Shear wave splitting in the Alpine region

Abstract

Summary To constrain seismic anisotropy under and around the Alps in Europe, we study SKS shear-wave splitting from the region densely covered by the AlpArray seismic network. We apply a technique based on measuring the splitting intensity, constraining well both the fast orientation and the splitting delay. 4 years of teleseismic earthquake data were processed, from 723 temporary and permanent broadband stations of the AlpArray deployment including ocean-bottom seismometers, providing a spatial coverage that is unprecedented. The technique is applied automatically (without human intervention), and it thus provides a reproducible image of anisotropic structure in and around the Alpine region. As in earlier studies, we observe a coherent rotation of fast axes in the western part of the Alpine chain, and a region of homogeneous fast orientation in the Central Alps. The spatial variation of splitting delay times is particularly interesting though. On one hand, there is a clear positive correlation with Alpine topography, suggesting that part of the seismic anisotropy (deformation) is caused by the Alpine orogeny. On the other hand, anisotropic strength around the mountain chain shows a distinct contrast between the Western and Eastern Alps. This difference is best explained by the more active mantle flow around the Western Alps. The new observational constraints, especially the splitting delay, provide new information on Alpine geodynamics.

Observations of mantle seismic anisotropy using array techniques: shear-wave splitting of beamformed SmKS phases

“…We find that the beam splitting intensity estimate is relatively independent of the single-station noise level, although the 95% confidence interval tends to increase as noise is added. The average single-station splitting intensity, on the other hand, decreases as noise is added, which is in agreement with the real-data results from the HLP region (Section 5.1) and is also in agreement with findings from previous papers (Monteiller & Chevrot, 2011;Hein et al, 2021). This implies that beam splitting parameters are more reliable at characterizing the anisotropy than single-station splitting measurements if noise levels are high.…”

“…For this other event, the differences in splitting intensity between beam and average single-station splitting are generally lower (Supplementary Figure S1), indicating that the SI differences for the HLP region can in fact be explained by the details of the data for the initially used event. The results of these tests suggest that for noisy data, SI values may generally be underestimated, consistent with conclusions drawn by other studies (e.g., Hein et al, 2021). We will further discuss this finding below in Section 7.…”

Present‐Day Upper‐Mantle Architecture of the Alps: Insights From Data‐Driven Dynamic Modeling

“…In the mean model, we observe a rotation in mantle flow around the Western Alps due to a return flow generated by the attached slab in the Northern Apennines. This is consistent with shear-wave splitting (SKS) measurements that also show a rotation of the fast axis around the Western Alps (Figure 3, Barruol et al, 2011;Hein et al, 2021). In the mean+std model, such a rotation is hindered by the presence of an attached slab beneath the Alps.…”

“…Horizontal flow in the asthenosphere at a depth of 220 km for (g) mean‐std, (h) mean, and (i) mean+std models are plotted as green lines scaled by velocity magnitude. Shear‐wave splitting measurements scaled by the delay time are plotted with black bars (Hein et al., 2021). White lines in the last three panels indicate the contour of the slabs at 220 km depth.…”

Present-day upper-mantle architecture of the Alps: insights from data-driven dynamic modelling

“…Horizontal flow in the asthenosphere at a depth of 220 km for (g) mean-std, (h) mean, and (i) mean+std models are plotted as green lines scaled by velocity magnitude. Shear-wave splitting measurements scaled by the delay time are plotted with black bars(Hein et al, 2021). White lines in the last three panels indicate the contour of the slabs at 220 km depth.…”