Microseismicity in the Large-N Swath-D Network: Revealing Seismic Sequences and Active Faults in the Eastern Alps

Abstract

Uniformly spaced large-N seismic networks like the Swath-D (2017–2019), a densified part of the AlpArray initiative, provide unmatched opportunities to study microseismic activity and fault structures. Here, we show how the combined analysis of spatially and temporally clustered seismicity, precise relocations, waveform-based clustering, and moment tensor solutions for 67 earthquakes (1.13.3) help to characterize the heterogeneous study region in the south-eastern Alps. We observe a strong zonation, with clustered microseismicity predominantly in the SE and NW parts of the study area. The identified sequences indicate a dominance of swarms in the NW compared to more mainshock-aftershock sequences in the SE, while both sequence types can occur in both regions. We identify multiple short faults in the NW with lengths of a few hundred meters, and distinguish two close fault structures activated in one sequence based on waveform similarity and focal mechanisms. Compared to predominant thrust faulting in the SE, normal and strike-slip faulting in the NW points to high regional complexity, with local stresses deviating from simple expectations of thrust faulting resulting from the Europe-Adria convergence. We find that zones of increased microseismic activity match zones of high P wave attenuation from a recent Qp model developed in the AlpArray initiative (Jozi Najafabadi et al., 2023, https://doi.org/10.1186/s40623-023-01942-0), supporting our interpretations of spatiotemporal patterns concerning crustal properties and tectonic activity. Our findings agree well with the occurrence of large historical earthquakes while simultaneously shedding light on much smaller seismogenic features.