High noise levels hamper teleseismic shear wave splitting measurements, which bandpass filtering does not always help. To investigate how robust splitting measurements are to noise, we analysed a set of synthetic records with known splitting parameters and added fixed levels of noise. In the presence of weak anisotropy, single‐waveform splitting measurements are unreliable when operating with noisy data sets. A practical rule in terms of S/N ratio and splitting delay time parameters is that splitting is confidently detectable at S/N > 8, regardless of the wave’s original polarization orientation. However, for the evidence of weak anisotropy to be detectable and measurable at an S/N value of 4, the backazimuth separation of the phases from the fast polarization direction needs to be higher than 20°. Stacks of individual measurements consistently yield reliable results down to S/N values of 4. Applying stacking to data from DSB (Dublin, Ireland), the fast polarization direction φ and lag time δt are 58° and 0.95 s. This orientation reflects surface trends of deformation in the area, as found elsewhere in the UK. Our result thus reinforces the proposed model that the detected anisotropy in the British Isles originates from lithospheric coherent deformation preserved from the last main tectonic episode.

S U M M A R YOccasionally, SKS and SKKS waveforms in the same seismogram are affected differently by anisotropy. One source of this discrepancy may be structures in D . In this study, we examine the discrepancy in order to determine where it arises in the propagation path and what it is due to. We find that D is the most likely source of the signal. The relatively minor differences in differential shear-wave splitting in SKS and SKKS limit large-scale azimuthal anisotropy in D to less than 2 per cent, though it may be locally stronger. The most effective way to develop the splitting differences is through polarization differences between SKS and SKKS imposed at the CMB. We examine how effective relief on and lateral gradients in CMB structure are at generating polarization anomalies and conclude that topography generates them best. Ramps and ridges are more effective generators of polarization anomalies than hills. Laterally extensive (500-1000 km) sloping CMB topography greater than 15 • can develop the observed polarization anomalies in the data. The topography required exceeds constraints from other sources, so is unlikely to be the major factor explaining the anomalies. We produce a global map of SKS/SKKS exit points from the core where anomalous polarization behaviour is found, in relation to the velocity structure known to exist in D : the anomalies appear to be restricted to faster than average areas of the CMB, suggesting a contribution from anisotropy related to the post-perovskite phase transition.

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Abstract. We present a focal mechanism catalogue of earthquakes that occurred in the southeastern Alps and surrounding areas from 1928 to 2019. The area involved in the process of convergence between the Adria microplate and Eurasia is one of the most seismically active regions in the Alpine Belt. The seismicity is minor, with the Ms= 6.5 Friuli earthquake being the strongest event recorded in the area, but the seismic risk is relevant because it is a highly populated region. For this reason, numerous studies have been carried out over time to investigate the stress field and the geodynamic characteristics of the region using focal mechanisms. To provide a comprehensive set of revised information, which is challenging to build quickly because the data are dispersed over many papers, we collected and revised the focal mechanisms that were previously published in the literature. Additionally, depending on the data quality and availability, we computed new focal mechanisms by first arrival polarity inversion or seismic moment tensor. Finally, we merged all the fault plane solutions to obtain a catalogue for a selection of 772 earthquakes with 1.8≤M≤6.5. For each earthquake, we reported all the available focal mechanisms obtained by different authors. We also suggested a preferred solution for users who need information provided expeditiously. The catalogue (https://doi.org/10.5281/zenodo.4660412; Saraò et al., 2021) is given as the Supplement of this paper and will be updated periodically (https://doi.org/10.5281/zenodo.4284970).