Shear-Wave Velocity Characterization of the Atlantic Coastal Plain Sediments via HVSR Inversion and Ambient Noise Dispersion

Accurate shear-wave velocity (Vs) structure in the Atlantic Coastal Plain (ACP) sediments is essential for evaluating site response and seismic hazard across the central and eastern United States (CEUS). The thick, unconsolidated ACP sediments strongly influence seismic wave amplification, shaking duration, and resonance. However, Vs models in this region remain under-constrained due to sparse borehole data, poor resolution at depth, and limitations in surface-based passive seismic methods. To address these issues, we implement a systematic horizontal-to-vertical spectral ratio (HVSR) inversion framework across 69 broadband stations of the Southeastern Suture of the Appalachian Margin Experiment (SESAME) distributed across the ACP of Georgia. Hourly HVSR curves are computed from three-component ambient noise records. These are subjected to multi-criteria quality control incorporating misfit-based filtering, amplitude clipping thresholds, and unsupervised machine learning models (Local Outlier Factor, Isolation Forest) to isolate stable high-fidelity observations. The resulting daily HVSR curves are then averaged and inverted using a three-stage non-linear optimization procedure: 1) initial parameter space sampling via Markov Chain Monte Carlo (MCMC), 2) gradient-free local minimization using the downhill simplex method, and 3) a final MCMC refinement. Resulting 1D Vs profiles resolve sedimentary thickness and discontinuities across the SESAME array. To complement HVSR constraints, we incorporate frequency-dependent ambient noise Rayleigh wave ellipticity for both fundamental and higher modes. Due to limitations imposed by station spacing (spatial aliasing), high-frequency multimode phase velocities (≥5 Hz) are poorly resolved. Instead, we extract low-frequency (≤0.5 Hz) fundamental-mode phase and group velocity dispersion from ambient noise cross-correlations to constrain deeper basin structure and upper crustal layering. Our integrated approach yields laterally consistent velocity models that improve regional characterization of sedimentary structure and seismic site conditions in the ACP. These results enhance the predictive capability of ground motion models and inform the development of site-specific amplification factors for CEUS seismic hazard assessments.