CODA OF LONG-RANGE ARRIVALS FROM NUCLEAR EXPLOSIONS

Abstract

Short-period, three-component recordings from peaceful nuclear explo-sions (PNE) of the profile QUARTZ, located in Russia, are used to constrain the nature of the coda of PNE arrivals. In particular, we examine the unusually strong and extensive coda of the long-range P n (interpreted as a whispering-gallery, WG) phase propagating to beyond 3000 km. Energy-balance considerations in three dimensions show that such an extensive coda is inherent not only to WG but to all other P-wave phases and can be explained by crustal scattering. The long coda is a result of ex-citation of short-period scattered waves (Pg, Sg, Lg, Rg) within the crust by the waves incident from the mantle, or, conversely, by generation of mantle phases from crustal guided waves within the source region. The resulting estimates of coda Q range between Q 380 near 2 Hz and Q 430 near 5 Hz and can be associated with crustal attenuation including the sediments. Our coda model also explains quantita-tively the observed build-up of the arrival amplitude with time and the apparent lack of a pronounced coda of the body-wave arrivals from the mantle transition zone. These effects result from adding up the energy of the later arrivals arriving during the codas of earlier arrivals. We "deconvolve" the overlapping coda patterns and show that the true relative energies of the arrivals are significantly lower than the apparent energies measured from the raw records. A whispering-gallery interpreta-tion of the long-range P n and crustal scattering accounts for the entire range of observations of kinematic, spectral, and amplitude pattern of the PNE wavefield and allows the derivation of constraints on attenuation.