ABSTRACT

Soil stiffness estimates are critical to geologic hazard and risk assessment in urban centers. Multichannel analysis of surface‐wave (MASW) data collection along city streets is now a standard, cost‐effective, and noninvasive soil stiffness approximation tool. With this approach, shear‐wave velocities (VS) are derived from Rayleigh‐wave signals. Although the current MASW practice is to neglect the effect of a high‐velocity road layer on soil VS estimates, our models show measurable impacts on Rayleigh‐wave amplitudes and phase velocities when seismic data are acquired on a road surface. Here, we compare synthetic models with field MASW and downhole VS measurements. Our modeling indicates that a road layer attenuates Rayleigh‐wave signals across all frequencies, introduces coherent higher‐mode signals, and leads to overestimated VS and VS30 values. We show that VS30 can be overestimated by more than 7% when soft soils underlie a rigid road surface. Inaccurate VS estimates can lead to improper soil classification and bias earthquake site‐response estimates. For road‐based MASW data analysis, we recommend incorporating a surface road layer in the Rayleigh‐wave inversion to improve VS estimate accuracy with depth.

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