Human activity causes vibrations that propagate into the ground as high-frequency seismic waves. Measures to mitigate the COVID-19 pandemic caused widespread changes in human activity, leading to a months-long reduction in seismic noise of up to 50%. The 2020 seismic noise quiet period is the longest and most prominent global anthropogenic seismic noise reduction on record. While the reduction is strongest at surface seismometers in populated areas, this seismic quiescence extends for many kilometers radially and hundreds of meters in depth. This provides an opportunity to detect subtle signals from subsurface seismic sources that would have been concealed in noisier times and to benchmark sources of anthropogenic noise. A strong correlation between seismic noise and independent measurements of human mobility suggests that seismology provides an absolute, real-time estimate of population dynamics.

Summary To constrain seismic anisotropy under and around the Alps in Europe, we study SKS shear-wave splitting from the region densely covered by the AlpArray seismic network. We apply a technique based on measuring the splitting intensity, constraining well both the fast orientation and the splitting delay. 4 years of teleseismic earthquake data were processed, from 723 temporary and permanent broadband stations of the AlpArray deployment including ocean-bottom seismometers, providing a spatial coverage that is unprecedented. The technique is applied automatically (without human intervention), and it thus provides a reproducible image of anisotropic structure in and around the Alpine region. As in earlier studies, we observe a coherent rotation of fast axes in the western part of the Alpine chain, and a region of homogeneous fast orientation in the Central Alps. The spatial variation of splitting delay times is particularly interesting though. On one hand, there is a clear positive correlation with Alpine topography, suggesting that part of the seismic anisotropy (deformation) is caused by the Alpine orogeny. On the other hand, anisotropic strength around the mountain chain shows a distinct contrast between the Western and Eastern Alps. This difference is best explained by the more active mantle flow around the Western Alps. The new observational constraints, especially the splitting delay, provide new information on Alpine geodynamics.

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. In dieser Studie untersuchen wir die Erdbebenserie von 2013 bei Ebreichsdorf im südlichen Wiener Becken. Hier wurden zwei Beben mit einer lokalen Magnitude von 4.2, sowie ca. 30 Nachbeben aufgezeichnet. Im ersten Schritt relokalisieren wir die Serie relativ zueinander, denn im Unterschied zu früheren Erdbebenserien ist der 2013er Datensatz wesentlich umfangreicher. Im Anschluss vergleichen wir die relokalisierten Erdbeben mit einem (ähnlichen) Bebenpaar des Jahres 2000. Um den Einfluss von unmodellierten Geschwindigkeitsänderungen zu reduzieren, verwenden wir den den HypoDD Algorithmus, welcher auf der Verwendung von Doppel-Differenz-Zeiten basiert. Zusätzlich verwenden wir unterschiedlich komplexe Geschwindigkeitsmodelle (1-D, 2-D und 3-D). Weiters testen wir die Stabilität der Ergebnisse mit unterschiedlichen Startlokalisierungen der Erdbeben. Nach der Relokalisierung befinden sich die beiden Hauptbeben von 2013 nur 40 m voneinander entfernt. Diese Kollokation wird von der hohen Kohärenz zwischen den Wellenformen der beiden Hauptbeben bestätigt. Die Nachbeben zeigen ein klares Muster, wobei die stärkeren Ereignisse in größeren Tiefen auftreten, und weiter im Südwesten, als die kleineren Erdbeben. Zusätzlich lokalisieren wir die beiden Hauptbeben von 2000-relativ zu den Hauptbeben von 2013 unter der Verwendung von S-P-Zeiten. Hier zeigt sich, das die beiden Bebenserien ca. 4 km voneinander stattfanden. Sie zeigen jedoch auch eine hohe Ähnlichkeit untereinander, wenn auch geringer als die Beben von 2013. Dies lässt darauf schließen, dass die frühere Vorstellung des...

Abstract. Site selection is a crucial part of the work flow for installing seismic stations. Here, we report the preparations for a countrywide temporary seismic network in Austria. We describe the specific requirements for a multi-purpose seismic array with 40 km station spacing that will be operative approximately three years. Reftek 151 60 s sensors and Reftek 130/130S digitizers form the core instrumentation of our seismic stations which are mostly installed inside abandoned or occasionally used basements or cellars. We present probabilistic power spectral density analysis to assess noise conditions at selected sites and show exemplary seismic events that were recorded by the preliminary network by the end of July 2015.

The project at hand is a field test around the KTB (Kontinentale Tiefbohrung) site in the Oberpfalz, Southeastern Germany, at the northwestern edge of the Bohemian Massif. The region has been extensively studied through the analysis of several seismic reflection lines deployed around the drilling site. The deep borehole had been placed into gneiss rocks of the Zone Erbendorf-Vohenstrauss. Drilling activity lasted from 1987 to 1994, and it descended down to a depth of 9101 m.In our experiment, we aim to recover structural information as well as anisotropy of the upper crust using the receiver function technique. This retrieved information is the basis for comparing the out-coming anisotropy amount and orientation with information of rock samples from up to 9 km depth, and with high-frequency seismic experiments around the drill site.For that purpose, we installed 9 seismic stations, and recorded seismicity continuously for two years from