Assessing the impact of noise source directivity and 3D basin structure on ambient seismic H/V ratios using numerical modelling

Ambient seismic noise H/V spectral ratios are widely applied in the assessment of local site effects, particularly in sedimentary basins, yet their reliability can be compromised in the presence of strong lateral heterogeneity and non-uniform noise source distributions. Disentangling these contributions remains a key challenge for the interpretation of H/V measurements in complex geological settings. In this study, we use three-dimensional numerical modelling to investigate how basin structure and ambient noise source characteristics jointly control H/V amplitude and polarization.

The work is motivated by pronounced temporal variability observed in long-term H/V measurements at a station in the Mygdonian sedimentary basin (northern Greece), including strong changes in amplitude and polarization and occasional shifts in the dominant frequency. The station is located above a blind fault that separates a shallow northern basin from a deeper southern basin, making it a suitable test case for studying site effects in a laterally heterogeneous environment. We compute 3D viscoelastic Green’s functions for a simplified yet geologically representative basin model and exploit source–receiver reciprocity to simulate ambient seismic wavefields for a range of spatially variable surface noise source distributions.

Synthetic H/V ratios are analyzed at receivers located on both sides of the fault to evaluate the sensitivity of site-response indicators to structural contrasts and source directivity. The simulations show that lateral heterogeneities associated with basin geometry and faulting significantly affect H/V amplitudes and polarization patterns, with the strongest effects observed near the fault zone and within the deeper basin. Variations in the spatial distribution of noise sources are identified as a first-order control on H/V measurements and can apparent spatial or temporal variations that mimic structural effects. In particular, sources located in the shallow basin preferentially excite surface waves trapped in the upper layers that propagate toward the deeper basin, imprinting the shallow basin signature on deep basin H/V ratios, while the reciprocal effect is not observed.

Polarization analysis reveals systematic differences across the fault, with preferred orientations generally parallel to the fault trace in the deep basin and perpendicular in the shallow basin, reflecting the underlying structural control. However, strongly directional noise sources can partially obscure this signature, underlining the need for caution when interpreting polarization results based on short time windows of H/V ratios. Finally, comparison with elastic Diffuse Field Theory (DFT) shows reasonable agreement near the H/V peak frequency for isotropic source distributions, but significant deviations arise in the presence of attenuation and strong lateral contrasts. These findings demonstrate the importance of 3D numerical simulations in separating the effects of source distribution and basin structure on H/V measurements, and emphasize the benefit of extended, spatially dense ambient noise monitoring in complex geological settings.