HR: 1330h
AN: S42E-0202    [PDF]
TI: Seismic Moment From Regional Surface Wave Amplitudes of Recent and Historic Earthquakes
AU: * Bernardi, F
EM: fabrizio@seismo.ifg.ethz.ch
AF: Swiss Seismological Service, Institute of Geophysics, ETH-Zurich, Zurich, 8093 Switzerland
AU: Braunmiller, J
EM: jochen@seismo.ifg.ethz.ch
AF: Swiss Seismological Service, Institute of Geophysics, ETH-Zurich, Zurich, 8093 Switzerland
AU: Giardini, D
EM: giardini@seismo.ifg.ethz.ch
AF: Swiss Seismological Service, Institute of Geophysics, ETH-Zurich, Zurich, 8093 Switzerland
AB: Accurate, consistent earthquake size estimates are important for seismic hazard assessment. In central Europe, seismic activity is low and long-term seismicity, available as intensities from written historical records, has to be incorporated for meaningful assessments. We determined the seismic moment $M_{o}$ of 25 stronger $20^{th}$-century earthquakes in Switzerland from surface wave amplitude measurements; these 25 events link the instrumental with pre-instrumental data and can be used to calibrate intensity-moment relations. We derived our amplitude-moment relation using digital regional data from 18 earthquakes in and near Switzerland for which we had obtained $M_{o}$ estimates from regional moment tensor analysis. The surface wave amplitude measurements were carried out at empirically determined, distance dependent dominant reference periods $T_{\Delta}$. The distance attenuation term of the standard surface wave magnitude ($M_{S}$) relation $S(\Delta)= log\, (A/T)_{max} + 1.66\, log \Delta$ is distance independent for maximum amplitudes measured at periods $T_{\Delta}$. Assuming a linear relation between $log\,M_{o}$ and $M_{S}$, we obtain $log\:M_{o} = S(\Delta) + 21.90$ for the digital data. Uncertainties of $\pm 0.3$ for the 21.90-constant term translate into $M_{o}$ uncertainties of a factor of 2, which were verified with an additional, independent data-set. Our relation allows fast, direct $M_{o}$ estimation for current earthquakes and, after re-calibration of the constant, can be applied in other regions of the World. We applied our relation to analog seismograms of 25 early-instrumental earthquakes in Switzerland. The analog data, recorded on mechanical and electromagnetic seismographs, were collected from several European observatories. Amplitude measurements from scans were performed at large amplifications on a computer screen and corrected for differences between $T_{\Delta}$ and the actual measurement periods. The resulting size estimates range from moment magnitude $M_{w} = 4.5$ to 5.8. The $M_{w} = 5.8$ 1946 event was the largest earthquake in Switzerland during the $20^{th}$-century. Uncertainties for the early-instrumental events are on the order of 0.4 magnitude units.
DE: 1734 Seismology
DE: 7215 Earthquake parameters
DE: 7223 Seismic hazard assessment and prediction
DE: 7255 Surface waves and free oscillations
SC: Seismology [S]
MN: 2003 Fall Meeting