Effects of a Descending Lithospheric Slab on Yield Estimates of Aleutian NuclearTests. Three Dimensional Structure of Subducted Lithospheric Slabs Constraints from the Amplitudes and Waveforms of S Waves

摘要

Theoretical amplitudes and travel times were computed using vicinity ray tracingin several different types of descending slab models, including two new models proposed for the Aleutian slab. In agreement with a similary study of focusing and defocusing at the Nevada Test Site, it was found that amplitudes correlate with broad regional anomalies in travel times. For a shallow focus source placed located to correspond to the relative locations of the Amchitka tests, a regional variation in P amplitudes was found that is similar to those found previously. Negative mb residual of -.2 to -.3 units are predicted at due North azimuths in the range of 70 to 90 great circle degrees. Smaller zones of positive mb residual are predicted in the distance range 42 to 53 great circle range at due North azimuth, which is also a zone of P wave multipathing. Multipathing is also predicted PcP over small geographic regions perpendicular to the strike of the slab near 12 great circle degrees. Many experiments were conducted in varying source location within and near slab structure. These demonstrate that multipathing is most easily induced if the source is located close to the high gradient zone defining the top of the slab. S and ScS amplitudes and travel time were also computed in these models, as part of a related study funded by NSF. For earthquakes located within slabs, it was assumed that regions of defocussing correspond to regions of maximum broadening and complexity in S waves. (jhd)