A numerical method is proposed to study the scattering of seismic shear waves induced by the presence of underground cavities in homogeneous soils. The method is based on the superposition of two solutions: the solution of the free-wave propagation problem in a uniform half-space, easily determined analytically, and the solution of the wave scattering problem due to the cave presence, evaluated numerically by means of an ad hoc code implemented by using the ANSYS Parametric Design Language. In the two-dimensional setting, this technique is applied to the case of a single cave, placed at a certain depth from the ground level. The frequency spectrum of the seismic shear oscillation on the ground surface is determined for different dimensions and depths of the cave and compared with the spectrum registered without caves. The influence of the cave dimensions and depth on the spectrum amplification is analyzed and discussed. 1. Introduction In the last decades a growing attention has been paid to the assessment of seismic vulnerability of existing ancient buildings and to the design of safe structures for seismic loadings. To this extent, a large variety of technical codes for designers and engineers have been developed, based on dynamical analysis or equivalent statical calculations. In all of them, crucial data to be taken into account is the frequency spectrum of the seismic oscillation registered at the ground level. The frequency content of a seismic signal depends on many factors such as the topography of the considered zone, the characteristics of the materials which the soil is made of, and the soil stratigraphy. These factors could cause amplifications and increase the seismic hazard for structures built on the nearby ground surface. Some recent studies [1–8] have pointed out that natural or man-made underground cavities could also be a source of seismic hazards for structures. Earthquakes in a calcareous area carved by underground karstic process, like grottoes, caves, and dolines, can cause the falling in or collapse of the vaults in the former or the removal of debris in the latter. Surface depressions or subsidence of the ground can occur with the resulting danger of collapse for the building above [9]. Some cases of serious damage were found in buildings over man-made cavities after the 1980-earthquake of Atella (Potenza, Italy) [1] and after the same earthquake, the most damaged buildings at Rionero in Vulture were mainly concentrated in areas where there was notable cave density [2]. Gizzi [3] highlight the relationship between the presence
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