Effects of site geometry on short-distance spatial coherency in Argostoli, Greece

Original Research Paper

Abstract

The spatial variation of the earthquake ground motion over short distances can significantly affect the dynamic response of large and extended engineered structures, especially on sites with inhomogeneity in surface geology and geometry. In current practices, such variation is taken into account in terms of coherency, a function of frequency and distance, established on an essentially empirical basis and difficult to extrapolate at different sites. Hence, a better understanding of its physical significance and its relationship with the underlying ground structure is indispensable. A two-dimensional dense array, deployed at the small and shallow Koutavos-Argostoli valley in Cephalonia, Greece, provided an abundance of data to study the stochastic characteristics of seismic ground motions over very short distances. A set of 46 magnitude 2–5 events at epicentral distances 0–200 km has been selected for the analysis. The lagged coherency of the S-wave dominating seismogram was computed for each station-pair within the array and was averaged over various distance intervals for the whole data set. The results indicate a lack of a clear dependence of the average coherency on the magnitude, back azimuth or site-to-source distance of the event. The most striking result concerns the influence of the site geometry: the coherency is systematically lower for the pairs aligned perpendicular to the axis of the valley (2D) compared to those aligned in the parallel direction. This finding is consistent with the dominance of valley-edge generated surface waves propagating from one edge to the other. The averaged coherency estimates are only weakly represented by the existing parametric models, indicating its strong site dependent nature.

Keywords

Lagged coherency Site geometry Surface waves Dense array Cephalonia Greece 

Notes

Acknowledgements

The authors would like to thank all NERA partners who participated in the seismological experiment, especially Nikos Theodulidis, Giovanna Cultrera, Tobias Boxberger, and Tommy Andreou. Seismological data used in this paper have been acquired by using the stations from SISMOB which is a member of French Seismologic and Geodetic Network (RESIF—Réseau Sismologique et Géodésique Français). RESIF is a national Research Infrastructure recognized as such by the French Ministry of higher education and research. It is additionally supported by a public grant overseen by the French national research agency (ANR) as part of the “Investissements d’Avenir” program (reference: ANR-11-EQPX-0040) and the French Ministry of ecology, sustainable development and energy. The authors would like to take this opportunity to thank the editor and two anonymous reviewers for their contributions in the review process of the article. The authors greatly appreciate the constructive suggestions and inspiring comments made by the reviewers, which have helped to improve the manuscript.

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Copyright information

© Springer Science+Business Media B.V., part of Springer Nature 2017

Authors and Affiliations

  1. 1.Seismic and Volcanic Risks Unit, Risks and Prevention DivisionBRGM, F-45060OrléansFrance
  2. 2.CNRS, IRD, IFSTTAR, ISTerreUniv. Grenoble Alpes, Univ. Savoie Mont-Blanc, CNRS, IRD, IFSTTAR, ISTerreGrenobleFrance
  3. 3.Department of Civil, Architectural and Environmental EngineeringDrexel UniversityPhiladelphiaUSA

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