Bollettino di Geofisica Teorica ed Applicata Vol. 59, n. 4, pp. 463-480; December 2018 DOI 10.4430/bgta0209
The seismogenic sources of the 1976 Friuli earthquakes: a new seismotectonic model for the Friuli area
M.E. Poli and A. Zanferrari Department of Agricultural, Food, Environmental and Animal Sciences, University of Udine, Italy
(Received: 24 September 2017; accepted: 31 October 2017)
ABSTRACT By matching interpretation of seismic profiles of the Friuli pre-Alpine area with the surficial data collected during the detailed geological survey (CARG - FVG project), we have reconstructed the deep arrangement of the Susans-Tricesimo thrust (which
is considered the seismogenic source of the 6 May 1976 earthquake, MW 6.45). The structural framework of the upper crust in Friuli is here updated, suggesting a new seismotectonic model based on the slip partitioning between the Idrija-Ampezzo right lateral strike-slip system and the SSW-verging thrust system of central-eastern Friuli. This new seismotectonic model may have crucial consequences for the assessment of seismic hazard in Friuli.
Key words: 1976 Friuli earthquakes, seismogenic sources, eastern Southern Alps, Dinaric strike-slip faults, slip partitioning, seismic hazard.
1. Introduction
The devastating earthquake that struck central Friuli on 6 May 1976 (Io IX-X), was one of the major seismic events (both historical and instrumental) recorded in north-eastern Italy and western Slovenia (Rovida et al., 2016). A second destructive seismic sequence struck the same area during
September with MW 5.60 (1976.09.11), MW 5.93 and 5.95 (1976.09.15) shocks, causing additional deaths and destruction. Numerous papers on the study of the seismological parametres of the 1976 Friuli earthquakes, such as the focal mechanism, the hypocentre and epicentre localization and the seismic moment of the sequence, have been produced during the last 40 years (see for example Ambraseys, 1976; Caputo, 1976; Console, 1976; Ebblin 1976; Finetti et al., 1976, 1979; Mueller, 1977; Cagnetti and Pasquale, 1979; Cipar, 1980; Slejko et al., 1989, 1999; Del Ben et al., 1991; Aoudia et al., 2000; Pondrelli et al., 2001; Peruzza et al., 2002). For a complete analysis of the bibliography, see Slejko (2018).
From a geological point of view, in spite of the main shock magnitude [MW 6.45 in Rovida et al. (2016)], the observed coseismic features of the earthquake were scarce. For a complete overview on the distribution of the geological effects (i.e. ground cracks and sand mounds) in the epicentre area, see Cavallin et al. (1977). Different sets of ground cracks were observed, in particular along the Gemona-Kobarid thrust (a segment of the Periadriatic thrust system). These features were variously interpreted as possible evidence of surface faulting (Bosi et al., 1976),
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or the effect of slope instability phenomena triggered by the earthquake (Martinis and Cavallin, 1978; Zanferrari et al., 2013). Concerning the sand mounds, they were mostly gathered north of the Susans hill, along the tip line of the Susans-Tricesimo thrust. The 1977 repetition of the 1952 high precision levelling and triangulation measurements along the IGM geodetic lines 36 and Aa, showed an uplift of 18 cm south of Venzone and subsidence of 7 cm near the Carnia locality, north of Venzone (Talamo et al., 1978). The inversion of these available geodetic data (Arca et al., 1985; Briole et al., 1986) suggested the activation of a set of low angle reverse faults with ruptures both east and west of the Tagliamento River. In 1993 and 2003, the University of Udine (Protezione Civile, 2005) repeated the precision levelling and triangulation measurements along the IGM geodetic lines 36 and Aa, proving that the observed geodetic signal was the sum of coseismic deformation induced by the main shock and by the strongest aftershocks and not only the effect of an interseismic regional uplift. By means of a new processing of the geodetic data, Cheloni et al. (2012) presented a new source model that simultaneously fits all the available geodetic measurements of the observed deformations. The model predicts the progressive activation of a single fault system for the entire sequence, the surface expression of which could be associated with the Buia blind thrust. The sequence hit a region affected by a complex fault pattern, made up by the south-verging, nearly E-W trending imbricated thrusts of the eastern Southern Alps (that deformed and partially re-used the old inherited NW-SE trending thrusts of the Paleogene External Dinarides) and by the active right lateral strike-slip systems of western Slovenia and eastern Friuli (Fig. 1). This complex tectonic framework and the lack of an operating seismological network during the seismic sequence did not permit until now a clear association between the earthquake/s and a specific seismogenic source. Immediately after the strongest seismic events of May and September, the epicentral areas were investigated. Initially, the main shock was attributed to the WNW-ESE trending reverse Dinaric structures located in the Buia-Tricesimo area (Amato et al.,
