible for the latest building of the Eastern Southalpine chain (ESC). Geomorphological investigations were performed to identify the surficial traces of recent fault activity, generally represented by gentle scarps connecting uplifted palaeolandscapes of Quaternary age with the flat and lower areas of the Venetian and Friulian plains. Surficial and subsurficial data (the latter from reflection seismic profiles) available for the investigated faults indicate that the thrusts have been responsible for the displacement of the entire wedge of Quaternary deposits. In the western sector of the investigated area, the six recognized fault segments represent portions of a 100‐km‐long thrust system, at the boundary between the Alpine relief and the plain areas. In the eastern sector, active tectonics is the result of parallel thrust segments, located both in the Alpine mountainous area and in the Friulian plain. The 3‐D geometry of the active thrust segments has been derived from new structural surficial surveys and the interpretation of reflection seismic profiles for a total length of 1700 km. On this basis, we defined the geometry of 10 seismogenic sources whose dimensions are consistent with the occurrence of earthquakes with M≥ 6. The comparison between the source geometry and the highest intensity data point distribution of large historical earthquakes has permitted to make hypotheses on the association of past seismic events to specific seismogenic sources. This procedure indicated that no large historical events can be attributed to three sources (Montello‐Conegliano, Arba‐Ragogna, Medea). This may indicate an elapsed time since the last activation of more than eight centuries, based on the completeness of the historical catalogues. The available data define, therefore, sources (and related areas) for which a high level of seismic hazard may be invoked.

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In recent years, new approaches for developing earthquake rupture forecasts (ERFs) have been proposed to be used as an input for probabilistic seismic hazard assessment (PSHA). Zone- based approaches with seismicity rates derived from earthquake catalogs are commonly used in many countries as the standard for national seismic hazard models. In Italy, a single zone- based ERF is currently the basis for the official seismic hazard model. In this contribution, we present eleven new ERFs, including five zone-based, two smoothed seismicity-based, two fault- based, and two geodetic-based, used for a new PSH model in Italy. The ERFs were tested against observed seismicity and were subject to an elicitation procedure by a panel of PSHA experts to verify the scientific robustness and consistency of the forecasts with respect to the observations. Tests and elicitation were finalized to weight the ERFs. The results show a good response to the new inputs to observed seismicity in the last few centuries. The entire approach was a first attempt to build a community-based set of ERFs for an Italian PSHA model. The project involved a large number of seismic hazard practitioners, with their knowledge and experience, and the development of different models to capture and explore a large range of epistemic uncertainties in building ERFs, and represents an important step forward for the new national seismic hazard model.

Abstract. We investigated the eastern corner of northeastern Italy, where a system of NW–SE-trending dextral strike-slip faults of western Slovenia intersects the south-verging fold and thrust belt of the eastern Southern Alps. The area suffered the largest earthquakes of the region, among which are the 1511 (Mw 6.3) event and the two major shocks of the 1976 seismic sequence, with Mw = 6.4 and 6.1. The Colle Villano thrust and the Borgo Faris–Cividale strike-slip fault have been here first analyzed by interpreting industrial seismic lines and then by performing morphotectonic and paleoseismological analyses. These different datasets indicate that the two structures define an active, coherent transpressive fault system that was activated twice in the past two millennia, with the last event occurring around the 15th–17th century. The chronological information and the location of the investigated fault system suggest its activation during the 1511 earthquake.