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Directional dependence of site effects observed near a basin edge at Aegion, Greece

Directional dependence of site effects observed near a basin edge at Aegion, Greece

Ktenidou, Olga-Joan, Chávez-García, Francisco-J., Raptakis, Dimitrios and Pitilakis, Kyriazis D. (2016) Directional dependence of site effects observed near a basin edge at Aegion, Greece. Bulletin of Earthquake Engineering, 14 (3). pp. 623-645. ISSN 1570-761X (Print), 1573-1456 (Online) (doi:

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We study site effects using 520 weak motion earthquake records from a vertical array in Aegion, Greece. The array is inside a basin, has four stations in soil, and one in bedrock (178 m depth). The site is marked by high seismicity and complex surface geology. We first use the records to establish the downhole accelerometer orientations and their evolution with time. Then we estimate site effects using empirical spectral ratios with and without a reference site (standard and horizontal-to-vertical spectral ratio). We find significant site amplification which cannot be accounted for by 1D model predictions, along with a significant difference in the amplification level between the two horizontal components. These are indications of 2D effects, namely surface waves generated at the basin edge. The difference in amplification between the horizontal components is maximised when these are rotated with respect to the orientation of the basin edge. The strongest amplification takes place in the direction parallel to the basin edge (SH, or out-of-plane motion), and is up to 2 times higher than in the perpendicular direction (SV, or in-plane motion). This directional effect on the amplification is corroborated by numerical 2D modelling using incident SH and SV waves, with the former possibly generating strong Love waves. In the records, the directionality is clear for windows containing the largest amplitudes of the records (S waves and strong surface waves), while it tends to vanish for coda-wave windows. This directionality is also observed when using response spectral ratios rather than Fourier ratios. We compute soil-to-rock amplification factors for peak ground acceleration (PGA) and find it is significantly higher than what is predicted by current design codes. We attribute this difference to the basin edge amplification, linear soil behaviour, and to the inability of simple scalar values like PGA to describe complex amplification effects. Finally, we analyse the earthquake records at a surface station near the slope crest and do not observe significant topographic amplification.

Item Type: Article
Uncontrolled Keywords: Site effects; Basin edge; Amplification; Directionality; CORSSA; Downhole array
Subjects: Q Science > QE Geology
T Technology > TA Engineering (General). Civil engineering (General)
Faculty / School / Research Centre / Research Group: Faculty of Engineering & Science > School of Engineering (ENG)
Last Modified: 17 Oct 2016 09:14

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