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Improving Ms Estimates by Calibrating Variable-Period Magnitude Scales at Regional Distances
WESTON GEOPHYSICAL CORP LEXINGTON MA
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During the final year of our project, we have calibrated the use of surface-wave magnitudes Ms, measured at regional distances as a rapid and robust estimator of seismic moment. We have used the Russell 2006 variable period surface-wave magnitude formula to convert Ms to seismic-moment magnitude, Mw, at local to regional distances using global datasets. In this pilot study, the Russell Ms technology was applied to 169 North American events with 3.2 Mw 6.5 at distances ranging from 48 to 5,268 km. The technique uses a time-domain magnitude estimation procedure that employs zero-phase Butterworth filters to effectively measure Rayleigh-wave Airy phase amplitudes at local and near-regional distances e.g., 1000 km. This allows for surface wave magnitudes to be estimated within minutes of the initiation of a seismic event. Of the 7,370 event-station pairs, more than half 4,051 of the measurements were at distances 1,000 km. The Ms estimates were regressed against moment magnitudes Mw estimated from P-wave modeling andor Rayleigh- and Love-wave spectral amplitudes Herrmann et al., 2008. Mw can be estimated using the relationship Mw 1.951 0.649 Ms. The observed scatter in the estimated Mw was approximately 0.2 magnitude units. The residuals between true and Ms-predicted Mw have a definable faulting mechanism effect, especially when strike-slip events are compared to those with other mechanisms. Preliminary results suggest that our MsMw relationship for North America is also transportable to the Middle East and the Korean Peninsula. We have also determined a methodology to estimate the Ms detection thresholds for the Russell formula. Broadband noise estimates for any seismic station, which are typically in units of acceleration m2s3 and decibels, can be converted to nanometers nm and input into the Russell equation for variable-period surface waves.
APPROVED FOR PUBLIC RELEASE