AlpArray Austria

AlpArray is an initiative to study the greater Alpine area with high-quality broadband seismological experiments. We gather infrastructure and expertise from institutions around Europe to work together on data acquisition, processing, and better understanding the subsurface under the Alps. Beyond the permanently installed stations, close to 400 portable broadband stations will be placed into a backbone to achieve a homogeneous network of broadband seismological instruments over the entire Alpine area with 40-60 km spacing, combined with ocean bottom seismometers in the Mediterranean Sea. This will give information on subsurface structure and deformation with homogeneous and even resolution. Beside the backbone, there will also be densely spaced stations along swaths across key parts of the Alpine chain. The main scientific goal of AlpArray is to better understand the structure, deformation, and evolution of the lithosphere beneath the Alps, from the surface down to the mantle transition zone, using the full set of seismological techniques for imaging structure and deformation, and for localizing seismicity. The geodynamic interpretation of the acquired data will eventually be complemented by other Earth Science subdisciplines such as gravity and magnetotellurics, structural geology, and numerical and analogue modeling. The role of the project AlpArray Austria is to assemble the Austrian portion of the backbone network. Instrumentation for the project is supplied by the University of Vienna (UW) and the Dublin Institute for Advanced Studies (DIAS, Ireland), complementing the permanent network maintained by the Central Institute for Meteorology and Geodynamics (ZAMG). The gathered dataset is unprecedented in the Alpine region, for its spatial coverage and quality, and it allows us to perform cutting-edge research for entire Alpine region, and especially for the Eastern Alps and their transition to the foreland and the Carpathians/Dinarides, where key questions pertaining to structure and deformation of the subsurface are still open. The dataset gathered in this project is of considerable value also for better understanding seismic hazard in the area. Earthquakes have in the past caused much damage in Austrian and neighboring cities (e.g., Villach in 1348, Vienna in 1590), and this will happen again. Gaining more base data about seismicity in and around Austria and the tectonic faults along which they occur is therefore of paramount importance.

The goal of AlpArray Austria and its international context is/was to better understand the structure, deformation, and evolution of the lithosphere beneath the Alps. A seismological network was constructed in a multinational effort, to cover the entire Alpine region with more than 600 stations. We have focused on the Eastern Alps, and have produced 3D images of subsurface structure for several key regions, e.g. the Vienna Basin and its surroundings, and the central- eastern part of the Alps. This was possible using a new technique based on natural ambient vibrations as seismic source. The 3D images show the structure of the basins clearly, as well as the different geological units in the Alps. They allow studying the depth continuation of these geological features. Surface waves from distant earthquakes have been used to trace wave arrivals across Central Europe. This has led to several surprises. The variations of (group) arrival time can be very large, and often the waves arrive from different directions than one would assume, if the Earth were (laterally-) homogeneous. Even more, the deviations show characteristic patterns that indicate finite-frequency effects on wave propagation. These effects allow pinpointing the heterogeneity in the subsurface better than before, and this will allow interesting new ways of structural imaging (also for the industrial practice). We have also studied seismic anisotropy and deformation below the Alps, and we have been able to locate interfaces at depth using the receiver-functions technique, in particular the crust-mantle boundary under the Alps. Many earthquakes have occurred during the experiment, and the new data have been very useful for better understanding these rupture phenomena in the subsurface. The earthquakes could be located better. Earthquakes occur infrequently in Austria, but occasionally they are disastrous (e.g., near Villach in 1348, and near Vienna in 1590). Gaining more information from the smaller events, e.g. about the faults that produce them, is rather important therefore. A less-expected result of the experiment was that rockfalls can be detected seismologically, up to distances of hundreds of kilometers. This is of interest since the remotely-applied seismic technique is rapid, and it does not require human observation of the rock fall. It can thus identify rock falls that would otherwise remain undetected. Last-but-not least the AlpArray has recorded acoustic waves that propagate above ground, e.g. those created by the gas explosion in Baumgarten in December, which has destroyed one of the major European gas hub 2017 to the east of Vienna. The AlpArray Austria project has allowed to provide a precise timing of the accident.