Seismogenic sources potentially responsible for earthquakes withM≥ 6 in the eastern Southern Alps (Thiene-Udine sector, NE Italy)

Abstract

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.