Effects of Source RDP Models and Near-source Propagation: Implication for Seismic Yield Estimation

Abstract

Ithas proven difficult to uniquely untangle the source and propagation effects on the observed seismic data from underground nuclear explosions, even when large quantities of near-source, broadband data are available for analysis. This leads to uncertainties in our ability to quantify the nuclear seismic source function and, consequently the accuracy of seismic yield estimates for underground explosions. Extensive deterministic modeling analyses of the seismic data recorded from underground explosions at a variety of test sites have been conducted over the years and the results of these studies suggest that variations in the seismic source characteristics between test sites may be contributing to the observed differences in the magnitude/yield relations applicable at those sites. This contributes to our uncertainty in the determination of seismic yield estimates for explosions at previously uncalibrated test sites. In this paper we review issues involving the relationship of Nevada Test Site (NTS) source scaling laws to those at other sites. The Joint Verification Experiment (JVE) indicates that a magnitude(m b )bias(6m b )exists between the Semipalatinsk test site (STS) in the former Soviet Union (FSU) and the Nevada test site (NTS) in the United States. Generally thisSmbis attributed to differential attenuation in the upper-mantle beneath the two test sites. This assumption results in rather large estimates of yield for largem b tunnel shots at Novaya Zemlya. A re-examination of the US testing experiments suggests that thisSm b bias can partly be explained by anomalous NTS (Pahute) source characteristics. This interpretation is based on the modeling of US events at a number of test sites. Using a modified Haskell source description, we investigated the influence of the source Reduced Displacement Potential (RDP) parameters LKand Bby fitting short-and long-period data simultaneously, including the near-field body and surface waves. In general, estimates of Band Kare based on the initial P-wave pulse, which various numerical analyses show to be least affected by variations in near-source path effects. The corner-frequency parameter Kis 20% lower at NTS (Pahute) than at other sites, implying larger effective source radii. The overshoot parameter Bappears to be low at NTS (although variable) relative to other sites and is probably due to variations in source conditions. For a low B,the near-field data require a higher value of to match the long-period M s and short-periodmbobservations. This flexibility in modeling proves useful in comparing released FSU yields against predictions based onm b andM s .Key wordsRDPNTSscaling relationshipssyntheticsGasbuggyRulison etc.