Abstract

<p>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 automatically (without human intervention), from 724 temporary and permanent broadband stations of the AlpArray deployment including ocean-bottom seismometers. We have obtained an objective image of anisotropic structure in and around the Alpine region, at a spatial resolution that is unprecedented. 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. 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 western and eastern Alps. This difference is best explained by the more active mantle flow around the Western Alps. We discuss earlier concepts of Alpine geodynamics in the light of these new observational constraints. </p>

Highlights

  • The Alps are formed by the collision of the Adriatic and European plates, (e.g. Trumpy 1960; Stampfli et al 2001; Schmid et al 2004; Handy et al 2010)

  • The most significant observation is the large-scale pattern of mountain-belt-parallel fast orientations, which continues in the outer Alpine regions

  • We have presented new SKS splitting parameters for the Alpine region that have been robustly determined

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Summary

Introduction

The Alps are formed by the collision of the Adriatic and European plates, (e.g. Trumpy 1960; Stampfli et al 2001; Schmid et al 2004; Handy et al 2010). While the surface expression of the mountain chain is well known, there remain many open questions that relate to its deeper portions, to the forces that drive the deformation, and to the deformation itself. Mantle flow must play an important role in mountain building, and in the formation of the wider Mediterranean tectonic system in general, for example Faccenna et al (2014). Its role needs to be better understood. The deformation that occurs during flow in the subsurface can result in preferred orientation of minerals, and becomes visible to seismic waves through the phenomenon of seismic anisotropy. Seismic anisotropy can be a tool for constraining the geometry of deformation (Silver 1996)

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