Improving Ms Estimates by Calibrating Variable-Period Magnitude Scales at Regional Distances
Abstract
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 and/or 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 Ms:Mw 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 m2/s3 and decibels, can be converted to nanometers (nm) and input into the Russell equation for variable-period surface waves.
Document Details
- Document Type
- Technical Report
- Publication Date
- Sep 01, 2008
- Accession Number
- ADA516374
Entities
People
- David G. Harkrider
- Harley M. Benz
- Jessie L. Bonner
- Michael E. Pasyanos
- Robert B. Herrmann