Assessment of the Spatial Variability of Site Response in Southern Ontario

In southern Ontario, Canada, site amplification plays a major role in the interpretation of ground motions and felt shaking during earthquakes. Sediments in the region vary from soft to stiff and overlie very hard glaciated bedrock, providing a strong impedance contrast that results in pronounced amplification of horizontal-component motions. Generally, the peak frequency of amplification (f peak) is inversely related to the depth to bed- rock. In southern Ontario and many other parts of eastern North America, f peak is the most important site-response param- eter. In this study, we measure the spatial variability of f peak within a 1 km distance of 10 seismograph stations in southern Ontario. Establishing spatial variability of f peak is useful for as- sessment of regional site response and its uncertainty; we can estimate site response if f peak is known, and thus it is important to know to what distances the value of f peak is spatially stable. Using ambient noise records, f peak is measured from horizon- tal-to-vertical (H/V) spectral ratios at a number of locations at increasing distance from each seismograph site. Ambient noise H/V amplification ratios are compared with those obtained using weak-motion earthquake recordings at the station. We find that, on average, log10f peak as measured by microtremor survey observations is within 0.13 units of the value recorded at the seismograph station (using H/V from earthquake records) for survey sites within 1 km of the station.