Scattering of long-period Rayleigh waves in Western North America and the interpretation of codaQmeasurements

Abstract

AbstractThe codas of long-period Rayleigh waves recorded at WWSSN and Canadian network stations in Western North America from eight underground explosions at NTS are examined in an effort to separate scattering and anelastic attenuation effects. Coda behavior of 0.1 and 0.2 hz Rayleigh waves follows coda characteristics seen in studies of short-period S waves. Coda decay rate is seen to be a stable observation over most stations in Western North America and is consistent with the hypothesis that backscattered surface waves from heterogeneities contained within the western half of the continent form the Rayleigh wave coda. The basic data observables of coda level and decay are interpreted using several plausible models. The single scattering model yields a coda Q consistent with previously determined Rayleigh anelastic attenuation coefficients. Separation of anelastic and scattering Q is possible using an energy flux model and shows that scattering Q is one to two orders of magnitude higher than anelastic Q. However, an energy flux model that incorporates a layer of scatterers over a homogeneous half-space shows that all Rayleigh-wave attenuation can be explained purely by scattering effects which include Rayleigh- to body-wave conversion. Coda can be fit equally well by these mutually incompatible models. It is not likely that the mechanisms of scattering or anelastic attenuation can be addressed by coda observations of a single homogeneous data set.