High-velocity surface layer effects on rayleigh waves: Recommendations for improved shear-wave velocity modeling

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 (V_S) are derived from Rayleigh-wave signals. Although the current MASW practice is to neglect the effect of a high-velocity road layer on soil V_S 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 V_S measurements. Our modeling indicates that a road layer attenuates Rayleigh-wave signals across all frequencies, introduces coherent higher-mode signals, and leads to overestimated V_S and V_S30 values. We show that V_S30 can be overestimated by more than 7% when soft soils underlie a rigid road surface. Inaccurate V_S 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 V_S estimate accuracy with depth.