1976; Barbano et al., 1985), while the 15 September event (MW 5.95) was related to the activation of the E-W striking Periadriatic thrust (Amato et al., 1976). At the beginning of the 21st century, Aoudia et al. (2000), Pondrelli et al. (2001), Peruzza et al. (2002) and Poli et al. (2002) processed again the seismological data related to the 1976 sequence. In particular, Aoudia et al. (2000) identified some possible superficial expressions of the seismogenic source along ridges (defined as Susans, Buia and La Bernadia mountains en echelon segments), which formed as the result of the blind thrusts activity. As for the shock of 15 September (09:21 GMT), the same authors proposed a focal mechanism consistent with the activation of a NE-SW thrust with a minor strike- slip component. By contrast, Pondrelli et al. (2001) proposed a localization consistent with the activation of the Periadriatic thrust for the main shock. Peruzza et al. (2002) and Poli et al. (2002), by means of new structural field analyses and a detailed review of seismic reflection profiles surveyed by ENI, integrated structural and seismological data and plotted hypocentres of the 1976 sequence over structural sections reporting the main thrust systems of the region. On these bases they proposed that: i) the Susans-Tricesimo thrust may be considered the causative source of the 6 May event; ii) some secondary high-angle back-thrust structures (i.e. south-dipping thrust planes) may have played a major role in triggering some of the aftershocks. Galadini et al. (2005) confirmed that the Susans-Tricesimo thrust could be the seismogenic source of the 6 May 1976 earthquake and proposed for the September main shock the activation
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Fig. 1 - Tectonic sketch map of the eastern Southern Alps in NE Italy and western Slovenia. Red rectangle refers to Fig. 2. CC’ trace of geological cross-section of Fig. 5. Legend: AR: Arba-Ragogna thrust; BE: Bernadia thrust; BU: Buia thrust; BV: Bassano-Valdobbiadene thrust; BFC: Borgo Faris-Cividale fault; CA: Cansiglio thrust; FS: Fella-Sava fault; GK: Gemona-Kobarid thrust; HS: Hochstuhl fault; IA: Idrija-Ampezzo fault; MD: Medea thrust; MT: Montello thrust; PA: Palmanova thrust; PE: Periadriatic thrust; PL: Periadriatic Lineament; PM: Polcenigo-Montereale thrust; PR: Predijama fault; PV: Pioverno fault; PZ: Pozzuolo thrust; RA: Resiutta-Ponte Avons fault; RP: Raune-Paularo fault; RS: Raša fault; ST: Susans-Tricesimo thrust; UB: Udine-Buttrio thrust; VB: Valsugana-Val Bordaglia thrust. of a deep ramp [i.e. the Trasaghis thrust in Galadini et al. (2005)]. A full discussion on the seismogenic sources of the Friuli earthquakes is also available in Burrato et al. (2008), who identify four seismogenic sources in the Gemona-Tarcento area. Among these, the “Gemona sud” (corresponding to the Susans-Tricesimo thrust) is considered the source of the main shock of the Friuli sequence and the blind “Gemona east” responsible for the 15 September (MW 5.95) shock. Starting from May 1977, the installation of the Seismometric Network of Friuli-Venezia Giulia enabled a better monitoring also for very low magnitude events. Its continuous improvement enabled studying the seismicity in NE Italy and neighbouring regions, such as Veneto and Trentino-Alto Adige, as well as Austria, Slovenia and Croatia. The inversion of the collected focal
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mechanisms indicate that the stress distribution of the Friuli region was not constant in time and in space: in particular, the principal stress axis sigma 1 ranges from NW-SE in the western part of the region, to the NNE-SSW in the eastern one (Bressan et al., 2003; Poli and Renner, 2004; Bressan and Bragato, 2009). 3-D images of P-velocity and P- to S-velocity ratio have been produced for the upper crust of the Friuli area using local earthquake tomography by Gentile et al. (2000). In particular, the Vp images show a high-velocity body below 6 km depth in the central part of the Friuli area (tectonic wedge), marked also by strong Vp/Vs heterogeneities. Comparison with the distribution of earthquakes supports the hypothesis that this tectonic wedge controls most of the seismicity and can be considered the main seismogenic zone in the Friuli area. Moreover, Perniola et al. (2004) studied the Coulomb stress changes induced by the main shock, and concluded that it likely triggered the occurrence and controlled the distribution of the following seismicity. After this review on the 1976 Friuli seismic sequence, we present the interpretation of seismic profiles of the Friuli pre-Alpine area, showing the deep arrangement of the Susans-Tricesimo thrust, that is considered the seismogenic source of the 6 May 1976 earthquake (MW 6.45). Then we present new geological data carried out by means of the geological survey of the 049-Gemona del Friuli and 066-Udine sheets CARG Project (new 1:50,000 Geological Map of Italy) in the Friuli Venezia Giulia region. Combining surface and deep data, we have reconstructed the structural framework of the upper crust in Friuli, suggesting a new seismotectonic model for the Friuli region based on the slip partitioning between the right lateral Idrija-Ampezzo strike-slip system and the SSE-verging thrust system of central Friuli.
