Long-period ground-motions for large European earthquakes, 1905-1992, and comparisons with stochastic predictions

Data from European earthquakes in the magnitude range 5 to 8 between 1905 and 1992 have been collected and collated, for distances between 200 and 3400 km. The data include both analog and digital records, with priority on wave paths traversing northern Europe. Historical analog records from Uppsala and De Bilt have been digitised, and appropriate response functions established. New estimates of seismic moments and moment magnitudes have been obtained, which together with moment magnitudes from other sources have been compared to surface wave magnitudes. The location of the largest north European earthquakes substantiate earlier suggestions that rifted regions (passive margins, rifts and grabens) may have the largest seismic potentials. A random-vibration (stochastic) model for prediction of observed peak amplitude, period and Fourier acceleration spectra has been developed and calibrated against intermediate and long-period observations. Reasonably good correspondence between predictions and observations are obtained when using a simple Brune source spectrum, new values for seismic moments and moment-magnitude relations, together with reasonable assumptions for stress drop, geometrical spreading and anelastic attenuation. The model is useful first of all for predicting broad regional averages, but as such it is robust, and it also has the potential to be used in an engineering context for predicting spectral response and peak ground accelerations. Some of the empirical data have also been studied in terms of pseudo-relative spectral velocity and compared to strong-motion response spectral prediction models established for northwestern Europe, again for low frequencies. Irrespective of these prediction models we emphasize, however, that the establishment of the data base itself has been an independent and important purpose of this study.