S U M M A R YSubducted slab roll-back, lithospheric instability and asthenospheric extrusion have all been proposed as mechanisms that explain the evolution of the extensional Pannonian Basin, within the convergent arc of the Alpine-Carpathian mountain system in central Europe. We determine the P-and S-wave velocity structure of the mantle to depths of 850 km beneath this region using tomographic inversion of relative arrival-time residuals from 225 (P waves) and 124 (S waves) teleseismic earthquakes recorded by 56 stations of the Carpathian Basins Project (CBP) temporary seismic network (16-month duration) and 44 permanent seismic stations. The observed median P-wave relative arrival-time residuals vary between −1.13 s (early) in the Alps and 1.12 s (late) at the western end of the Carpathians; S-wave relative arrival-time residuals are about twice as large (−2.13 s and 3.39 s). We tested the effect of deterministic corrections on our relative arrival-time residuals using crustal velocity models from controlled source experiments, but show that the use of station terms in the inversion provides a robust method of correcting for near-surface crustal variation. Our tomographic models reduce the P-wave rms residual by 71 per cent to 0.130 s and our S-wave rms residual by 59 per cent to 0.624 s. At shallow sublithospheric depths we image several localized lower velocity regions, correlated with higher heat flow and interpreted as upwelling asthenosphere. We image a high velocity structure down to depths of about 350 km beneath the Eastern Alps. Further east, beneath the Pannonian Basin, a deeper continuation of the Eastern Alps fast anomaly is imaged trending E-W from ∼300 km depth and extending into the mantle transition zone (MTZ). In the MTZ we image a fast anomaly extending outwards as far as the Carpathians, the Dinarides and the Eastern Alps. This higher velocity mantle material is interpreted as being produced by a mantle downwelling, whose detachment from the lithosphere above may have triggered the extension of the Pannonian Basin.
We develop and apply an imaging procedure for simultaneous location and characterization of seismic source properties called Moment Tensor Migration Imaging. The procedure constructs images for moment tensor components using a weighted diffraction stack migration, and combines ray‐theoretical Green's functions with a reverse time moment tensor imaging methodology. By applying an approximation we term the ‘ray‐angles only approximation’, we form an expression for Moment Tensor Migration Imaging where the migration weights depend only on the take‐off and arrival angles for rays leaving receiver positions and incident upon the image points. Moment Tensor Migration Imaging retains the benefits of diffraction stack procedures for source location and characterization, namely speed, flexibility, and the potential for incorporating non‐linear stacking procedures, whilst also providing the benefits of moment tensor imaging such as: the inclusion of multiple phase and multiple component data; the collapsing of the source radiation pattern; estimation of the moment tensor.
We examine variations of the imaging procedure through a synthetic test. We show that although the assumptions required for the imaging and ray‐angles only approximation may not be strictly valid for realistic survey geometries, a simple weight adjustment can be used to obtain more accurate and stable results in these situations. In our synthetic example we find that the use of a P‐wave only migration without this reweighting structure produces poor results, whereby the resulting images show activity upon incorrect moment tensor components. However, many of these effects are mitigated by use of the reweighting scheme and the results are further improved through the introduction of non‐linear stacking operators such as semblance weighted stacks. The highest quality moment tensor images (for the synthetic test examined here) are obtained through the use of both P‐wave and S‐wave wave fields. This highlights the importance of multicomponent data and multiphase modelling when characterizing seismic sources. We also find that the imaged moment tensor components vary proportionately when the input velocities are perturbed by a scale factor. This suggests, for the geometry investigated here, derived source properties such as fault‐plane solutions and shear‐tensile components will not be influenced by bulk changes in seismic velocities. Finally, we show the application to a real microseismic event observed using a surface array during hydraulic fracturing. We find that the procedure collapses the seismic radiation pattern into an anomaly with a maximum at the hypocentre and our derived mechanism is consistent with the observed radiation pattern from the source.
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