Imaging the Alpine crust with ambient-noise tomography: linking surface observations to deep structures

By making use of the data coverage from the AlpArray and Swath D networks, a dense set of ambient-noise measurements of Rayleigh and Love waves is extracted. This data is used to investigate the Alpine crustal and uppermost mantle structure using different approaches: (1) Azimuthal anisotropy from Eikonal tomography for the AlpArray network. We show how Eikonal tomography can be used to extract azimuthal anisotropy from surface-wave data. The methodological advantages and difficulties are discussed in detail. It is found that strong velocity heterogeneities can be the source of a major bias by causing strongly deformed wavefronts. By averaging contributions from many azimuthal directions and careful data correction, most of this bias can be removed. The results indicate a mostly orogen parallel upper and orogen perpendicular lower layer of anisotropies. In the forelands, we find good agreement with SKS studies from which we infer that lithospheric and asthenospheric anisotropies are mostly parallel. (2) Azimuthally anisotropic 3D shear velocity structure of the eastern Alps from rjMcMC tomography. With this innovative approach, we go beyond what is is shown in (1) and are able resolve the depth structure of the azimuthal anisotropy and estimate the model uncertainties. It is shown that under the orogen, a two layer anisotropic structure exists that separates the upper crust which is dominated by arc-parallel anisotropy from the lower crust and uppermost mantle which mostly show arc-perpendicular fast axis. We find that the anisotropy in the upper crust is largely controlled by major fault structures. The isotropic velocity distribution indicates a fast anomaly in the Giudicarie zone that may be related to Permian magmatism and causes a small offset in the Moho proxy. The estimated Moho structure closely resembles the positions of the underlying subudction slabs with a lateral offset between Central and Eastern Alps. (3) 3D joint inversion of surface and body wave data. To better image the crust-mantle transition zone under the Alps, we apply a 3D rjMcMC imaging approach that combines different datasets and resolves the Vp and Vs structure from the surface to 600 km depth. With this approach there is no need for crustal corrections applied to the body wave travel times since the crustal structure is constrained by ambient noise data. Preliminary results of this model indicate that the slabs are more vertical and vertically more continuous as compared to a pure P-traveltime inversion. The Python scripts used to obtain the results are already or will be published on the author’s github page (github.com/ekaestle)

