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Gaetano Vitale

strata with velocity inversion

My name is Gaetano, and i have a question about the velocity inversion which take place at the conglomerates-clay interface; this is my velocity profile: thickness (m) – vs (m/sec) 4 – 224 (shallow layer) 34 – 700 (conglomerates and sand) 30 – 331 (clay) 25 – 418 (clay) 30 – 501 (clay) 20 – 574 (clay) 28 – 656 (clay) bedrock – 900 (clay)

I did some simulation with strata, obtaining deamplificazioni in the frequency range 2,5 – 8 Hz. My question is this: in the presence of strong velocity inversion the results of simulations are reliable? Thank you.

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    Ellen M. Rathje

    Thank you for your question. Before answering, let me say that by deamplification I assume you mean that the spectral acceleration at the surface is smaller than the input.

    If the input motion is large enough, the induced strains could be damping out the frequencies between 2.5 – 8 Hz (or 0.125 to 0.4 s period). This will happen with or without the velocity inversion, but the large change in velocity from 700 below to 224 above could generate very large strain in that surface layer.

    If you mean by deamplification that the response spectrum has a trough between 2.5 – 8 Hz, I think that this could be caused by your velocity inversion. I ran your profile through Strata and saw a trough here…but only when the soil was linear elastic. It was not there when I ran at as equivalent linear.

    Finally, I think the simulations are as reliable for this profile as they are for any other. However, there are always limitations to 1D site response.

    Good luck!

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