Site amplification factors for deep deposits and their application in seismic hazard analysis for Central US

This report presents the results of probabilistic seismic hazard analysis with nonlinear site effects, PSHA-NL (FF)(2011), for Central and Eastern United States. This approach consists of the following steps: (a) Develop a synthetic earthquake catalog over a period of simulation years to reflect the seismic sources and their recurrence rates based on 2008 USGS hazard map, (b) Use attenuation relationships to calibrate point source (SMSIM) and finite fault models (EXSIM) for generation of synthetic ground-motion time histories corresponding to the synthetic earthquake catalogue at hard rock, (c) Propagate ground motion through soil columns of varying thickness using equivalent linear and nonlinear site response analysis (DEEPSOIL) to develop a uniform hazard response spectrum, (d) compute soil column depth dependent NEHRP style site amplification factors.
This approach was applied to soil column thickness of 30m, 100m, 200m, 300m, 500m, 1000m, and for site classes C, D and E. Computed UHRS for 30m soil-column profiles of NEHRP site-class D are consistent with NEHRP design spectra. But those of Site class C and D differ somewhat. For all site classes, the site factors exhibited a clear depth dependence, and thus dependent site coefficients are proposed. Comparisons with factors proposed in the literature are also made. It is recommended that future revisions of site factors account for the depth of the soil column to hard rock.