ABSTRACT

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 (fpeak) is inversely related to the depth to bedrock. In southern Ontario and many other parts of eastern North America, fpeak is the most important site‐response parameter. In this study, we measure the spatial variability of fpeak within a 1 km distance of 10 seismograph stations in southern Ontario. Establishing spatial variability of fpeak is useful for assessment of regional site response and its uncertainty; we can estimate site response if fpeak is known, and thus it is important to know to what distances the value of fpeak is spatially stable.

Using ambient noise records, fpeak is measured from horizontal‐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, log10(fpeak) 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.

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