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 M0 of 25 stronger twentieth-century events in Switzerland from surface-wave amplitude measurements. These M0 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 M0 estimates exist. The surface-wave amplitudes were measured at empirically determined distance varying reference periods TΔ. For amplitudes measured at TΔ, the distance attenuation term of the surface-wave magnitude relation S(Δ) = log (A/T)max + 1.66 log Δ is independent of distance. For log M0 = MS + CE, we get log M0 = S(Δ) + 14.90. Uncertainties of ±0.3 for the 14.90-constant correspond to a factor of 2 M0 uncertainty, which was verified with independent data. Our relation allows fast, direct M0 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 TΔ and actual measurement periods. The resulting magnitudes range from Mw = 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.
Online material: Moment-tensor analysis of 14 recent earthquakes.