Shear-Wave Velocity Models of the Subsurface Critical Zone at Mpala, Kenya, from Ambient Noise

Abstract In this study, we develop 1D shallow subsurface velocity profiles from one year of ambient-noise measurements collected at the Mpala Research Centre (Mpala), a wildlife preservation area in Laikipia County, Kenya. Mpala is located in a rangeland shared by wildlife, livestock, and people, and experienced extreme drought in 2023, during which water access depended largely on shallow wells influenced by subsurface hydrogeology. In addition, growing interest in seismic wildlife monitoring relies on interpreting ground-coupled animal vibrations, for which amplitudes and dominant frequencies are affected by local site effects. Both issues highlight the need for information on shallow subsurface structure. Mapping of site-specific shear-wave velocities (VS) provides a common framework to relate stratigraphy to shallow groundwater availability and to correct for spatial variability in amplification that biases wildlife signal detectability. Motivated by these needs, we develop the first locally constrained VS models in Mpala based on 12 months of seismic data recorded by a 15-station broadband seismic array using ambient-noise horizontal-to-vertical spectral ratio (HVSR) and passive seismic interferometry. The HVSR and Rayleigh-wave dispersion measurements from the two methods are jointly inverted. The dispersion curves’ frequency band (≈2–9 Hz) provides depth sensitivity of ∼70 to 430 m, whereas HVSR constrains near-surface impedance contrasts. Across our seismic network, we detect three consistent velocity contrasts in the upper 100 m, first at ∼1 to 4 m (VS∼280 to 500 m/s), second at ∼5 to 20 m (VS∼345 to 900 m/s), and finally at ∼15 to 50 m (VS∼1160 to 2600 m/s). The resulting VS models support well-siting and inform how to account for local site-amplification effects for monitoring and modeling ground-coupled wildlife sensing. For future work, we recommend multiscale seismic array configurations with both locally denser and targeted wider station spacing to reliably estimate shallow depths and the geological structure.