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Cohérence spatiale des mouvements sismiques et structure géométrique du proche sous-sol: un exemple à Argostoli, Grèce

Abstract : The spatial variation of the earthquake ground motion over short distances can significantly affect the dynamic response of large and extended engineered structures, particularly when they are located at sites with lateral heterogeneity. In current practices, it is taken into account in terms of coherency, as a function of frequency and distance, established on an essentially empirical basis, making it difficult to extrapolate at different sites. So, in order to improve our understanding of the physical significance of coherency and its relationship to the geometrical and mechanical characteristics of the ground structure, a very dense network was installed in seismically active region of Argostoli-Koutavos (Cephalonia, Greece). Consisting of 21 velocimeters deployed on four concentric circles of radii 5, 15, 40 and 80 m around a central station in a small valley (~50m thickness, width 1.5 km), the array recorded several hundred events in the period from September 2011 to April 2012. A subset of 46 events having a homogeneous distribution in terms of epicentral distance (0-200 km), back-azimuth (0-360 °) and magnitude (2 to 5) has been selected for a systematic analysis of the coherencyof all station pairs observed on 'the most energetic phase' containing the S-wave. 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, and on the length of chosen signal window provided that it contains the same S-phase energy. The most striking result concerns the influence of the site geometry;the coherency is systematically higher for the pairs aligned parallel to the axis of the valley (2D), and lower for those aligned in the perpendicular direction. This result is consistent with the formation of the wavefield in the valley, dominated by surface waves propagating from one edge to the other. The observed coherency estimates are,however,weaklyrepresented by the existing parametric models. MOTS-CLÉS :variabilitéspatiale, réseau dense, cohérence, champ d'ondes.
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Afifa Imtiaz, Cécile Cornou, Pierre-Yves Bard, Aspasia Zerva. Cohérence spatiale des mouvements sismiques et structure géométrique du proche sous-sol: un exemple à Argostoli, Grèce. 9ème Colloque National AFPS2015, AFPS / IFSTTAR, Nov 2015, Marne-la-Vallée, France. ⟨insu-01742644⟩

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