, Riahi 2015, Inbal et al., 2018, Mi et al., 2022). Interestingly, the three spectral peaks, which presumably originated from a common source (or sources), do not overlap.  Similar behavior had previously been observed for ground motion modulated by remote wind-turbine activity \citep{Inbal2018}, and may be the result of variability of the thicknesses of the upper layer encountered along the path to the stations.  To qualitatively assess these effects, I have used a discrete wavenumber reflectivity method \citep{Cotton1997} to calculate synthetic spectra excited by a vertical force acting on the surface of a plane-layered model consisting of a low-velocity thin layer overlaying a homogeneous uniform elastic half-space.  The spectra computed for upper-layer thicknesses of 10 and 100 m (Figure \ref{FIG_SYN_SPEC} ; see table S1 for the layer properties), consistent with the width of the upper soft sedimentary layer in the study area \citep{Gardosh2011}, peak at frequencies close to the ones observed to peak in the IS data.  Other factors that can affect the spectra include the spatial distribution of sources, whose extent is close to the aperture of the IS stations, the local topography, and the depth to the water table.