2. Regional structural setting
The Friuli region belongs to the north-eastern edge of the Adria microplate that includes the thrust-belt of the eastern Southern Alps (ESA), with its foreland basin (i.e. the Veneto-Friuli plain) at present shared with the northern Apennines and the right lateral, strike-slip system of western Slovenia (Fig. 1). The current geological and structural framework of the Friuli region originates from a complex paleogeographic evolution linked to the Alpine orogenic cycle. During the Mesozoic, the study area belonged to the passive margin of Africa with the development of basins and carbonate platforms. In particular, starting from the Middle Jurassic a wide platform (the Friuli Carbonate Platform) developed, limited by the Belluno and Tolmin basins toward the west and the NE respectively. The inversion of the relative motion between Africa and Europe started during the Early Cretaceous, giving rise to a compressive and/or strike-slip regime at the north-eastern Adria margin. Therefore, after the Mesozoic extension, ESA underwent a polyphase compressional evolution during the Cenozoic. The first tectonic event developed from the Upper Cretaceous to the Upper Eocene. It corresponds to the westward propagation of the fold and thrust belt of the External Dinarides. Deformation is well documented in Carnia and Friuli and represents even now the basic arrangement in the easternmost area (i.e. Karst and Julian foothills). Starting from the Late
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Oligocene - Early Miocene, Dinaric tectonic structures were involved (that is folded, cut and/ or re-utilised) in the southward Neogene propagation of the eastern Southern Alps polyphase fold and thrust belt (Doglioni and Bosellini, 1987; Caputo, 1996; Castellarin and Cantelli, 2000; Fantoni et al., 2002; Toscani et al., 2016). The fundamental evolution of the ESA chain started from the Middle Miocene, by means of a few compressive phases with maximum horizontal stress axis ranging from NW-SE to N-S (Castellarin and Cantelli, 2000; Caputo et al., 2010). From the Serravallian to the Messinian in response to a strong tectonic uplift and the exumation of the Variscan crystalline basement along the Valsugana thrust (Zattin et al., 2003), a southward shifting foredeep developed, following the progressive tectonic growth of the ESA chain and the remarkable northward tilting of its foreland (Fantoni et al., 2002; Toscani et al., 2016). A polyphase thrust system, mostly NW-SE striking, with ramp-flat geometries and frontal splays, progressively propagated southwards causing considerable shortening (Doglioni, 1992; Selli, 1998; Schönborg, 1999) and the progressive filling of the foredeep. The ESA growth and its southward propagation occurred also during the Pliocene-Quaternary with a NW-SE maximum compressional axis (Venturini, 1990; Castellarin and Cantelli, 2000; Caputo et al., 2010), giving rise to a NE-SW striking, SE-verging fold and thrust belt well exposed in the Friuli region. At present, GPS data show 2-3 mm/yr of N-S convergence between Adria and Eurasia (e.g. Bechtold et al., 2009; Devoti et al., 2011; Cheloni et al., 2014). This is absorbed by the WSW- ENE trending, SSE-verging thrust front of the eastern Southern Alps and by NW-SE trending, right-lateral strike-slip fault systems in western Slovenia (Vrabec and Fodor, 2006). The external sector of the ESA in Veneto and Friuli is still in evolution, as confirmed by both historical and instrumental seismicity (Slejko et al., 1989; Burrato et al., 2008; Bressan and Bragato, 2009) and the widespread deformations of the Upper Pleistocene-Holocene deposits cropping out in the piedmont plain (Benedetti et al., 2000; Galadini et al., 2005; Zanferrari et al., 2008a, 2008b; Poli et al., 2009; Monegato and Poli, 2015; Poli et al., 2015).
