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Spectral Shear-Wave Ground-Motion Scaling in Switzerland

ETH Zurich
Institute for Geophysics
8093 Zurich, Switzerland
bay{at}seismo.ifg.ethz.ch
Istituto Nazionale di Geofisica e Vulcanologia
00143 Rome, Italy
(L.M.)

Manuscript received 25 August 2001.

Three-component recordings of local and regional earthquakes and explosions are used to assess the spectral characteristics of attenuation, excitation, and duration of ground motion in Switzerland. The data set consists of 292 events in the magnitude range from 2.0 to 5.2, with a total of 2958 waveforms recorded in Switzerland and the German border region from 1984 to 2000. Distance ranges from 5 to 350 km. Stations are located on National Earthquake Hazard Reduction Program 1994 site classes A and B outcrops. Empirical excitation, site, and attenuation terms are derived for the Fourier spectra and peak ground velocities of the ground motion in the frequency range 1-15 Hz by applying an iterative damped least-squares regression. These results are used to calibrate effective theoretical attenuation and excitation models. A grid search through the parameter space is then applied to obtain the quality factor Q(f) and a piecewise linear geometrical spreading function G(r), allowing complex behavior of attenuation. Optimum results are obtained for Q(f) = 270 f^0.50, G(r) = r^-1.1 for 0-50 km, r^-0.6 for 50-70 km, r^+0.2 for 70-100 km, and r^-0.5 for distances greater than 100 km. The increased amplitudes in the distance range 70-100 km can be explained by the reflection of shear waves at the Moho. This reflected wave energy also leads to an increased duration of ground motion for the same range. The inverted excitation terms are modeled, based on Brune's source spectrum, applying an effective stress parameter of 5-10 bars and a high-frequency roll off (e^-₀f_k) ₀ = 0.015. The small values of ₀ agree with the mean site condition. A regional site difference between the Alps and the Alpine Foreland site is obtained with a factor of two higher amplitudes in the latter.

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