Advances in imaging the Alpine crust and mantle

The dense coverage of the AlpArray Seismic Network and related targeted arrays (LOBSTER, Swath D, EASI) has led to numerous new models of the Alpine orogenic lithosphere, slabs and mantle above the Mantle Transition Zone. We highlight some novel features of these models, how they may help to answer old questions, as well as pose new questions: (1) P-wave images from teleseismic travel-time tomography (Paffrath et al., 2021a,b; Handy et al., 2021) use an innovative approach to include the highly heterogeneous Alpine orogenic crust in their model. In addition to confirming previous models for partial slab detachment beneath the Western Alps, they find evidence for a long (≥300 km), subvertical slab anomaly underneath the Eastern Alps that is detached from the orogenic lithosphere at 70-150 km depth. The latter corroborates images from surface-wave tomographic studies (Kaestle et al., 2018), but contrasts with other past- and present images indicating deeper slab detachment (Handy et al., 2015) and/or a through-going connection of the slab with Adriatic lithosphere (e.g., Plomerová et al., 2022). Cooperation of the Bochum and Prague groups to explain these disparate features reveal that crucial features, e.g., the connection of slabs with the orogenic lithosphere, depends strongly the geometry of the model area and the chosen crustal model. (2) New receiver function (RF) studies extracted signals in the Eastern Alps where previous work only imaged a ‘Moho gap’ (Hetényi et al., 2018; Kind et al., 2021; Mroczek et al., 2023, Michailos et al., 2023). These studies confirm the notion of marked, along-strike variations in structure: in the west (TRANSALP, 11.9°E), the European Moho is clearly down-going (e.g., Kummerow et al., 2004), whereas in the east (14°E), competing interpretations range between an underlying Adriatic Moho (Hetényi et al., 2018) and a downgoing European interface to more than 100 km depth (Mroczek et al., 2023). All methods indicate that the upper-plate Moho shallows from the E. Alps to the Pannonian Basin. (3) The internal structure of the Eastern Alpine crust is imaged with local earthquake (Jozi Najafabadi et al., 2021) and ambient noise tomography (Molinari et al., 2020; Qorbani et al., 2020; Sadeghi-Bagherabadi et al., 2021; Kästle et al., this vol.). The LET models show a bulge-shaped fast anomaly just to the south of the western Tauern window, possibly indicating stacking of lower crustal nappes, probably of both European and Adriatic affinity (McPhee et al., this vol.), and a fast anomaly east of the Giudicarie Fault that may be related to a Permian magmatic body, as also indicated by gravity studies (Spooner et al., 2021). (4) AlpArray has opened the door to study crustal and mantle anisotropy in unprecedented detail (Kästle et al., 2022; Soergl et al., 2022; Kästle et al., this vol.). SKS studies (e.g., Hein et al., 2021) suggest that mantle flows around slabs and potentially through slab tears, in the Western and Eastern Alps. Newest results indicate that crustal anisotropy in the Eastern Alps is layered, with an upper layer with fast directions oriented mainly orogen-parallel, approximately following major Neogene oblique-slip faults exposed at the surface. The studies also show a clear distinction between the fast-axis orientations within the Alps and in its foreland. The latter results are in excellent agreement with findings from SKS studies, indicating similar dynamics affecting the entire lithosphere. The detailed analysis of Swath-D data conducted by Link et al. (2021) has further been able to show a sharp transition in SKS splitting orientations at around 13° longitude, that is indicative of the separate evolution of central and eastern Alps. (5) Preliminary results from the joint inversion of surface- and body-wave data provide a better understanding of the different sensitivities of P- and S-waves to the upper mantle structures under the Alps. Initial results of a P-wave velocity model from teleseismic full-waveform inversion (FWI, Friederich et al., this vol.) provide surprisingly high resolution in the crust and uppermost mantle with clear images of the Alpine orogenic roots and anomalies within the crust (e.g., Ivrea Body, E. Alps lower crustal bulge). The resulting FWI model is independent of any crustal correction and may provide a vital contribution to ongoing discussions on slab origin and detachment. Taken together, the diversity of seismological images in the same area with often contrasting tectonic implications underscores the need for serious benchmarking of seismological models. Large arrays like AlpArray provide an excellent opportunity to conduct such comparative studies

Joint inversion of seismic data for temperature and lithology in the Eastern Alps

The high density SWATH-D and AlpArray seismic networks provide a unique opportunity in the Eastern Alps to resolve the complex plate configuration and investigate how the crustal structure seen today reflects the dramatic changes in mountain building style and reorganisation of plate boundaries at about 20 Ma. This study complements the partner project where scattered wave tomography is applied to the same area (presented in the poster ‘Applying scattered wave tomography and joint inversion of high-density (SWATH D) geophysical and petrophysical datasets to unravel Eastern Alpine crustal structure’, Tilmann et al). In order to bring together the seismological and geological-mineralogical constraints in a probabilistic self-consistent way, we employ the joint inversion of seismological and petrophysical data sets. Receiver functions and surface wave dispersion curves, calculated in partner projects, are usually jointly inverted for elastic properties. By utilising the strengths of Markov Chain Monte Carlo inversion, we are able to instead parameterise our model by temperature and mineral assemblage. By inverting seismic data directly for the crust’s constituent mineral assemblages, we are led to a deeper understanding of intra-crustal structure, temperature, and petrophysical properties of crustal layers. A further significant advantage is in interpretation where the probabilities of certain lithologies being present allows for a more seamless integration of qualitative geological data and a reduction in interpretation biases compared to when only seismic velocities are presented

Two-receiver measurements of phase velocity: cross-validation of ambient-noise and earthquake-based observations

