Sensitivity of Seismic Amplification to Shear-Wave Velocity Variability and Lithological Complexity: Insights from the Mt. Etna Volcanic Area (Italy)

Abstract

Seismic microzonation in volcanic environments is strongly affected by epistemic uncertainty due to the marked heterogeneity of volcanic deposits and the complex stratigraphic architecture of the shallow subsurface. In these contexts, simplified parameters such as equivalent shear-wave velocity (V_SE) or VS30 are commonly adopted for seismic site classification, although they often fail to adequately represent the internal variability and impedance contrasts controlling local seismic response. This study investigates the relationship between equivalent shear-wave velocity (V_SE), its associated variability, such as the standard deviation (Sd), and the seismic amplification factor (AF) in the Mt. Etna volcanic area (eastern Sicily, Italy). The analysis is based on ten high-resolution down-hole seismic surveys collected within the framework of the Regional Seismic Microzonation activities of the Mt. Etna area and distributed across different geological sectors of the volcanic edifice, including lava-dominated successions, volcaniclastic deposits, and sedimentary–volcanic transition zones. Shear-wave velocity data were analyzed using statistical indicators and lithological classification based on Engineering Geological Units (EGUs), while amplification factors were estimated through 1D equivalent-linear numerical simulations for different spectral period intervals. The results show that sites characterized by similar V_SE values may exhibit significantly different amplification behavior, demonstrating that V_SE alone is not sufficient to describe the seismic response of heterogeneous volcanic successions. In contrast, the standard deviation of V_S (Sd) represents an effective indicator of internal velocity variability and provides complementary information for interpreting amplification effects.The relationship between Sd and AF is non-linear and strongly dependent on the spectral period range. Moderate heterogeneity tends to enhance amplification through the development of impedance contrasts and resonance effects, whereas highly heterogeneous profiles tend to reduce coherent amplification due to wave scattering and stratigraphic complexity. Distinct heterogeneity thresholds were identified for different VSE classes, beyond which amplification behavior becomes increasingly site-specific and no longer predictable through simplified equivalent-velocity approaches. Although the proposed Sd–AF relationship is specific to the investigated geological setting, the methodology can also be applied to other volcanic and heterogeneous geological environments to better constrain seismic amplification effects.