Regional-scale site characterization mapping in high impedance environments using soil fundamental resonance (f0): New England, USA

Abstract

In this work, we develop regional-scale site characterization maps of New England, a glaciated region in the Eastern United States, using site fundamental frequency (f0). Due to the strong impedance contrast that creates strongly resonant site response behavior in New England, f0 is the preferred site response proxy for the region and best characterizes spatial variability in soil amplification. We first develop a database of 1577 f0 values collected from the literature (1313) and picked from HVSR curves collected during an additional field campaign (487). Using the surficial geologic units from the conterminous US surficial geology map, we compute distributions of f0 for each of the surficial geologic units mapped in New England. We find that the thick glaciofluvial ice-contact sediments and thick proglacial sediments of Cape Cod and Long Island are characterized by f0 distributions with the lowest medians (1.06 and 1.03 Hz respectively) and narrowest interquartile ranges (0.23 and 0.28 Hz respectively) in New England, which we interpret as being the thickest sediments in the region. The f0 distribution of the thin, proglacial sediments, mostly fine-grained in the Boston Basin, the coast of Lake Champlain and the Maine coast has a relatively low median (2.70 Hz), however it has high variability (3.65 Hz interquartile range) since the sediment thickness varies widely in this geologic unit. The f0 distribution of the thin alluvial sediments of the Connecticut River Valley also has a low f0 median (1.83 Hz) however, it has less variability than the proglacial sediments, mostly fine-grained thin (1.59 Hz). We present maps of the median and interquartile range of the f0 distributions and their corresponding Vs30 distribution approximations. Vs30 distributions are developed for each of the surficial geologic units by assuming a layer-over-halfspace model for site response and establishing estimates of sediment average shear-wave velocity (Vsavg) for each of the units based on a limited number of shear wave velocity profiles from the region. We compare our results against two existing site characterization maps for the region, Wald and Allen (2007) and Becker et al. (2011) and find that our updated maps result in median Vs30 values that tend to be higher than Becker et al. (2011) except in geologies with consistently deep profiles, and lower than Wald and Allen (2007) median values, with the exception of coastal zone sediments.