Relocation of earthquakes in the Southern and Eastern Alps (Austria, Italy) recorded by the dense, temporary SWATH–D network using a Markov chain Monte Carlo inversion

Abstract. Local earthquakes with magnitudes in the range of 1–4.2 (ML) in the Southern and Eastern Alps (2017–2018) registered by the dense, temporary SWATH-D network and the AlpArray network reveal seismicity in the upper crust (0–20 km). The seismicity is characterized by pronounced clusters along the Alpine frontal thrust, e.g., Friuli-Venetia (FV) region, in the Giudicarie-Lessini (GL) and Schio-Vicenza domains, as well as in the Austroalpine Nappes and the Inntal area. Some seismicity also occurs along the Periadriatic Fault. The general pattern of seismicity reflects head-on convergence of the Adriatic Indenter with the Alpine orogenic crust. The deeper seismicity in the FV and GL regions indicate southward propagation of the Southern Alpine deformation front (blind thrusts). The first arrival-times of P- and S-waves of earthquakes are determined by an automatic workflow and then visually/manually checked and corrected. We applied a Markov chain Monte Carlo inversion method to achieve precise hypocenter locations of the 344 local earthquakes. This approach simultaneously calculates hypocenters, 1-D velocity model, and station-corrections without prior assumptions such as initial velocity models and earthquake locations. A further advantage of the method is the derivation of the model parameter uncertainties and noise levels of the data. The accuracy of the localization procedure is checked by inverting a synthetic travel-time dataset from a complex 3-D velocity model and using the real stations and earthquakes geometry. The location accuracy is further investigated by the relocation of quarry blasts. The average uncertainties of the locations of the earthquakes are below 500 m in the epicenter and ∼1.7 km in depth when using the average V_P and V_P/V_S models and the station-corrections from the simultaneous inversion.