International audiencePhase velocities derived from ambient-noise cross-correlation are compared with phase velocities calculated from cross-correlations of waveform recordings of teleseismic earthquakes whose epicentres are approximately on the station–station great circle. The comparison is conducted both for Rayleigh and Love waves using over 1000 station pairs in central Europe. We describe in detail our signal-processing method which allows for automated processing of large amounts of data. Ambient-noise data are collected in the 5–80 s period range, whereas teleseismic data are available between about 8 and 250 s, resulting in a broad common period range between 8 and 80 s. At intermediate periods around 30 s and for shorter interstation distances, phase velocities measured from ambient noise are on average between 0.5 per cent and 1.5 per cent lower than those observed via the earthquake-based method. This discrepancy is small compared to typical phase-velocity heterogeneities (10 per cent peak-to-peak or more) observed in this period range.We nevertheless conduct a suite of synthetic tests to evaluate whether known biases in ambient-noise cross-correlation measurements could account for this discrepancy; we specifically evaluate the effects of heterogeneities in source distribution, of azimuthal anisotropy in surface-wave velocity and of the presence of near-field, rather than far-field only, sources of seismic noise. We find that these effects can be quite important comparing individual station pairs. The systematic discrepancy is presumably due to a combination of factors, related to differences in sensitivity of earthquake versus noise data to lateral heterogeneity. The data sets from both methods are used to create some preliminary tomographic maps that are characterized by velocity heterogeneities of similar amplitude and pattern, confirming the overall agreement between the two measurement methods

Slab break-offs in the Alpine subduction zone

After the onset of plate collision in the Alps, at 32–34 Ma, the deep structure of the orogen is inferred to have changed dramatically: European plate break-offs in various places of the Alpine arc, as well as a possible reversal of subduction polarity in the eastern Alps have been proposed. We review different high-resolution tomographic studies of the upper mantle and combine shear- and body-wave models to assess the most reliable geometries of the slabs. Several hypotheses for the tectonic evolution are presented and tested against the tomographic model interpretations and constraints from geologic and geodetic observations. We favor the interpretation of a recent European slab break-off under the western Alps. In the eastern Alps, we review three published scenarios for the subduction structure and propose a fourth one to reconcile the results from tomography and geology. We suggest that the fast slab anomalies are mainly due to European subduction; Adriatic subduction plays no or only a minor role along the Tauern window sections, possibly increasing towards the Dinarides. The apparent northward dip of the slab under the eastern Alps may be caused by imaging a combination of Adriatic slab, from the Dinaric subduction system, and a deeper lying European one, as well as by an overturned, retreating European slab.GRNE graduate schoolDeutsche Forschungsgemeinschaft (DE)H2020 Marie Skłodowska-Curie Actions ()http://www.orfeus-eu.org/eid

Two-receiver measurements of phase velocity: cross-validation of ambient-noise and earthquake-based observations

Phase velocities derived from ambient-noise cross-correlation are compared with phase velocities calculated from cross-correlations of waveform recordings of teleseismic earthquakes whose epicentres are approximately on the station–station great circle. The comparison is conducted both for Rayleigh and Love waves using over 1000 station pairs in central Europe. We describe in detail our signal-processing method which allows for automated processing of large amounts of data. Ambient-noise data are collected in the 5–80 s period range, whereas teleseismic data are available between about 8 and 250 s, resulting in a broad common period range between 8 and 80 s. At intermediate periods around 30 s and for shorter interstation distances, phase velocities measured from ambient noise are on average between 0.5 per cent and 1.5 per cent lower than those observed via the earthquake-based method. This discrepancy is small compared to typical phase-velocity heterogeneities (10 per cent peak-to-peak or more) observed in this period range.We nevertheless conduct a suite of synthetic tests to evaluate whether known biases in ambient-noise cross-correlation measurements could account for this discrepancy; we specifically evaluate the effects of heterogeneities in source distribution, of azimuthal anisotropy in surface-wave velocity and of the presence of near-field, rather than far-field only, sources of seismic noise. We find that these effects can be quite important comparing individual station pairs. The systematic discrepancy is presumably due to a combination of factors, related to differences in sensitivity of earthquake versus noise data to lateral heterogeneity. The data sets from both methods are used to create some preliminary tomographic maps that are characterized by velocity heterogeneities of similar amplitude and pattern, confirming the overall agreement between the two measurement methods.Applied Geophysics and Petrophysic