On the time-stability of resonance frequencies in deep basins

Determining the resonance frequencies of sediment-filled basins is important in seismic site effects assessment and to infer information about the geometrical and mechanical properties of the basins. Being intrinsic properties of elastic bodies, resonance frequencies are not expected to change over time, at least in the short term and under small excitations, in this type of basins. By analysing multi-annual time-series at some seismic stations located on markedly alpine and subalpine 2-D basins, we first state under what type of exciting function (ambient noise) these resonances can be identified and with what uncertainty. The analysis will reveal a clear annual and daily oscillation of the resonance frequencies, increasing in the summertime and at daytime (i.e. directly correlated with temperature). We attempt to provide different explanations to this not yet so systematically documented experimental evidence. A clear and unique answer is yet to come.