Nonlinear ground response of granular materials

Approximately 600 million people live in proximity to active volcanoes,. However, volcanic activity poses substantial risks to nearby populations and infrastructure. Mitigating the associated hazards requires continuous monitoring of volcanic activity, primarily through seismic observations. Yet, interpreting seismic data in complex and heterogeneous volcanic environments demands a thorough understanding of near-surface mechanical properties and ground behaviour. Recent developments in environmental and nonlinear seismology have revealed that Earth's near-surface materials exhibit dynamic, nonlinear behaviours that classical, linear frameworks cannot describe. This offers new perspectives on understanding processes associated with natural hazards. At the same time, emerging sensing technologies, such as distributed fibre-optic systems, are transforming seismic monitoring by enabling dense spatio-temporal and direct measurements of one wavefield gradient (strain), opening new avenues for studying complex geological systems like volcanoes. Integrating nonlinear seismology with advanced sensing technologies provides an opportunity to better characterise the behaviour of in-situ geomaterials under dynamic conditions. A deeper understanding of this behaviour enhances our ability to interpret volcanic processes and improve hazard assessments. This thesis investigates the nonlinear ground response of near-surface granular materials (scoria) at volcanic settings, using Mt. Etna (Sicily) as a study case. The research comprises three complementary studies; the first two published as peer-reviewed journal articles and the third one submitted for publication. The first study is instrumented oriented. It evaluates the technical performance and limitations of Distributed Dynamic Strain Sensing. It demonstrates that combining multiple fibre instances within a single cable enhances sensitivity, while the internal cable structure significantly influences strain transfer, leading to amplitude discrepancies in strain rate of up to 66\% between co-located cables. It further shows that acquisition parameters such as gauge length and sampling rate impose trade-offs between dynamic range and strain rate resolution. These findings establish a methodological framework for optimising DDSS deployments and correcting instrument-related biases, analogous to established calibration protocols for broadband seismometers. The second study is observation-driven using the technical insights from the first study. It explores the physical mechanisms governing nonlinear ground response events triggered by volcanic explosions at Mt. Etna. It shows that the scoria exhibits either linear elastic or hyperelastic behaviour. The case is dependent on the interplay between the pressure rate of volcanic explosions in air and the strain rate measured in the ground. Although slope stability is not immediately threatened under the observed conditions, the results indicate that nonlinear softening of scoria could facilitate landslide initiation along steep volcanic flanks near the material’s friction angle. This study links DDSS-derived wavefield gradients to micromechanical processes in unconsolidated media, underscoring their significance for volcanic hazard assessment. The third study is modelling-based. It extends the findings from the second study through nonlinear elastic lattice modelling combined with wave propagation simulations. By deriving spring constants from the pressure–strain rate relations (established in the second study) it quantifies the nonlinear elastic properties of the scoria and analyses their influence on the seismic wavefield. Incorporating higher-order Hookean terms reveals that granular nonlinearity produces harmonic distortion, fundamental mode amplification, and high-frequency enhancement consistent with ground response events observed in 2018–2019. The modelling further demonstrates that linear site effects do not explain the observed ground response, confirming that nonlinear elasticity is essential to reproduce the field data. This approach establishes a pathway to retrieve nonlinear elastic constants from field observations and integrate them into predictive numerical models. Overall, this thesis demonstrates that nonlinear effects in near-surface granular materials can be observed and quantified using fibre-optic sensing. It marks a significant step toward incorporating nonlinear mechanics into applied seismology and hazard monitoring. By bridging micromechanical processes with field-scale geophysical observations, this work advances both the scientific understanding and the practical monitoring of dynamic Earth systems.