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As in the case of structures, also on soils we can observe a general ‘basic’ damping value under low shear strain ( 10—4) which is 2–3% below 10 Hz and which means, by recalling Eq. (6),a 12–18% amplitude decay per cycle. This value commonly reaches 6% ( 430% amplitude decay per cycle) at high frequencies (e.g. Figs. 9 and 10). The fact that damping increases with frequency is expected.

In Fig. 12C we show the damping ratios at 1 Hz (red circles) and 10 Hz (cyan circles) measured at the different strain levels in the field and compare them with the damping values measured in the laboratory on the same sediment types in undisturbed samples. We note that the difference is of one order of magnitude (0.3–0.5% damping measured in the laboratory vs. 3–5% measured at the geophysical scale). As a further confirmation, we plot our findings
(i.e. soil damping between 2% and 5% below 10 Hz for ϵ = [10—4, 10—3]) against standard damping curves commonly used in site
response modeling and well known in the literature (Fig. 13). We see that in all cases damping measured in situ is one order of magnitude larger than damping measured in the laboratory at small strain rates.