Looking for active and capable faults in Friuli: a multidisciplinary approach to study the Maniago thrust (western Carnic Prealps, NE Italy).

In the framework of the agreement between the Friuli Venezia Giulia Region (Geological Survey), I.S.P.R.A. and University of Udine, in order to improve knowledge about the active and capable faults in the Friuli Venezia Giulia Region and update the ITaly HAzard from CApable faults (ITHACA) database, new geological, morphotectonic, geophysical and paleoseismological studies were carried out on the Maniago thrust (western Carnic Prealps, Friuli) near Meduno locality (PN). The Maniago thrust belongs to the Quaternary front of the eastern Southern Alps (ESA), a polyphase fold and thrust belt in evolution from the Middle Miocene to the Present. The development of this polyphase belt is linked to the N-ward indentation Adria microplate and, starting from the Miocene-Pliocene transition, to its NE-ward counter-clockwise rotation (Vrabec and Fodor , 2006). Up today Adria moves northward about 2-3 mm/yr (Caporali et al., 2013) and its motion is accommodated not only by the WSW-ENE trending, SSE-verging thrust-front of the ESA in Veneto and Friuli regions, but also by the NW-SE trending, right-lateral strike slip fault-systems in W-Slovenia. In the western Carnic Prealps the ESA-external front is arranged in a set of WSW-ENE trending, SSE-verging, middle angle, mostly blind thrusts involving both the uppermost Pleistocene (i.e. LGM) and Holocene sediments cropping out in the pre-alpine area and in the Friuli piedmont Plain. Morphotectonic investigations identified surficial traces of the recent fault activity, generally represented by drainage anomalies and gentle scarps connecting uplifted and back-tilted Quaternary paleo-landscapes with the plain. Tectonic activity is also testified by the historical and instrumental seismicity (Rovida et al., 2016). Historical earthquakes hit the western Carnic Prealps causing widespread damage and losses: the 1776/7/6 Prealpi Friulane (Mw: 5.82; Imax: 8-9); the 1812/10/25 Pordenonese (Mw: 5.7; Imax: 7-8); the 1873/18/10 Alpago (Mw: 6.3; Imax: 9-10) and the 1936/10/18 Cansiglio (Mw: 6.1; Imax: 9). In the last decades maximum recorded magnitude is the Claut 1996/4/13 event (Mw 4.43; Imax: 5-6). The study area is located at the lower reach of the Meduna valley, where the incision of the Meduna River crosses the Miocene succession and cuts the Pliocene-Holocene alluvial deposits of the piedmont plain. The study shows that the valley configuration has been shaped during the Pliocene-Quaternary with long lasting steady intervals, spaced out by periodic tectonic pulses linked to the activity of the thrust front of the ESA. The most recent pulse related to the Maniago thrust shows an Upper Pleistocene – Holocene slip-rate of about 0.5 mm/yr (Monegato and Poli, 2015). During the last pulse the activity of the Maniago thrust gave rise to a tectonic scarp (about 900 m long and 1-4 m high) shaped in the LGM Rivalunga terrace near Meduno. Integrated geophysical investigations (Electrical Resistivity Tomography, seismic refraction and reflection, Ground Penetrating Radar (GPR), passive seismic (HVSR, ReMi) and MASW) were performed across this morphological feature. All the geophysical data showed, at different scale, remarkable lateral anomalies across the morphological scarp, highlighting that it is compatible with a tectonic origin. Two paleoseismological trenches were dug across the scarp. Progressive thickening of the colluvial sediments toward the tectonic scarp, gentle anticline-trend of LGM and Early Holocene (historical) deposits, middle to high angle reverse faults with clasts-rotation along the shear zones involving the historical sediments and extrados cleavage, indicated that the Maniago thrust is not only an active fault but also a capable one. Moreover the very recent age of the deformed stratigraphic units (1460 AD), compared with the DBMI-15 Catalogue allowed us to consider the 1776.07.10 earthquake (Imax=8-9), as the last seismic event linked to the Maniago thrust tectonic activity. These new data cast new light on the seismic hazard assessment in the western Friuli.