Seismic Site Characterization and Region-Specific Seismic Site Parameter Relationships of Essex County, Ontario, Canada

Abstract

We perform a multimethod in situ seismic field campaign to assess variability in local site conditions throughout Essex County, Ontario, Canada. Fundamental peak frequencies (f0HV) determined at 86 sites from microtremor horizontal-to-vertical spectral ratios (MHVSRs) increase southward from Windsor (∼1.7 Hz) to Amherstburg (>4 Hz) and eastward within Amherstburg (up to 17 Hz). We determine similar VS for individual subsurface layers at the 11 array sites from constrained shear wave velocity (VS) depth (z) profiles obtained by joint inversion of the site’s fundamental-mode Rayleigh-wave dispersion curve and f0HV. This indicates that the spatial variation in f0HV is driven primarily by the resonator depth, which shallows southward. We compile our 11 noninvasive VS depth profiles with an additional 86 VS profiles from previous invasive field testing, to develop a VS–depth relationship for Essex County’s postglacial sediments. Use of most other VS–z relationships available in eastern Canada underpredict the fundamental site frequency in Essex County because the average VS typical of those regions is lower. We find that the velocities for Montreal and Charlevoix (Canada) are closest to the velocities of our study area and consistent with Holocene alluvium deposits in the U.S. Geological Survey national crustal model. Compilation of in situ measurements at 172 sites throughout Essex County enables determination of three region-specific predictive relationships of important seismic site characterization metrics that are applied to generate regional seismic microzonation maps.