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

We use the random decrement method (RDM) to determine the damping (ξ) and dominant period (Td) of soils using earthquake records and ambient noise, and compare the values of ξ with the spectral decay parameter kappa (κ0) 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: ξ=(0.50±0.178)×κ0, which predominate in the study area, and is comparable with that obtained for stations on conglomerates (ξ=(0.43±0.130)×κ0). We also find a theoretical equation ξ=(Vs/2H)κ0 useful for any region, that depend on the shear wave velocity (VS), the decay parameter (κ0) 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 ξ and Td simultaneously with the RDM; consistent estimates of ξ and Td can be obtained using the whole record or parts of it; the RDM avoids the calculation of spectral ratios to estimate Td and ξ 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.

► We test random decrement method to estimate damping and dominant period at different sites. ► Consistent estimates are obtained using whole earthquakes records or parts of them. It is not necessary to calculate H/V ratios. ► We found equations related site-specific spectral decay parameter and damping in the region. ► Different seismic response is due probably to different degrees of fracturing and weathering of rocks.