Site characterization for urban seismic hazards in lower Manhattan, New York City, from microtremor array analysis

We characterize the shear-wave velocity (Vs) of soil deposits in lower Manhattan at six sites using the SPAC and HVSR microtremor methods. The soil Vs ranges from 140 m/s to 300 m/s over bedrock. We believe that bedrock depth is constrained to ±15% with these microtremor data, but an accurate quantification of bedrock Vs is unattainable because of low spectral coherency at frequencies below 3 Hz. HVSR data corroborate the SPAC microtremor results at sites based on good alignment of theoretical Rayleigh ellipticity peaks, 1D SH amplifications, and peak HVSR. Resonant frequencies vary between 1.4 and 5.5 Hz between the sites from both observed HVSR and predicted 1D SH amplification. Shear-wave velocities to 30 m classify the sites as C, D, or E. This study demonstrates that given relatively shallow bedrock depth the SPAC method can work in extremely urbanized areas.