Insights in nonlinear ground response in volcanic environments from distributed dynamic strain sensing

Volcanic environments are often characterized by frequent explosive activity and complex ground features. Explosions can couple into the ground, triggering ground response (GR) influenced by near-surface properties. While GR resulting from seismic input is well-studied, GR generated by air-to-ground coupling of volcanic explosions remains poorly understood. Investigating this phenomenon is crucial for understanding near-surface material dynamics and improving volcanic hazard assessments. To study explosion-induced GR, a multi-parametric network was deployed near Mt. Etna's summit craters in 2019, where GR had been previously observed. The network includes broadband seismometers, infrasound sensors, and a fibre optic cable for distributed dynamic strain sensing (DDSS). Over 65,000 explosions were recorded, with some triggering high-frequency GR signals (10-50 Hz) in the DDSS data. These high-frequency signals, embedded in low-frequency explosions (0.7-4 Hz), amplify upon coupling into the ground. We also classified the explosions using waveform similarity, and GR signals were analysed using an adapted approach incorporating temporal and spatial dimensions. Strain rate vs. pressure rate relationships derived from classified signals were interpreted in terms of either linear elastic or hyperelastic near-surface behaviour. Despite no clear consensus towards which mechanical model describes best the ground behaviour, we suggest a nonlinear site amplification driven by mechanical particle interactions rather than near-surface layer resonance.