The Alland earthquake sequence in Eastern Austria: Shedding light on tectonic stress geometry in a key area of seismic hazard

We present our results on the fault geometry of the Alland earthquake sequence in eastern Austria (Eastern Alps) and discuss its implications for the regional stress regime and active tectonics. The series contains 71 known events with local magnitudes 0.1 ≤ ML ≤ 4.2 that occurred in between 2016 and 2017. We locate the earthquakes in a regional 3D velocity model to find absolute locations. These locations are then refined by relocating all events relative to each other using a double-difference approach, based on relative travel times measured from waveform cross-correlation and catalogue data. We also invert for the moment tensor of the ML = 4.2 mainshock by fitting synthetic waveforms to the recorded seismo-grams using a combination of the L1- and L2-norms of the waveform differences. Direct comparison of waveforms of the largest events in the sequence suggests that all of them ruptured with very similar mechanisms. We find that the sequence ruptured a reverse fault, that is dipping with ~30° towards ~north-northeast (NNE) at 6–7 km depth. This is supported by both the hypocentres and the mainshock source mechanism. The fault is most likely located in the buried basement of the Bohemian massif, the “Bohemian Spur”. This (reverse) fault has a nearly perpendicular orientation to the normal-fault structures of the Vienna Basin Transfer Fault System further east at a shallower depth, indicating a lateral stress decoupling that can also act as a vertical stress decoupling in some places. In the west, earthquakes (at a larger depth within the upper crust) show compressive stresses, whereas the Vienna Basin to the east shows extensional (normal-faulting) stress. This provides insight into the regional stress field and its spatial variation, and it helps to better understand earthquakes in the area, including the “1590 Ried am Riederberg” earthquake.