A Preliminary Model of Site Amplification for use in Ground-Motion Modeling in Southern Ontario

English Abstract:

A critical component in the understanding and interpretation of earthquake ground motions is the role that site effects play. In southern Ontario, Canada, the soil 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 in southern Ontario. We show that site response can be modeled using two key descriptive variables that are readily obtainable: (i) peak resonant frequency (f_peak), as determined from H/V or soil depth; and (ii) overall soil type (or stiffness). We use these variables to create a preliminary model of site amplification across southern Ontario that can be used in the development of ground-motion prediction equations (GMPEs) and in real-time interactive ground-motion (IGM) map applications.

The key to the site characterization is the relationship between f_peak and drift thickness (depth to bedrock), which we derive using H/V data from earthquakes in the region, combined with a detailed digital drift thickness map available online from the Ontario Geological Survey (OGS). The OGS map also provides information on soil type, which is correlated with peak amplitudes (A_peak) of response. H/V spectral shapes may be associated with four main soil categories, which in decreasing order of stiffness are: bedrock, till, sand/clay, and organic soil/fill. The value of A_peak increases as stiffness decreases. We model site response by defining a generic site amplification curve, which is dependent only on f_peak and soil category. The generic curve enables an estimate of site amplification to be made over the entire frequency band from 0.1 to 20 Hz, knowing just the site depth and type. These site amplification curves can be applied in the development of regional GMPEs, and in the construction of robust near-real-time IGMs.