The Lassee segment of the Vienna Basin fault system as a potential source of the earthquake of <Emphasis Type="Italic">Carnuntum</Emphasis> in the fourth century <Emphasis Type="SmallCaps">a.d.</Emphasis>

The Vienna Basin fault system is a slow moving (1–2 mm/y) active sinistral fault extending from the Alps through the Vienna Basin into the Carpathians. It comprises an array of NE-striking sinistral strike-slip segments, which differ both by their kinematic and seismologic properties. Among these, the Lassee segment 30 km east of Vienna is of particular interest for seismic hazard assessment as it shows a significant seismic slip deficit. The segment is located about 8 km from the Roman city of Carnuntum, for which archaeological data indicate a destructive earthquake in the fourth century a.d. (local intensity about 9 EMS-98). Mapping of the Lassee segment using 2D seismic, GPR, tectonic geomorphology and Pleistocene basin analysis shows a negative flower structure at a releasing bend of the Vienna Basin fault. The hanging wall of the flower structure includes a Quaternary basin filled with up to 100-m thick Pleistocene growth strata. Faults root in the basal detachment of the Alpine-Carpathian floor thrust at about 8 km depth. The active faults east of the flower structure offset a Middle Pleistocene terrace of the Danube River forming an up to 20-m high composite fault scarp. High-resolution GPR (40, 500 MHz) mapped at least four distinct surface-breaking faults along this scarp including three faults, which are covered by about 2 m of post-tectonic strata. The youngest fault offsets these strata and coincides with a 0.5-m high scarp. This scarp may be interpreted as the product of a single surface-breaking earthquake, provided that the mapped fault offset formed during coseismic surface rupture. Data indicate that the Lassee segment may well be regarded the source of the fourth century earthquake. The interpretation is in line with local attenuation relations indicating a source close to the damaged site, observed fault dimensions and the fault offsets recorded by GPR and morphology.