HR: 0800h
AN: S31B-1066    [Abstracts]
TI: Explosion Shear Wave Generation and Scattering
AU: * Baker, G E
EM: glenn.e.baker@saic.com
AF: SAIC, 10260 Campus Pt Dr MS-A3, San Diego, CA 92121
AU: Stevens, J L
EM: jeffry.l.stevens@saic.com
AF: SAIC, 10260 Campus Pt Dr MS-A3, San Diego, CA 92121
AU: Xu, H
EM: heming.xu@saic.com
AF: SAIC, 10260 Campus Pt Dr MS-A3, San Diego, CA 92121
AB: We use observations of explosion-generated Lg together with three separate types of numerical models to determine how underground nuclear explosions generate shear wave phases. This question is fundamental to how Lg phases are interpreted for use in explosion yield estimation and earthquake/explosion discrimination. A simple point explosion in a uniform medium generates no shear waves, so the Lg phase is generated entirely by non-spherical components of the source and conversions through reflections and scattering. Our results indicate that the most important sources of high frequency explosion shear waves are P to S conversions at the free surface and S waves generated directly by a realistic distributed explosion source including nonlinear effects due to the free surface and gravity. In addition, Rg scattering may contribute to lower frequency Lg. Near source S is observed on both radial and tangential component records from a diverse set of explosion data. The data sets include 1) Degelen Mountain explosions recorded at distances less than 100 km and corresponding recordings at Borovoye (BOR) at 650 km; 2) recordings from Russian deep seismic sounding experiments; 3) Nevada Test Site (NTS) explosion sources including the Nonproliferation Experiment (NPE) and nuclear tests covering a range of source depths and media properties. We model the overburied NPE, and underburied and overburied Degelen explosions, using point sources and two-dimensional nonlinear finite difference calculations to quantify the source effects. We use energy conservation to determine an upper bound on Rg to Lg scattering. Results indicate that Rg to Lg scattering may be important at frequencies less than 1 Hz, and in Lg coda, but is less than Lg generated directly by the explosion at higher frequencies. We use 2D and 3D finite difference calculations, using the known topography and velocity structure at Degelen Mt. and lateral heterogeneities within the crust, to estimate the effect of surface and crustal scattering on Lg. Results indicate that topographic scattering can significantly disrupt the surface pS scattered phase, so more shear wave energy is trapped in the crust. The topography also appears to be the most significant scatterer of Rg, and has a profound effect on its dispersion.
DE: 7219 Nuclear explosion seismology
DE: 7260 Theory and modeling
DE: 7200 SEISMOLOGY
DE: 7203 Body wave propagation
SC: Seismology [S]
MN: 2004 AGU Fall Meeting