Attenuation of seismic shear wave energy in Switzerland

P>Modelling the attenuation of shear wave energy is an important component of seismic hazard analysis. Previous studies have shown how attenuation, particularly in the uppermost layers of the crust, is regionally dependent. The impact of this is that the decay of energy radiating from an earthquake will vary from place to place. To quantify the regional attenuation in Switzerland we model the Fourier spectral amplitude of small-to-moderate earthquakes, recorded on the local seismic networks. High-frequency decay is parametrized by Q and kappa, while apparent geometrical spreading models account for the frequency-independent decay of energy. We analyse ground motion encompassing the significant duration of shaking to provide models that are useful for the purpose of seismic hazard analysis. Two methods are used to estimate the whole path attenuation parameter, t*: first, a simultaneous fit of the source model and attenuation effects across the entire spectral bandwidth for earthquakes with M > 2; and secondly, a linear fit of an attenuation model to the high-frequency part of the spectrum for earthquakes with M > 3.5. The t* parameter is found to vary with hypocentral distance consistent with a weakly attenuating crust and strongly attenuating uppermost layer. 1-D tomographic inversions indicate a profile of increasing Q with depth down to the Moho. Frequency-independent decay is parametrized using a three-part model which allows for the inclusion of Moho reflection phases in the spectrum in the range of 20-140 km in the Swiss Foreland and from 70 to 140 km in the Swiss Alps.