3. Regional seismicity
The Friuli Venezia Giulia region and western Slovenia are two of the most seismically active areas in the western Mediterranean region. The seismicity in the eastern Southern Alps shows that both the south-verging Southalpine thrusts and the NW-SE trending Dinaric strike-slip faults are active within the same stress field even if there are significant distinctions between the Venetian area (sigma 1 about NW-SE), the central Friuli area (sigma 1 about N-S) and western Slovenia (sigma 1 about NNE-SSW) (Slejko et al., 1999; Bressan et al., 2003; Poli and Renner, 2004, Bressan and Bragato, 2009). Fault plane solutions for western Slovenia typically show dextral strike-slip kinematics on steep NE-dipping fault planes (Poljak et al., 2000; Kastelic et al., 2008), while the eastern Southern Alps generally show NNW-dipping reverse faults planes, with local strike-slip component. Two major historical earthquakes struck the study area on March 1511 [maximum intensity IX MCS in Rovida et al. (2016)] and on January 1348 [maximum intensity IX-X MCS in Rovida et al. (2016)]. Concerning the 1511 quake, in spite of numerous studies (Ambraseys, 1976;
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Ribaric, 1979; Fitzko et al., 2005; Košir and Cecič, 2011), many issues remain unsolved. A recent reappraisal of macroseismic data led to a new distribution of intensities (Camassi et al., 2011), where values are strongly decreased in Slovenia. In particular, the intensity of X assigned to Idrija (Fizko et al., 2005) has been removed. Moreover, Camassi et al. (2011) proposed a new epicentre for the 1511 event in the eastern Friuli, near Tarcento, and defined MW 6.30 (Rovida et al., 2016). New geological and paleoseismological data (Falcucci et al., 2018) indicate the Borgo Faris -
Cividale right lateral fault as the seismogenic source for the 1511 quake. The 1348 event [MW 6.60: Rovida et al. (2016)] strongly hit the Carinthia, Carnia and Friuli regions: recently Rovida et al. (2016) have localized the epicentre near Tarvisio along the Fella-Sava fault. Concerning the instrumental seismicity, western Slovenia was hit by two medium-size earthquakes in 1998 [MW 5.60: Bajc et al. (2001)] and in 2004 [MW 5.20: Zĭvcĭć and Krn-2004 team (2006)]. The focal mechanisms for both earthquakes show almost pure dextral strike-slip (Bernardis et al., 2000; Zupancĭč et al., 2001). Concerning the Friuli region, only moderate size earthquakes were recorded after the installation of the Seismometric Network of Friuli Venezia
Giulia: among these, the January to April 1996 Claut seismic sequence with ML 4.30 (Bernardis et al., 1997), the 1998 Trasaghis event (ML 4.10) and the 2002 Mount Sernio earthquake [ML 4.90; Barnaba and Bressan (2013)].
4. Structural setting of the Osoppo-Tricesimo area
South of the Gemona-Kobarid thrust (Fig. 2), the study area is characterised by a complex structural arrangement linked to the superimposition of different (in space and time) tectonic events. The basic structural setting is represented by the tectonic units of the west-verging, NW- SE striking Paleogene External Dinarides thrust belt (see for example the Bernadia tectonic unit, BE in Fig 2). These structures were cut, deformed and/or reactivated by the Neogene-Quaternary polyphase tectonics that built up the eastern Southern Alps (s.s.) In the study area, according to macro- and mesoscopic structural investigations, two main Pliocene-Quaternary deformational events were recognised: the first, with a NW-SE-directed maximum horizontal compression, gathered NE-SW striking thrusts and folds (Pliocene event in Caputo et al., 2010); the second (Quaternary), with sigma 1 about N-S (Monegato, 2006; Caputo et al., 2010), that deforms the previous structures and generates the present external thrust front. At the surface, the main active structures are the Susans-Tricesimo (ST) and the Buia (BU) thrusts. The ST thrust is a NNE-dipping, WNW-ESE striking mostly blind thrust running for about 20 km under the Upper Pleistocene deposits of the Tagliamento LGM glacier between Cimano in the west and Savorgnano in the east (Fig. 2). It crops out near Susans (Zanferrari et al., 2013), giving rise to a series of tectonic slices stacking in the Upper Miocene Montello Conglomerate. Westwards, it cuts the active Arba-Ragogna thrust (Poli et al., 2009). Eastwards, secondary WNW-ESE striking faults related to the ST thrust are responsible for the discontinuous cropping out of the Lower Eocene turbiditic sequences. Near Savorgnano, the thrust is cut and displaced by a set of NW-SE dextral strike-slip faults. The hangingwall of the ST is also deformed by the south-verging Buia thrust that develops from the Tagliamento River to Torlano locality (Zanferrari et al., 2013). To the west, it is blind with a roughly W-E trend (deduced from the ENI seismic lines). To the east (WSW-ENE trending),
