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Abstract

The area extending from the upper Mürz Valley to Semmering, and on to the southern Vienna Basin, belongs to the seismically most active regions in Austria. Because of the population density and sensitive infrastructure, seismic hazard assessment is an important issue. Routine location of earthquakes carried out by the Central Institute for Meteorology and Geodynamics, Austria, images well the general seismicity pattern. However, a correlation with individual faults cannot be resolved. In this study, recordings from passive seismic monitoring projects (ALPASS and CBP) and permanent seismic network data are used together with a new 3D seismic velocity model to locate earthquake hypocentres in the Vienna Basin area. Three different location methods were applied. Focal coordinates of the 44 earthquakes determined by these methods depend on the applied method and differ considerably from the routine locations. Analysis of the residual travel-times and the location of quarry blasts showed that the best results were achieved with a probabilistic location method based on the new 3D velocity model for P- and S waves. An absolute accuracy of ~3 km was obtained by this method. Clustering and linear alignment of epicentres as well as their probability density functions allow for the correlation with local fault systems. Hypocentres with increasing focal depth closely follow the Mur-Mürz in the Semmering area and its extension to the Pottendorf fault in the southern Vienna Basin. Further, the continuation of the Salzach-Ennstal-Mariazell- Puchberg fault towards the Vienna Basin has been imaged clearly by epicentres. Implementation and further improvement of the 3D velocity model has the potential to enhance accuracy of routine epicentre location.