Submerged notches, coastal changes and tectonics in the Rijeka area, NW Croatia Stathis C. Stiros ⁎, Fanis Moschas Dept. of Civil Engineering, Patras University, Patras 26500, Greece abstract article info Article history: Received 9 July 2011 Received in revised form 4 October 2012 Accepted 11 October 2012 Available online 23 October 2012 Communicated by: D.J.W. Piper Keywords: notch sea-level faulting elastic dislocation Croatia Rijeka earthquake seismic risk Submerged coastal notches characterize the coast of the wider Rijeka area, NW Croatian coast, Adriatic Sea. These notches originate from a notch formed approximately 2,000 years ago, their depth ranges between 0.50 and 1.15 m and their subsidence history is a matter of debate. A detailed study of all the available data in relation to the tectonics of the area was made using signal analysis techniques. Our analysis revealed that the notch depth reflects the superimposition of two different effects, a quasi-regional relative sea-level rise of about 55 cm, and of a subsequent, local, tectonic subsidence with amplitude gradually increasing to 60 cm in the vicinity of the major thrust in the Bakar Bakar–Vinodol area. Elastic dislocation modeling permitted to explain this last local sub- sidence in terms of loading of the footwall of this thrust and provide evidence for an unrecorded earthquake of minimum magnitude M w 6.8, which occurred most probably in the last few centuries. This result indicates that the convergence between the Southern Alps and the Adria is at least partly accommodated by earthquakes, and puts some constraints in the estimations of the seismic risk in the wider region. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Coastal notches represent erosion marks in carbonate coasts, formed in response to biological and mechanical erosion. The notch vertex is ad- justed to the mean sea level, and for this reason, fluctuations of the sea level are printed on the notch profile and elevation and their study per- mits to put some constraints on the sea-level change history (Fig. 1; Pirazzoli, 1982, 1986; Laborel and Laborel-Deguen, 1994). Submerged coastal notches, evidence of a fossil sea-level approximately 0.5 m lower than the present one, have since long been identified along the Cro- atian coast, in the Adriatic Sea, along a distance of more than 150 km, be- tween Zadar and Rijeka (Fig. 2 Pirazzoli, 1980; Fouache et al., 2000). The submerged notches in the wider Rijeka area, in the innermost part of the Adriatic, covering an area 30 × 40 km wide, have been recent- ly studied in details by Benac et al. (2004, 2008) who showed that their depth ranges between 0.50 and 1.15 m below present mean sea level. The variable depth of this notch in the Bakar area has been assigned to tectonic events and earthquakes by Benac et al. (2004), but in absence of major earthquakes in coastal Croatia in the last centuries (Del Ben et al., 1991; Camassi and Stucchi, 1996; Herak et al., 1996; Slejko et al., 1999; Albini, 2004; Guidoboni et al., 2007; Stucchi et al., 2007), the pro- cesses responsible for their subsidence became a matter of debate and these notches have been even regarded as an “enigma” (Antonioli et al., 2004; Pirazzoli, 2005). The aim of this article is to shed some light on the relationship be- tween earthquakes, tectonic movements and coastal changes, and hopefully provide a solution to this enigma. 2. Geomorphological and Seismotectonic background The study area is at the innermost part of the Adriatic Sea, and is un- derlain by a Mesozoic carbonate platform marked by sub-parallel, usually oblique-slip north-dipping thrusts or reverse faults. These faults belong to a main fault zone, shown for simplicity as a line in Fig. 2 and which accom- modates crustal shortening at the border of the Adria-Eurasia plates at a rate of approximately 2 mm/year (Anderson and Jackson, 1987; Carulli et al., 1990; Del Ben et al., 1991; Caporali et al., 2003; Serpelloni et al., 2005; Pondrelli et al., 2006; Meletti et al., 2008). Tectonic activity controls to some degree the relief; for instance, the Bakar Bay–Vinodol Valley rep- resents a topographic depression between two fault-controlled folds (Fig. 3). However, there is no clear evidence of major (M w > 6) earth- quakes, nor of seismic faulting during the Holocene in the study area (Del Ben et al., 1991; Camassi and Stucchi, 1996; Herak et al., 1996; Slejko et al., 1999; Albini, 2004; Guidoboni et al., 2007; Stucchi et al., 2007). From the geomorphological point of view the study area can be clas- sified as an area of marine transgression, as is the case with the adjacent Dalmatian coasts, which are characterized by ria-type shorelines, i.e. a re- lief produced by aerial or sub-aerial erosion, subsequently modified by marine transgression. The flat-floor, 50 m-deep Bay of Rijeka and the 30 m-deep nearby Bay of Trieste have since long been recognized as Marine Geology 329–331 (2012) 103–112 ⁎ Corresponding author. E-mail addresses: stiros@upatras.gr (S.C. Stiros), fmoschas@upatras.gr (F. Moschas). 0025-3227/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.margeo.2012.10.005 Contents lists available at SciVerse ScienceDirect Marine Geology journal homepage: www.elsevier.com/locate/margeo