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Fig. 2 - Geological map of the Tricesimo-Osoppo area (simplified from 066-Udine and 049-Gemona del Friuli sheets - CARG Project). AA’ and BB’ traces of the two seismic lines of Figs. 3 and 4, respectively. Legend: 1) slope deposits (Pleistocene-Holocene); 2) Holocene alluvial deposits; 3) Upper Pleistocene (LGM) glacial and fluvioglacial deposits; 4) Lower Pleistocene-Upper Pleistocene (pre-LGM) alluvial deposits; 5) Osoppo Conglomerate (Pliocene); 6) Upper Miocene Molassa (Serravalian p.p.-Messinian p.p.); 7) Cavanella Group (Aquitanian-Serravalian p.p.); 8) Upper Paleocene-Lower Eocene turbidite successions; 9) Middle Jurassic-Upper Cretaceous Friuli carbonate platform; 10) Lower Jurassic carbonate platform; 11) Jurassic-Cretaceous basin successions; 12) Upper Triassic carbonate platform. Boreholes: BE: Bernadia; CB: Casali Benedetti; CV: Col Vergnal; GE: Gemona 1; RE: Reana del Roiale; SM: Santa Margherita. it is responsible for the outcropping of the Eocene Flysch (Buia hills) arranged in a series of N80° striking, south-verging asymmetrical folds. Near Tarcento, the Buia thrust overlaps the Lower Eocene Flysch on the Aquitanian Preplans Fm. (Cavanella Group). In the eastern edge, it cuts the Paleogene Bernadia thrust overlapping the Grivò Flysch (Upper Paleocene-Lower Eocene p.p.) on the Marls and Arenites of Savorgnano [Lower Eocene p.p. in Zanferrari et al. (2013)]. In the Osoppo area (Fig. 2), a set of NE-SW trending, SE-verging thin skinned-thrusts involving flysch and Cavanella Group, develop with evidence of active deformations both in the surface and at depth (from ENI seismic lines). The CARG-FVG Col Vergnal borehole (Zanferrari et al., 2013) showed the Montello Conglomerate at 90 m under the topographic surface. Here,
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Fig. 3 - Frontal south-verging thrust system in Friuli: a) Reflection seismic section; b) its geological interpretation. Legend: PQ: Pliocene-Quaternary undifferentiated succession; OSP: Osoppo Conglomerate (Pliocene); UM: Upper Miocene Molassa (Serravalian p.p.-Messinian p.p.); CAV: Cavanella Group (Aquitanian-Serravalian p.p.); FLY: turbidite successions (Paleocene-Lower Eocene); CP: undifferentiated Upper Triassic-Upper Cretaceous carbonate platforms; CV: Col Vergnal borehole. the deltaic Pliocene Osoppo conglomerate (Monegato, 2006) lies unconformably on the Miocene deposits (Zanferrari et al., 2013).
4.1. The seismic lines The interpretation of the seismic profiles (kindly supplied by ENI) crossing the piedmont plain between Tricesimo and Osoppo (Fig. 2), allowed drawing the deep geometry (almost until the first 7 km) of ST and BU thrusts. In the following, we present the geological interpretation of two seismic lines AA’ and BB’ (Figs. 3 and 4).
4.2. AA’ profile The profile is NNW-SSE trending and crosses the ST thrust near the Casasola locality (Figs. 3a and 3b; for location see Fig. 2). In this seismic profile we can investigate the subsurface of the study area until 2-3 km in depth (about 1.5 s TWT).
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a
b Fig. 4 - Frontal south-verging thrust system of the Friuli region: a) Reflection seismic section; b) its geological interpretation. Legend: PQ: Pliocene-Quaternary undifferentiated sediments; UM: Upper Miocene Molassa (Serravalian p.p.-Messinian p.p.); CAV: Cavanella Group (Aquitanian-Serravallian p.p.); FLY: Paleocene-Lower Eocene turbidite succession (GRI: Grivò Flysch, SVO: Arenites and marls of Savorgnano); CP: undifferentiated Upper Triassic-Upper Cretaceous carbonate platforms; TVZ: Upper Carnian mostly terrigenous-evaporitic succession (Travenanzes Fm.); Cu: Carnian unconformity.
In the footwall of the ST, the Cavanella Group (CAV: Aquitanian-Langhian) is about 500 m thick, while the Upper Miocene Molassa (UM: Serravallian-Messinian), reached more than 1 km. Both the Miocene sequences are deformed by a low angle north-dipping thrust named San
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Daniele thrust in Zanferrari et al. (2013). Pliocene?-Quaternary deposits (about 100 m thick) lie unconformably on the Miocene succession. Moving to the north, close to the ST thrust, the entire stratigraphic succession rises up, as a result of the strong Neogene-Quaternary tectonic activity along this fault. The Cavanella Group shows a minimum vertical throw of 1 s: since it outcrops in the hangingwall of the ST near Cimano bridge (156 m a.s.l., Monte Baldo Fm.), we can calculate for this unit a post-Langhian vertical throw of about 1.5 km. Also the Pliocene?-Quaternary succession is deformed, and shows a minimum vertical displacement of about 200 m in correspondence of the ST thrust. Subsurface data confirm the Quaternary activity of the ST: the Casali Benedetti borehole (CB in Fig. 2, located in the hangingwall of the ST), reaches the Lower Eocene Flysch at about 135 m a.s.l., while the Santa Margherita borehole (SM in Fig. 2), located in the footwall of the ST, with the bottom at 41 m a.s.l., does not reach the bedrock [see geological section E-E’ in the 066-Udine geological sheets: Zanferrari et al. (2008b)]. Northwards, the Buia thrust involves both Flysch and Molassa successions. Moreover, its hangingwall is deformed by a set of NE-SW trending thin-skinned thrusts involving the Cavanella Group, the Upper Molassa (see Col Vergnal borehole) and the Osoppo conglomerate.
