Understanding the Bias in Ms and MsVmax Magnitude Estimates using Regional and Upper-Mantle Synthetic Seismograms
Abstract
This paper lays a theoretical framework towards explaining the bias observed in Ms and MsVmax magnitudes measured from the regional, upper-mantle and teleseismic seismograms. Problems with the magnitude estimates are the influence of interfering non-dispersive surface waves at a given period, the effects caused by attenuation, and the presence of spectral holes, which are both depth and velocity structure dependent. To quantify these effects, we focused on tracking both Ms at 20s period and MsVmax (Russell, 2006), the later estimated using the largest amplitude at a period between 8 and 25s after correcting for the interfering signals. To this end, we collected waveforms at distances up to 100deg, applied the published magnitude formulae to a global data sets consisting of earthquakes and explosions. Our empirical study suggests that the bias in the magnitude estimates are distance and period dependent. Analogous to the empirical analysis, we generated both the frequency-wavenumber and mode summation synthetic seismograms up to 50deg for all kinds of seismic sources: pure explosions, mixed explosions comprising of multiple mechanisms, fundamental faults with sources embedded at various depths. We used synthetic seismograms from both elastic and anelastic crustal models representing the geologic environments of the western and eastern United States, and mixed paths that include both oceanic and continental paths. The synthetic study suggests that magnitude is influenced by the depth and focal mechanisms; especially for the earthquake sources making magnitudes dependent on the formula's ability to correct for the distance, attenuation and propagation effects. The synthetic results were used to explain the magnitude bias in the observed data sets. A promising note of this investigation is that the mb:MsVmax may be a reasonable event discriminant at a regional distance similar to its counterpart mb:Ms discriminant at teleseismic distances.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.S23A2539S
- Keywords:
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- 7219 SEISMOLOGY / Seismic monitoring and test-ban treaty verification;
- 7255 SEISMOLOGY / Surface waves and free oscillations