Pliocene-Quaternary activity of the Dalmatian unit basal thrust – a potential seismogenic source in the Dubrovnik epicentral area (Croatia)

This study focused on the identification of seismically active structures in the southeastern part of the External Dinarides around the city of Dubrovnik. The wider region is characterized by a relatively high seismicity rate, with intermittent occurrences of strong events, indicating ongoing tectonic activity. Historical, instrumental, and paleoseismological records indicate that this area has been affected by at least a dozen strong earthquakes over the last 3000 years. Among these, the most significant was the Great Dubrovnik earthquake from 1667 (M_L = 6.9), which devastated the region. To identify potential seismogenic sources and active faults in the Dubrovnik offshore area, 106 offshore 2D seismic lines and three deep exploration wells were analysed. Identified offshore faults were mapped, and a regional balanced seismotectonic cross-section was constructed by correlating the hypocentral distribution of recorded seismicity and onshore geological and structural data with offshore 2D seismic lines and well logs. Our results suggest that the main seismogenic source offshore of Dubrovnik is related to the NE-dipping frontal thrust and fault splays of the Dalmatian tectonic unit in the External Dinarides. The ongoing propagation of this fault system is documented by syn-tectonic deposition and folded Plio-Quaternary sediments that can be traced for 40 km along the strike. Based on kinematic modelling, the average slip rate estimated for the active frontal thrust and its splays is 1.13 mm/yr, while average shortening is 0.9 mm/yr from the Late Miocene to recent times. These findings are thus suggestive of repeated seismogenic events, such as the one in 1667, to accommodate shortening in the area.