4.3. BB’ profile The profile is WSW-ENE trending and crosses the ST near Tricesimo locality (Fig. 2). Unlike the AA’ section, this seismic profile highlights the deep arrangement of the Friuli piedmont plain down to 7-km deep (Figs. 4a and 4b). We can observe that: a) the Carnian unconformity (Cu), that is 6-7 km in depth; b) above the Carnian unconformity, we can identify a thick sedimentary succession consisting of about 3 km of carbonates (i.e. the undifferentiated Upper Triassic-Upper Cretaceous carbonate platforms: CP), about 2 km of Paleocene-Lower Eocene turbiditic sequence; more than 1 km of Miocene Southalpine Molassa and at the top, a few hundred metres of post-Messinian deposits unconformably lying on the Molassa succession; c) in the seismic profile the ST thrust cuts and displaces the Meso-Cenozoic sedimentary succession. In particular, the vertical throw of the Friuli Carbonate Platform (FCP) on the ST is more than 2 km, while the vertical throws on the Cavanella Group (the Preplans Fm. crops out near Tarcento locality, about 200 m a.s.l.) is about 1.5 km. This means that the ST thrust is an inherited Dinaric thrust, re-used during the Neogene-Quaternary tectonics; d) the thickness of the Pliocene?-Quaternary succession strongly decreases crossing the ST thrust: in the footwall it locally reaches about 500 m [see the Reana del Roiale borehole: RE in Fig. 2: Zanferrari et al. (2008b)], but in the hangingwall it shows an average thickness of a few tens of metres.
5. Upper crust geological cross-section
In this chapter, we propose and discuss a new geological section crossing the upper crust of the eastern Southern Alps from the Fella valley to the piedmont plain south of Udine (Fig. 5). This cross-section refers to Zanferrari et al. (2013), modifies Burrato et al. (2008), and was made using the following data set:
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Fig. 5 - N-S regional geological cross-section across the ESA thrust belt and its foreland in Friuli (modified from Zanferrari et al., 2013). The section highlights the relationships between the active south-verging thrusts of the ESA and the Idrija-Ampezzo dextral strike-slip system. West-verging Dinaric thrust: capped lines. For location see Fig. 1. Source data in the text. Legend: MB = magnetic basement (Cati et al., 1989); PT = undifferentiated Permo-Triassic succession; TVZ = Upper Carnian mostly terrigenous-evaporitic succession; CP = undifferentiated Upper Triassic-Upper Cretaceous/Paleocene carbonate platforms; JKB = Jurassic-Cretaceous basin successions; FLY = Upper Cretaceous- Lower Eocene turbiditic succession; CAV = Lower-Middle Miocene Cavanella Group; UM = Middle-Upper Miocene Molassa succession; PQ = Pliocene-Quaternary succession; BU = Buia thrust; GK = Gemona-Kobarid thrust; IA = Idrija-Ampezzo fault; MA = Magnano thrust; MD = Medea thrust; MV = Musi-Verzegnis thrust; PA = Palmanova thrust; PV = Pioverno fault; PZ: = Pozzuolo thrust; RA = Resiutta-Ponte Avons fault; ST = Susans-Tricesimo thrust; UD = Udine-Buttrio thrust.
1) the depth and the structural setting of the magnetic basement. According to Cati et al. (1989), the magnetic basement in Friuli deepens toward the north (following the Alpine thrusting) and the NE (following the Dinaric thrusting). In particular, it dips from the coastline area (where it is about 6-km deep) to Gemona del Friuli (about 11-km deep), but along the roughly E-W striking structural alignment Sappada-Tarvisio it rises up, reaching 8 km in depth. This structural step was ascribed by Carulli and Ponton (1992) to the Neogene reverse activity of a hypothetical north-steeply dipping blind thrust named Alto Tagliamento; 2) the interpretation of the seismic profiles calibrated on the ENI Cargnacco 1 (Venturini, 2002) and Lavariano 1 wells in the piedmont area (Nicolich et al., 2004). These data, joined with the top of the magnetic basement (Cati et al., 1989), allowed evaluating the thickness of the Permo-Triassic succession in the Southalpine foreland (about 3 km). The anomalous thickness of the Permo-Triassic succession in the Carnic area (more than 7 km), can be justified only by means of a strong tectonic stacking (see also Carulli and Ponton, 1992); 3) the CARG Project (new Geological Maps of Italy, at scale 1:50,000). The realization of the 066-Udine and 049-Gemona del Friuli geological sheets (Zanferrari et al., 2008b, 2013), allowed collecting new superficial data on the arrangement of the Southalpine chain in Friuli. The main structural data highlighted during the geological survey are summarised in the following; 3a) the External Dinarides structures represent a pervasive framework inside the eastern Southalpine chain in Friuli (Merlini et al., 2002; Poli, 2008). On the basis of the geological and structural surveys, the upper crust in Friuli is arranged in a stack of Paleogene tectonic units (among which the Gemona-Kobarid, the Susans-Tricesimo, and the Palmanova
