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

Attenuation is a fundamental process of seismic wave propagation, yet its role in site–city
interaction remains poorly constrained and rarely quantified. In particular, un- derstanding
how buildings collectively dissipate seismic energy through scattering and absorption is essential
for assessing earthquake impact in urban areas. 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 lack- ing, leaving open questions about how
such mechanisms manifest in practice.
In this study, we address this gap using the META-FORET experiment, in which a dense
pine forest is considered as a natural analogue of an urban environment. Trees act as distributed
reso- nant scatterers, allowing us to investigate urban-like scattering and attenuation processes
under well-characterized and repeatable conditions. We analyze both ambient noise and active
shot data to extract key ground motion parameters that are directly relevant to seismic hazard
assessment, including horizontal-to-vertical spectral ratios (H/V), spatial variability of ground
motion, wave attenuation and intensity indices. Passive data reveal frequency-dependent
scattering signatures around tree resonances (20 and 50 Hz), includ- ing perturbations of H/V
curves, reduced coherence and absorption.
Active shot analyses further show a systematic reduction of Arias intensity and a strong
increase in Trifunac duration within the forest compared to the open field, especially near
resonance frequencies. These observations indicate that resonant scatterers redistribute seismic
energy, reducing direct-wave amplitudes while enhancing coda wave durations.
This study provides the first experimental quantification of urban-like scattering and
attenuation from real seismic data. By bridging fundamental wave physics and ground motion
indicators, we propose a noise-based technique to characterize seismic wave atten- uation in
urban environments.

Keywords: urban-like scattering, wavefield coherence, absorption, seismic attenuation,
spectral ratios.