Experimental characterization of urban-like scattering and attenuation from a dense nodal array: implications for seismic ground motion

Summary Attenuation and scattering play a key role in the propagation of seismic waves through complex media, yet their specific contribution to site–city interactions remains poorly understood. Numerical studies have recently introduced the concepts of urban attenuation and urban mean free path to describe these processes. However, observational evidence based on real data is still lacking. In this study, we experimentally investigate these processes using a forest as a natural analogue of an urban environment, where trees, organized as a resonant metamaterial (the METAFORET experiment), act as distributed resonant scatterers analogous to buildings. Using a dense nodal array that combines ambient noise and active-source measurements, we evaluate several key indicators including Arias duration, H/V spectral ratios, coherence, and spatial variability—alongside parameters describing wave attenuation and scattering. The observations reveal a redistribution of seismic energy, characterized by reduced direct-wave amplitudes and prolonged coda-wave durations. These results provide the first experimental evidence of urban-like attenuation and scattering derived from real seismic data, and they pave the way for a passive, noise-based approach to characterize seismic attenuation effects in urban environments.