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one, i.e. the external front of the Paleogene Dinaric belt, buried under the Friuli plain), progressively incorporated and merged in the eastern Southalpine chain by means of the polyphase Neogene-Quaternary compressive tectonics. The Dinaric units frequently show evidence of south-verging Neogene-Quaternary reactivation; 3b) the identification of a wide, steeply north-dipping, right lateral strike-slip fault system along the Resia and Tagliamento valleys. This is a broad and anastomosed right lateral N110°-115° strike-slip fault system, coupled with synthetic (N135° striking) and antithetic (N30° striking) faults, NW- or SE- verging reverse faults, en echelon fold systems, strike- slip duplexes, contractional and releasing bends, positive or negative flower structures (Zanferrari et al., 2013). This strike-slip system is the continuation in the Friuli region of the Idrija system affecting western-Slovenia and therefore it was named the Idrjia- Ampezzo strike-slip system (IA in Fig. 1). Tectonic activity across the Resia and Tagliamento valleys has been related to the complex structural setting in which compressional and strike-slip movements occur. In this context, we hypothesise that the roughly 3 km throw of the magnetic basement along the Tagliamento valley (Cati et al., 1989), may be entirely or partially linked to the Pliocene- Quaternary transpressive activity of the Idrjia-Ampezzo system. Moreover, this tectonic setting can produce local subsidence along the valley [i.e. transtensive activity: see for example Barnaba et al. (2010)], where remnants of the fluvial succession may have been preserved (Monegato and Vezzoli, 2011).
6. Discussion and conclusion
Starting from the Mesozoic, the Friuli upper crust has been subjected to multiple tectonic phases, leading to the development of structures with variable orientations and kinematic histories and, therefore, variable potential for reactivation under the present stress conditions. In such a complex tectonic setting, it is hard to associate a single seismic event to a specific tectonic source. This correlation is even more difficult if there is no well-organised seismometric network, as happened during the 1976 Friuli seismic sequence (see Slejko, this volume). Up to this point, in order to answer the question “Is the Susans-Tricesimo thrust the seismogenic source of the 6 May Friuli earthquake?” we can make the following considerations: 1) type of focal mechanisms. All the researchers agree on a reverse focal mechanism with roughly W-E striking planes for the 6 May event. One plane gently dips toward the north, the second steeply dips toward the south; 2) the epicentre of the earthquake. In this case, most authors (Slejko et al., 1989; Peruzza et al., 2002; Poli et al., 2002) suggest that the seismogenic source is located in the pre- Alpine area, south of the Gemona-Kobarid thrust (i.e. a segment of the Periadriatic thrust or Barcis-Staro Selo thrust). Only Engdhal et al. (1998) and Pondrelli et al. (2001), propose an epicentre about 40 km northwards, along the Resia Valley; 3) inversion of geodetical data. The inversion of the geodetic data fits with the activation of low-angle reverse faults (Arca et al., 1985; Briole et al., 1986; Cheloni et al., 2012).
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Therefore, at the present state of knowledge, the tectonic structure that best fits these requisites seems to be the Susans-Tricesimo thrust that shows considerable evidence of Quaternary deformations both on the surface and in the seismic profiles. The geometrical constraints acquired by means of geological and geophysical investigations confirm that this fault is able to generate
MW 6.50 seismic events (Wells and Coppersmith, 1994; Galadini et al., 2005). Also the damage distribution (Rovida et al., 2016), mostly located in the hangingwall of the ST thrust, seems to be congruent with the activation of this source. Recently, Cheloni et al. (2012), proposed a progressive activation of a single fault (i.e. the Buia blind thrust, with strike/dip/rake: 282°/20°/112°) for the entire sequence. On the basis of the structural observations (Fig. 5), we cannot exclude that also the Buia thrust had a relevant role in the evolution of the 1976 Friuli sequence, even if the evidence for Quaternary deformations are more significant for the Susans-Tricesimo thrust (Figs. 3 and 4). Concerning the possible role played by the Periadriatic thrust (here Gemona-Kobarid segment), a few authors (e.g. Engdhal et al., 1998; Pondrelli et al., 2001) locate the 6 May epicentre to the north, in an area between Venzonassa and Resia valleys, linking the main-shock to the activation of the Gemona-Kobarid thrust, which besides is considered an active fault because of its morphotectonic evidence (see Galadini et al., 2005). Different hypotheses for the 15 September (09:21 GMT) source were indicated by Galadini et al. (2005), that proposed the activation of the deep ramp of Trasaghis thrust and by Moratto et al., (2012), that suggested the activation of the Buia thrust. However, the only indications of relevant coseismic deformations (Talamo et al., 1978) were noted in correspondence of two high angle strike-slip faults of the Idrija-Ampezzo system located between Venzone and Carnia localities, cited in Zanferrari et al. (2013) as Pioverno fault and Resiutta-Ponte Avons fault respectively (PV and RA in Figs. 1 and 5). The Idrija-Ampezzo strike-slip fault-system shows clear evidence of Pliocene-Middle Pleistocene tectonic activity, documented by the strong deformations of the continental
Table 1 - MW≥5 seismic events that struck the central Carnia starting from the 1700 A.D. (from Rovida et al., 2016). Location in Fig. 6.
