ALBERT

Description: A critical component in the interpretation of earthquake ground motions is the role that site effects play. In eastern Canada, the sediment layers which overlie glaciated bedrock produce strong and highly variable site responses. We use horizontal-to-vertical (H/V) response spectral ratios as the indicator variable by which to characterize the salient characteristics of site response. We show that site response can be modeled using two key descriptive variables that are readily obtainable: (1) peak frequency ( f peak ), as determined from H/V or sediment depth; and (2) overall sediment type (or stiffness). We use these variables to create a preliminary model of site amplification that can be used in the development of ground-motion prediction equations (GMPEs) and in regional-scale ShakeMap-type applications. The key to the site characterization is the relationship between f peak and sediment thickness (depth-to-bedrock), which we derive using H/V data from earthquakes in the region, combined with compiled geophysical logs and a digital sediment thickness map from the Ontario Geological Survey (OGS). The OGS map also provides information on surficial sediment type, which is correlated with peak amplitudes ( A peak ) of response. H/V spectral shapes may be associated with four main site categories, which in decreasing order of stiffness are: bedrock, till, sand/clay, and organic sediment/fill. The peak amplitudes of response are generally shown to increase as stiffness decreases, at least for the weak motions that are the focus of this study. We model site response by defining a generic site amplification curve, which is dependent only on f peak and site category. These site amplification curves can be applied in the development of regional GMPEs, and in the construction of near-real-time ShakeMaps.