Preliminary results on Seismic Site Response Assessment in Presence of a Lava Tube: the Lamponi grotto study case, Etna volcano

Seismic site response describes how ground motion changes as seismic waves pass through different types of soil, rock, or structures. This study aims to enhance understanding of local seismic response in areas with cavities, particularly in lava tubes. The seismic response in lava tubes is unique and depends on factors such as the geological characteristics of the tube, its size, and its interaction with surrounding materials. Lava tubes, typically cylindrical or tunnel-shaped, have dimensions - such as length, diameter, and wall thickness - that influence how seismic waves travel through and around them.
The study focuses on the Lamponi grotto, located on the northern side of Mount Etna. The grotto runs in a northeast-southwest direction, extending roughly 600-700 meters. It is divided into two sections: an upstream section, which is about 300-400 meters long and better preserved, and a downstream section, which is shorter and shows signs of roof collapse. 
A topographic survey of the lava tube was conducted using a LiDAR system to create a detailed 3D model of both the interior and exterior of the grotto. This high-resolution point cloud data was analyzed to measure the roof thickness, which will be crucial for future numerical modeling and structural analysis.
For seismic site characterization, single-station noise measurements were taken at 50 locations on top of the grotto, with a reference station inside the lava tube. The data were processed using standard spectral ratio methods (comparing outcave vs. incave), vertical-to-horizontal spectral ratios, and Fast Fourier Transform (FFT). Preliminary results indicated strong vertical amplification above the lava tube at frequencies greater than 10 Hz.