PEER has just published Report No. 2013/18 titled “Identification of Site Parameters that Improve Predictions of Site Amplification” as a new addition to the PEER Report Series. It was authored by Ellen M. Rathje and Sara Navidi of the Department of Civil, Architectural, and Environmental Engineering at The University of Texas at Austin. The research and report were sponsored by PEER’s Program of Applied Earthquake Engineering Research of Lifelines Systems supported by the California Department of Transportation and the Pacific Gas and Electric Company.
The effects of the local soil conditions on earthquake shaking are often quantified via an amplification factor, which is defined as the ratio of the ground motion at the soil surface to the ground motion at a rock site at the same location. Site amplification models are empirical equations that predict site amplification based on the general characteristics of the site. Most of the current site amplification models predict amplification based on the average shear wave velocity in the top 30 m (VS30). However, additional site parameters influence site amplification and should be included in site amplification models.
To identify site parameters beyond VS30 that influence site amplification and to develop an empirical site amplification model that includes these parameters, site response analyses are performed for a large suite of shear wave velocity profiles. These analyses identified the parameter Vratio, defined as the ratio of the average shear wave velocity from 20 m and 30 m to the average shear wave velocity in the top 10 m, as an important site parameter that influences site amplification. An empirical site amplification model is developed based on the site response results that predicts amplification as a function of Vs30, Vratio, spectral acceleration on rock, and depth to rock.