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Seismic Moment from Regional Surface-Wave Amplitudes: Applications to Digital and Analog Seismograms

¹ Institute of Geophysics
ETH-Hönggerberg
CH-8093 Zurich, Switzerland
 (F.B.)

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 T. For amplitudes measured at T, the distance attenuation term of the surface-wave magnitude relation S() = log (A/T)_max + 1.66 log is independent of distance. For log M₀ = M_S + C_E, we get log M₀ = S() + 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 T 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.

Online material: Moment-tensor analysis of 14 recent earthquakes.