Abstract. Intraplate regions characterized by low rates of seismicity are challenging
for seismic hazard assessment, mainly for two reasons. Firstly, evaluation of
historic earthquake catalogues may not reveal all active faults that
contribute to regional seismic hazard. Secondly, slip rate determination is
limited by sparse geomorphic preservation of slowly moving faults. In the
Vienna Basin (Austria), moderate historical seismicity
(Imax,obs/Mmax,obs=8/5.2) concentrates along the
left-lateral strike-slip Vienna Basin Transfer Fault (VBTF). In contrast,
several normal faults branching out from the VBTF show neither historical nor
instrumental earthquake records, although geomorphological data indicate
Quaternary displacement along those faults. Here, located about 15 km
outside of Vienna, the Austrian capital, we present a palaeoseismological
dataset of three trenches that cross one of these splay faults, the
Markgrafneusiedl Fault (MF), in order to evaluate its seismic potential.
Comparing the observations of the different trenches, we found evidence for
five to six surface-breaking earthquakes during the last 120 kyr, with the
youngest event occurring at around 14 ka. The derived surface displacements
lead to magnitude estimates ranging between 6.2±0.5 and 6.8±0.4. Data
can be interpreted by two possible slip models, with slip model 1 showing
more regular recurrence intervals of about 20–25 kyr between the
earthquakes with M≥6.5 and slip model 2 indicating that such earthquakes
cluster in two time intervals in the last 120 kyr. Direct correlation
between trenches favours slip model 2 as the more plausible option. Trench
observations also show that structural and sedimentological records of strong
earthquakes with small surface offset have only low preservation potential.
Therefore, the earthquake frequency for magnitudes between 6 and 6.5 cannot
be constrained by the trenching records. Vertical slip rates of
0.02–0.05 mm a−1 derived from the trenches compare well to
geomorphically derived slip rates of 0.02–0.09 mm a−1. Magnitude
estimates from fault dimensions suggest that the largest earthquakes observed
in the trenches activated the entire fault surface of the MF including the
basal detachment that links the normal fault with the VBTF. The most
important implications of these palaeoseismological results for seismic
hazard assessment are as follows. (1) The MF is an active seismic source,
capable of rupturing the surface despite the lack of historical earthquakes.
(2) The MF is kinematically and geologically equivalent to a number of other
splay faults of the VBTF. It is reasonable to assume that these faults are
potential sources of large earthquakes as well. The frequency of strong
earthquakes near Vienna is therefore expected to be significantly higher than
the earthquake frequency reconstructed for the MF alone. (3) Although rare
events, the potential for earthquake magnitudes equal or greater than M=7.0
in the Vienna Basin should be considered in seismic hazard studies.
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