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Neotectonics of the Dinarides – Pannonian Basin Transition Area and possible sources of the ML 6.5 Banja Luka Earthquake of 1969

The fold-and-thrust belt of the Dinarides on the western Balkan Peninsula hosts some of the strongest instrumentally recorded earthquakes in Europe. Seismicity distribution (e.g., D’Agostino et al., 2008) and GPS-derived shortening rates between Italy and the Pannonian Basin amount to c. 3-5 mm/a (e.g., Bennett et al., 2008) and suggest that the Adria- Europe convergence is largely taken up within the external Dinarides near the Adriatic Sea coast. However, instrumentally strong earthquakes were also recorded within the internal Dinarides, as evidenced by the ML 6.5 Banja Luka earthquake (northern Bosnia and Hercegovina) of 27 October 1969. This suggests that the convergence zone is diffusely distributed across the entire Dinarides up to the Pannonian Basin. Our study first provides a neotectonic framework for this transitional area. We then review possible source mechanisms for the still very poorly understood 1969 earthquake sequence. Subduction of Meliata-Vardar oceanic lithosphere and subsequent collision of the European-derived Tisza-Dacia and Adriatic plates in the late Cretaceous - early Paleogene led to the development of the SW-vergent Dinarides fold-andthrust belt. Severe extension affected the northern Dinarides in conjunction with the formation of the Pannonian Basin since the latest Paleogene (24 Ma) and throughout much of the Miocene. This extension reactivated former thrust faults related to foregoing collision as low-angle extensional detachments and led to the formation of core complexes, exposing greenschist- to amphibolite grade rocks. This is documented on several inselbergs along the southern margin of the Pannonian Basin, i.e. along the southern margin of the Sava depression between Bosnia and Hercegovina and Croatia. The Sava depression formed as a supradetachment basin on top of the north-dipping detachment. The initially coherent detachment was disrupted by both N- and S-dipping high-angle normal faults since about Ottnangian times (c. 18 Ma). We presume that the Miocene extensional fault system extended at least to mid-crustal depths based on the fact that amphibolite-grade rocks are exposed in the footwall of the detachment. After the Pontian (c. 5 Ma), numerous high-angle normal faults were compressively or transpressively inverted, rejuvenating an ‘inselberg’ morphology in the southwestern Pannonian Basin that had initiated during Miocene extension. The available focal mechanisms for the main 1969 Banja Luka earthquake and its largest foreshock on 26 October 1969 (ML 6.0) indicate reverse faulting along c. E-W-trending nodal planes and c. N-S trending pressure axes based on pwave first-motion polarities. We analyzed the aftershock distribution in order to better understand which fault ruptured during the main event, restricting the analysis to events with M ≥ 2 and an epicenter location uncertainty < 10 km. The events during the first 36 hours following the largest foreshock cluster in an ENE-trending area, 40 km long and 10 km wide. This cluster parallels the trend of the frontal thrust of the East Bosnian - Durmitor thrust sheet cropping out a few km to the south. We hence suspect that this thrust could have been compressionally or transpressionally reactivated during the recorded seismic events. Alternatively, the event could have nucleated along one of the Neogene detachment faults itself. We next analysed the seismicity in the larger epicentral area during the entire time span covered by the earthquake catalogue. This analysis was restricted to ML ≥ 3.0 earthquakes, for which the determination of hypocenter depth has smaller uncertainties. The average depth distribution for the considered events tends to parallel the ENE- to NE-oriented thrusts in the area. The shallower events tend to occur close to the surface trace of the thrusts and deepen towards NE. This is taken as circumstantial evidence for ongoing seismic activity along the East Bosnian - Durmitor thrust in the internal Dinarides.

Neotectonics; Dinarides; Pannonian basin; Banja Luka earthquake

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