Y/M/D Io MW Epicentral area 1700/07/28 VIII-IX 5.7 Gorto valley 1788/10/20 VII-VIII 5.2 Tolmezzo 1853/02/19 VII 5.1 Amaro 1908/07/10 VII-VIII 5.3 Aupa valley 1920/05/05 VI-VII 5.3 Moggio Udinese 1924/05/12 VI 5.0 Ampezzo 1924/12/12 VII 5.4 Tolmezzo-Degano valley 1928/03/26 – 5.7 Aupa valley 1928/03/27 VIII-IX 6.0 Verzegnis 1956/11/05/ VI 5.1 But valley 1959/04/26 VII-VIII 5.2 But valley 1959/06/13 – 5.3 Ampezzo
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Fig. 6 - Tectonic sketch of the central Friuli and Carnia areas across the Idrija-Ampezzo strike-slip system (modified from Zanferrari et al., 2013). Historical and instrumental epicentres (from Rovida et al., 2016): MW≥5.0 (green circles), MW≥6.0 (red circles). Focal mechanisms (underlined my/m/d dates) from: Slejko et al. (1989), Bernardis et al. (2000), Peruzza et al. (2002), Zĭvcĭć and Krn-2004 team (2006), Saraò (2008). Legend: AM: Ampezzo; TO: Tolmezzo; GE: Gemona del Friuli; UD: Udine. successions cropping out along the Tagliamento valley (Monegato, 2006; Monegato and Stefani, 2011; Zanferrari et al., 2013). Because of the lack of deposits, more difficult is to demonstrate the tectonic activity of the Idrija-Ampezzo system during the Late Pleistocene-Holocene. However, the area was hit by medium to high both historical and instrumentally recorded events (Fig. 6 and Table 1).
The strongest historical event struck the Tolmezzo area on March 27, 1928 [MW 6.02; Io IX MCS in Rovida et al. (2016)]. The focal mechanism of this event (Ebblin, 1976; Cagnetti and Pasquale, 1979; Slejko et al., 1989) shows two steeply dipping conjugate planes: the first NW-SE striking with right lateral movement, the second NE-SW striking with left lateral movement. Both planes fit with the activation of the Idrija-Ampezzo strike-slip system. In this frame, we wonder if the low angle SSW-verging thrusts of the Julian Prealps (i.e. Gemona-Kobarid, Susans-Tricesimo, and Buia thrusts) and the Idrija-Ampezzo strike-slip fault system, may be structurally joined. Based on the evidence of active deformation along the Idrjia-Ampezzo strike-slip fault and the available knowledge on the kinematics of the region, we suggest that the transpressive kinematics is partitioned between the Idrjia-Ampezzo strike-slip fault-system and the contractional thrust- belt developing along the Julian pre-Alpine area (Fig. 5). Slip partitioning displacement in the frame of oblique convergence has been observed in many cases, for both the coseismic and long-term displacements (e.g. Wesnousky and Jones, 1994; Henyey et al., 1999; Walker et al., 2003; Bemis et al., 2015). In particular transpressive deformation was observed along the Borgo
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Faris-Cividale right lateral strike-slip system in Julian pre-Alpine area (Falcucci et al., 2018). Here morphotectonic and paleoseismological investigations suggest that transpressive slip is accommodated by the subvertical strike-slip Borgo Faris-Cividale fault and the Colle Villano low angle thrust that acts as a synthetic splay. This new seismotectonic model may have crucial implications for earthquake hazard assessment in NE Italy and western Slovenia, where Serpelloni et al. (2016) identifies, by means of the analysis of geodetic and seismological data, two belts of higher deformation rates.
Acknowledgements. Many thanks to D. Slejko and M. Riuscetti, for asking us to take part in this volume and to ENI S.p.a. - Exploration and Production Division, for permission to analyse the seismic profiles. A. Marchesini is acknowledged for the graphic design. The paper benefited from the suggestions of G. Monegato. We would like to thank R. Caputo and M. Riuscetti for their constructive suggestions, which helped improve the manuscript.
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Corresponding author: Maria Eliana Poli Department of Agricultural, Food, Environmental and Animal Sciences University of Udine, Italy Via del Cotonificio, 114, Udine, Italy Phone: +39 0432 558747; e-mail: [email protected]
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