The tectonic structure of the Eastern Alps is heavily debated with successive geophysical studies that are unable to resolve areas of ambiguity (e.g., the presence of a switch in subduction polarity and differing crustal models). In order to better understand this area, we produce a high resolution Moho map of the Eastern Alps based on a dense seismic broadband array deployment. Moho depths were derived from joint analysis of receiver function images of direct conversions and multiple reflections for both the SV (radial) and SH (transverse) components, which enables us to map overlapping and inclined discontinuities. We observe the European Moho to be underlying the Adriatic Moho from the west up to the eastern edge of the Tauern Window. East of the Tauern Window, a sharp transition from underthrusting European to a flat and thinned crust associated with Pannonian extension tectonics occurs, which is underthrust by both European crust in the north and by Adriatic crust in the south. The Adriatic lithosphere underthrusts northward below the Southern Alps and becomes steeper and deeper towards the Dinarides where it dips towards the north-east. Our results suggest that the steep high velocity region in the mantle below the Eastern Alps, observed in tomographic studies, is likely to be of European origin.

The Late Cretaceous collision of the Adriatic microplate with Eurasia resulted in an overall SW-vergent and in-sequence structural architecture of the Dinarides. In the Paleogene the deformation propagated from the Internal towards the External Dinarides, associated with ca. 130 km of crustal shortening. Fault kinematic data and balanced cross-sections across the External Dinarides suggest contrasting styles along-strike the orogen, separated by a ca. 250 km long dextral transpressive fault. This fault delimits the southern, SW-vergent nappe stack segment from a northern, NE-vergent backthrust-dominated Velebit segment. Based on the distribution of the flexural foreland basin sediments, it is known that these two domains deformed contemporaneously, which marked the end of the Paleogene Dinaric orogeny.

Within these Eo- to early Oligocene syntectonic and older Mesozoic carbonate platform rocks horizontal marine terraces are preserved at elevations of up to 600 m. We extracted terrace surfaces along the entire Adriatic coast from DEMs. All these flat surfaces are degradational, not related to bedding or faults, and they are located on the western side of the main drainage divide facing the Adriatic Sea. Their spatial correlation is in agreement with the position of a reported negative P-wave tomography anomaly, which in turn correlates with the thinned part of the Adriatic lithosphere. Our findings suggest an orogen-wide surface uplift affecting the Dinarides in the Neogene due to mantle delamination.

Our overarching results show that the Paleogene Dinaric orogeny was related to high crustal shorten- and thickening, whereas the Neogene was related mainly to surface uplift.

Current tectonic activity in the eastern Southern Alps is driven by the ongoing collision of Adria with Europe at a rate of ca. 2-3 mm/yr. While the South Alpine Front is well-studied, the deformation in the hinterland of the orogenic front is still not well-understood. Structurally, this area is characterised by dominantly E-W-trending, south-vergent thrusts and dextral strike-slip faults of the eastern Periadriatic fault system and the Dinaric system in Slovenia. Here we present new data on active faulting from tectonic geomorphology studies, field mapping, paleoseismology, and Quaternary dating techniques. We show that in Slovenia, crustal deformation is accommodated by a system of NW-SE striking right-lateral strike slip faults in a more than 60 km-wide zone. While the largest of those faults might be capable of magnitude M≥7 earthquakes, there is no historical or geological record for such large events. Several shorter faults with lengths of less than 15 km also show postglacial activity, but very little is known about their earthquake history. In Italy, most of the deformation is accommodated by thrusting at the South Alpine orogenic front and in the Friulian Plain. However, historical reports and our geomorphological observations indicate that strong earthquakes (M>6) occurred in the interior of the mountain chain, but these events are probably very rare. In Austria, the geological record of active faulting is sparse, although damaging historical quakes are known. New dating results from undisturbed geomorphic markers allow us to place constraints on the maximum amount of deformation that is accommodated here.

The eastern Southern Alps are a seismically active region. Research has been conducted to identify seismically active faults, with the sources of several catastrophic earthquakes still unknown.

In this study, we conducted a paleoseismological investigation around the Fella-Sava Fault in the trijunction of Austria, Italy, and Slovenia, within the epicentral area of the destructive 1348 earthquake (M_W 6.6-7.0). The Fella-Sava Fault is a ca. 100 km long, E-W- to WNW-ESE trending dextrally transpressive fault related to the Periadriatic fault system. Using digital elevation models (DEMs), we tried to identify direct evidence for surface rupturing earthquakes like fault scarps or offset geomorphic markers. Additionally, we put a special emphasis on sackungen. Sackungen are created by the gravitational collapse of mountain flanks, both aseismically and coseismically. Hillshades and aspect-maps derived from the DEMs proved suitable for mapping sackungen on a regional scale covering an area of ca. 1500 km².

A systematic remote sensing-based mapping of sackungen (of which several were subsequently ground-proven) in an area 15 km to both sides of the western segment of the Fella-Sava Fault revealed their clustering within 5 km on either side of the fault. The sackungen correlate neither with lithology nor valley depth. A directional analysis shows that the preferred trend of the sackungen is parallel to the Fella-Sava Fault and doesn’t correlate with the distribution of regional slope orientations. Thus, we suspect that the higher number sackungen in proximity of the Fella-Sava-Fault provides geomorphological evidence for its postglacial seismic activity, including the 1348 earthquake.

The Periadriatic Fault System (PAF) is among the largest post-collisional structures of the Alps. Recent studies using GPS velocities suggest that Adria-Europe convergence is still being accommodated in the Eastern Alps. However, according to instrumental and historical seismicity records, earthquake activity is mostly concentrated along structures in the adjacent Southern Alps. This is also the case for the PAF, which except for ambiguous historical events presents little to no earthquake record. Electron spin resonance (ESR) and Optically Stimulated Luminescence (OSL) are ultra-low temperature thermochronometers (closing temperature below 100 °C), with a Quaternary dating range (a few decades up to ~2 Ma). Both have the potential to date shear heating episodes. In this contribution, we present a first approach to narrow down earthquake activity during the Quaternary in the Eastern Alps by combining the application of ESR and OSL. Specifically, we aim to show which segments of the PAF system accommodated seismotectonic deformation by directly dating quartz and feldspar from fault gouges. For ESR, we measure the signals from the Al center in quartz following the single aliquot additive (SAAD) and single aliquot regenerative (SAR) protocols, focusing on the 4-11 µm and 100-150 µm grain size fractions. For OSL, we measure the IR₅₀ and pIRIR₂₂₅ signals on K-feldspar aliquots of the 100-150 µm grain size fraction. Our ESR results indicate the PAF accommodated seismic slip during the Quaternary with a maximum age ranging from 1-0.5 Ma, and OSL minimum ages of around 0.3 Ma.

The 2019, Mw 6.4 earthquake struck Albania at the Adriatic port city of Durres, 30 km away from the capital Tirana. It caused significant destruction and more than 50 deaths. The mainshock had thrust mechanism typical for the western Balkan margin along which the Adriatic micro plate collides with Europe. A dense temporary network of 30 seismic stations was deployed for 9 months to record the aftershock sequence. We applied a fully automated, machine learning-based detection and phase-picking routine to analyse the seismic data. This way more than 19,000 events were detected and located. Deriving cross-correlation-based differential travel times and relocating the entire catalogue of events with the HypoDD algorithm and a newly derived 1D velocity model reveals the fine-scale structure of the thrust fault. The rupture occurred on a ~30° NE dipping fault between approx. 10 and 20 km depth. The aftershocks collapse to several additional synthetic and antithetic structures highlighting a complex fault network.