The many tectonic plates and distinct interacting units within the Alps lead to dramatic changes in geology over a relatively small area. This poses a unique scientific challenge where understanding the evolution of the Alps and the active seismotectonics is vital to inform seismic hazard models in the highly populated greater Alpine region. Despite a long tradition of study, fundamental questions about present-day geological structure, particularly in the Eastern Alps, remain unanswered. Diverse results from separate seismological studies have often led to conflicting interpretations of the Alpine structure, and thus to dissimilar models of Alpine orogeny.
To gain a deeper understanding of the Alps, the multidisciplinary AlpArray initiative was founded. The core dataset of AlpArray was gathered with the AlpArray Seismic Network, which consisted of 628 seismic stations in the wider Alpine area. This network was supplemented by several complementary deployments; among these was the SWATH-D network, which delineates the region of the Eastern Alps that is the focus of this thesis. Thus, with a total of 168 densely spaced stations straddling Austria and Italy, SWATH-D was ideally located to investigate the Eastern Alps and provides the bulk of the seismic data that we investigate. We used three distinct methods to analyse the seismic data; the autocorrelation of ambient seismic noise, migration receiver functions, and the probabilistic inversion of surface waves and P-wave tomography.
In the field of seismic interferometry, autocorrelations can be used to extract the Green's function from ambient noise data at a single station. With this autocorrelation variation of the method, we are, in principle, able to retrieve zero-offset reflections in a stratified Earth (like cross-correlations for pairs of stations). These reflections are valuable as they do not require an active seismic source and, being zero-offset, are better constrained in space than passive earthquake-based measurements. By applying a depth-velocity stacking scheme to the autocorrelations and receiver functions together, we can resolve the ambiguity between these parameters. Their application to temporary AlpArray stations showed some success in identifying reflections from the Moho (boundary between the crust and mantle) in the (structurally simpler) Alpine foreland, thereby demonstrating that the method is also applicable for temporary seismometer deployments.
By migrating receiver functions from SWATH-D stations to depth, we produced a high-resolution Moho depth map of the Eastern Alps. This was facilitated by the joint analysis of receiver function images of direct conversions and multiple reflections for both the SV (radial) and SH (transverse) components. This combination of multiples and components enabled us to map overlapping and inclined discontinuities. We observed the European Moho to be underlying the Adriatic Moho from the west up to the eastern edge of the Tauern Window. East of the Tauern Window, there is a sharp transition from underthrusting European crust to a flat and thinned crust. This thinner crust is associated with Pannonian extension tectonics and is underthrust by both European crust in the north and Adriatic crust in the south. The Adriatic lithosphere underthrusts northward below the Southern Alps and becomes steeper and deeper toward the Dinarides, where it dips towards the north-east. These observations suggest that the steep high-velocity region in the mantle below the Eastern Alps, observed in tomographic studies, is likely to be of European origin.
Probabilistic inversion with multiple geophysical observations is a natural progression from the observations that we make with receiver functions. Rather than the standard inversion for seismic velocity, we employ petrophysical constraints to directly invert for lithology and temperature within the crust in a probabilistic manner through Markov-chain Monte Carlo. The application of this method leads to a deeper understanding of the intracrustal structure, temperature, and petrophysical properties. Changes in these quantities across geological boundaries can help reconstruct tectonic history. A further significant advantage of this method is in interpretation, where the probabilities of certain lithologies being present allow for a more seamless integration of qualitative geological data and a reduction in interpretation biases compared to when only seismic velocities are presented. We show the strength of this approach by deriving the temperature, structure, and lithological probabilities for a transect of seven SWATH-D stations crossing a major tectonic fault and observe an associated change in both temperature and composition.
All three of these techniques and their application in the Alps required the simultaneous advancement and development of new seismological methods. AlpArray and its complementary seismic networks are used in conjunction with these methods to produce more complete and higher resolution images of the subsurface and to probe deeper into the properties of the crust. These advancements and associated observations contribute to answering the geological questions posed by AlpArray.
