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Abstract

Accurate, consistent earthquake size estimates are fundamental for seismic hazard evaluation. In central Europe, seismic activity is low and long-term seismicity, available as intensities from written historical records, has to be included for meaningful assessments. We determined seismic moments M-0 of 25 stronger twentieth-century events in Switzerland from surface-wave amplitude measurements. These M-0 can be used to calibrate intensity-moment relations applicable to preinstrumental data. We derived the amplitude-moment relation using digital data from 18 earthquakes in and near Switzerland where independent M-0 estimates exist. The surface-wave amplitudes were measured at empirically determined distance varying reference periods T-&UDelta;. For amplitudes measured at T-&UDelta;, the distance attenuation term of the surface-wave magnitude relation S(&UDelta;) = log (A/T)(max) + 1.66 log &UDelta; is independent of distance. For log M-0 = Ms + C-E, we get log M-0 = S(&UDelta;) + 14.90. Uncertainties of ± 0.3 for the 14.90-constant correspond to a factor of 2 M-0 uncertainty, which was verified with independent data. Our relation allows fast, direct M-0 determination for current earthquakes, and after recalibration of the constant, the relation can be applied anywhere. We applied our relation to analog seismograms from early-instrumental earthquakes in Switzerland that were collected from several European observatories. Amplitude measurements from scans were performed at large amplifications and corrected for differences between T-&UDelta; and actual measurement periods. The resulting magnitudes range from M-w = 4.6 to 5.8 for the largest earthquake in Switzerland during the twentieth century. Uncertainties for the early-instrumental events are on the order of 0.4 magnitude units.