Soil damping and site dominant vibration period determination, by means of random decrement method and its relationship with the site-specific spectral decay parameter kappa

摘要

We use the random decrement method (RDM) to determine the damping (ζ) and dominant period (T_d) of soils using earthquake records and ambient noise, and compare the values of ζ with the spectral decay parameter kappa (k_0) previously estimated at several sites in the San Bernardino Valley, northeastern Sonora, Mexico. The dominant period take values from 0.12 s at sites on igneous rocks, to 0.17-0.20 s at sites on continental deposits (Tertiary conglomerates). The damping varies between 2.8 and 5.7%. We find the empirical relationship: ζ = (0.50 ± 0.178) × k_o, which predominate in the study area, and is comparable with that obtained for stations on conglomerates (ζ=(0.43 ± 0.130) × k_0). We also find a theoretical equation ζ = (V_s/2H)k_0 useful for any region, that depend on the shear wave velocity (V_s), the decay parameter (k_0) and the thickness of the soil-layer considered (H). We calculate the thickness of the surface layer at each site and find that it varies from 80- to - 150-m. We conclude that: it is possible to obtain ζ and T_d simultaneously with the RDM; consistent estimates of ζ and T_d can be obtained using the whole record or parts of it; the RDM avoids the calculation of spectral ratios to estimate T_d and ζ as well as any other pre-processing of the signal; the vertical component of ground motion reflects the properties of shallow layers at the analyzed sites.