Ivan Wong of LCI has co-authored a technical paper “Evaluating, kappa, Q(f), and stress parameter in the Southern Rocky Mountains of Colorado” with Bob Darragh and Walt Silva of Pacific Engineering & Analysis. The paper has been published ahead of print and will appear in an upcoming issue of the Bulletin of the Seismological Society of America.
The paper presents the results of an analysis to assess whether ground motion models for the tectonically active western U.S. (WUS) or the more stable central and eastern U.S. (CEUS) should be used for areas in the Rocky Mountains and Colorado Plateau. In the National Seismic Hazard Maps, the USGS uses ground motion models appropriate for both the CEUS and the WUS for the Rocky Mountains and Colorado Plateau which includes much of Colorado and Wyoming, eastern Utah, northeastern Arizona, and northwestern and eastern New Mexico. Geologically and geophysically, much of this region would be considered to be more similar to tectonically active regions to the west. To evaluate the validity of this approach we have analyzed earthquake source (stress parameter), path (frequency-dependent Q), and site parameters (kappa) for 24 earthquakes of M 1.5 to 3.3 that have been recorded at 84 seismic stations in the Southern Rocky Mountains of central Colorado. The parameter values that were computed are more consistent with typical values found in the WUS and not the CEUS. These results suggest that the use of ground motion models for tectonically active regions like the WUS are more appropriate for use in seismic hazard analyses in regions like the Southern Rocky Mountains. Furthermore our results coupled with several other studies would argue that the boundary used by the USGS in the National Seismic Hazard Maps should be located further to the east to at least align with the traditional boundary separating the Rocky Mountains and the Great Plains.