Ebreichsdorf 2013 earthquake series: Relative location

We study recent moderate-size earthquakes in the Southern Vienna Basin, focusing on the 2013 series of two earthquakes with local magnitudes of 4.2 and their aftershocks. Furthermore, we compare them to a similar series of earthquakes from 2000. Due to the superior dataset, we can jointly relocate all earthquakes from 2013 datasets. To reduce the influence of unmodeled velocity inhomogenities, we use the "double-difference-times" implemented in the HypoDD software. Additionally, we use velocity models with different degrees of complexity (1-D to 3-D). We also test the stability of the results with different sets of initial locations. After relocation the main shocks are located only 40 m apart; the colocation is confirmed by the high inter-event coherence. Moreover, the aftershocks show a clear pattern with larger earthquakes having deeper hypocenters and location in the South West and shallower, smaller earthquakes in the northeast. We also locate the two main shocks from 2000 relative to the main shocks from 2013 using S-P-times. The main shocks from 2000 are located 4 km to the northeast of the 2013 main shocks. This suggests that the earlier notion of "event clustering" in the Southern Vienna Basin needs to be reconsidered, since at least some of the earthquakes, here the aftershocks, seem to occur between the clusters that have been proposed previously. Still the question why earthquake collocation within short time intervals occurs, remains open.