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Ellipticity of Rayleigh waves in basin and hard-rock sites in Northern Italy

Berbellini, A; Morelli, A; Ferreira, AMG; (2016) Ellipticity of Rayleigh waves in basin and hard-rock sites in Northern Italy. Geophysical Journal International , 206 (1) pp. 395-407. 10.1093/gji/ggw159. Green open access

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Abstract

We measure ellipticity of teleseismic Rayleigh waves at 95 seismic stations in Northern Italy, for wave period between 10 and 110 s, using an automatic technique and a large volume of high-quality seismic recordings from over 500 global earthquakes that occurred in 2008–2014. Northern Italy includes a wide range of crustal structures, from the wide and deep Po Plain sedimentary basin to outcropping sedimentary and crystalline rocks in the Northern Apennines and Alps. It thus provides an excellent case for studying the influence of shallow earth structure on polarization of surface waves. The ellipticity measurements show excellent spatial correlation with geological features in the region, such as high ellipticity associated with regions of low seismic velocity in the Po Plain and low ellipticity values in faster, hard rock regions in the Alps and Apennine mountains. Moreover, the observed ellipticity values also relate to the thickness of the basement, as highlighted by observed differences beneath the Alps and the Apennines. Comparison between observations and predicted ellipticity from a reference crustal model of the region show substantial fit, particularly for T ∼ 38 s data. Discrepancy for shorter wave period suggests that slight modifications of the model are needed, and that the ellipticity measurements could help to better constrain the shallow crustal structure of the region. Predictions for the Po Plain are larger than the observations by a factor of four or more and transition from retrograde to prograde Rayleigh wave motion at the surface for periods of T ∼ 10–13 s is predicted for seismic stations in the plain. Analysis of corresponding real data indicates a possible detection of teleseismic prograde particle motion, but the weak teleseismic earthquake signals are mixed with ambient noise signals at the predicted, short, transition periods. Detection of the period of polarity inversion from the joint analysis of earthquake and ambient noise ellipticity measurements may provide further, stringent, constraints on the structure of sedimentary basins.

Type: Article
Title: Ellipticity of Rayleigh waves in basin and hard-rock sites in Northern Italy
Open access status: An open access version is available from UCL Discovery
DOI: 10.1093/gji/ggw159
Publisher version: http://dx.doi.org/10.1093/gji/ggw159
Language: English
Additional information: This article has been accepted for publication in Geophysical Journal International (http://dx.doi.org/10.1093/gji/ggw159). Copyright © The Authors 2016. Published by Oxford University Press on behalf of The Royal Astronomical Society. All rights reserved.
Keywords: Surface waves and free oscillations; Site effects; Seismic tomography; Wave propagation
UCL classification: UCL
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Earth Sciences
URI: https://discovery.ucl.ac.uk/id/eprint/1496137
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