Understanding nonlinear ground response using air-to-ground wave interactions from explosions; an example from Mt. Etna, Sicily.

The near-surface can exhibit complex, nonlinear behavior when seismic wavefields interact with unconsolidated materials. Traditional linear site-effect models often fail to explain amplitude-dependent ground response, highlighting the need to resolve the physical mechanisms that control nonlinear processes. Improving this understanding is essential for predicting near-surface behavior during strong ground motions and other seismo-acoustic sources.

Here, we investigate the mechanism of nonlinear ground response using volcanic explosions at Mt. Etna (Sicily) as a natural laboratory. We deployed a multi-parameter network near the summit craters, consisting of broadband seismometers, infrasound sensors, and a buried fiber-optic cable at 30 cm depth for distributed dynamic strain sensing (DDSS). The observatiosn show how aereal explosion waves from Etna’s main vents couple into shallow, unconsolidated scoria deposits. The coupling generates a characteristic ground response signal marked by an amplification of emergent high-frequency energy (10–50 Hz) embedded by the predominantly low-frequency (<10 Hz) explosion waves.

To mechanically characterise the near surface under nonlinear excitation, we compiled a catalog of more than 8,000 volcanic explosions. We analise the relationship between peak-to-peak stress-rate amplitudes measured from infrasound recordings of the explosions, and peak-to-peak strain-rate amplitudes of the associated ground response measured with DDSS. This relationship reveals an hyperelastic behavior of the scoria deposits, expressed by three distinct, consecutive elastic stages: (i) semi-linear elasticity, (ii) softening, and (iii) subsequent stiffening.

The resulting hyperelastic curves allow us to estimate key nonlinear elastic parameters, to model the nonlinearity of the scoria using a lattice mesh. Wave-propagation simulations using this constitutive description reproduce the observed ground response at Mt. Etna. We further validate the approach by modeling explosion–ground interactions for events in which nonlinear ground response is not observed, using the same nonlinear material properties. Our results demonstrate that strain-rate measurements can be used to derive nonlinear near-surface properties of complex geomaterials. Such approach enables an improved modeling of ground behavior that cannot be captured by linear site-effect approaches.