Near Real Time Estimation of Seismic Event Magnitude and Moment via P and Lg phases
Local magnitude and moment of a seismic event can be estimated in near real time by means of signal detector parameters being defined as rms trace estimates. The advantage of such a signal detector is accurate prediction of maximum signal amplitude via the Random Vibration Theory (RVT) which implies a relationship between the maximum amplitude and the rms value for stationary signals. Although seismic signals are non-stationary, the RVT validity was tested on many seismic signals with excellent results. For moment and magnitude estimation, the geometrical spreading and attenuation effects are accounted for by using empirical correction curves which are often unavailable for P g and P n -phases and sometimes for L g -waves. Using source theory, we have computed such correction curves for different frequency ranges using P and L g propagation parameters as published by Sereno et al. (1988). This novel approach was used for automatic maximum phase amplitude measurements and subsequently event magnitude and moment estimation using 50 local events recorded by NSN (Norwegian Seismological Network). The results here are in very good agreement with reference magnitudes from NORSAR. L g based magnitudes appear more stable than those tied to P g and P n amplitudes. Also, at distances below 100 km P magnitudes are consistently below the corresponding L g magnitudes. This may be explained by too small corrections at short distances as proper spreading and attenuation terms remain problematic at such ranges.
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