HR: 1330h
AN: S32B-0846    [PDF]
TI: Rg-Lg coupling as a Lg-wave excitation mechanism
AU: * Ge, Z
EM: ZGE@es.ucsc.edu
AF: University of California, Santa Cruz, Earth Sciences, 1156 High Street, Santa Cruz, CA 95064 United States
AU: Xie, X
EM: xie@es.ucsc.edu
AF: University of California, Santa Cruz, Earth Sciences, 1156 High Street, Santa Cruz, CA 95064 United States
AB: Regional phase Lg is predominantly comprised of shear wave energy trapped in the crust. Explosion sources are expected to be less efficient for excitation of Lg phases than earthquakes to the extent that the source can be approximated as isotropic. Shallow explosions generate relatively large surface wave Rg compared to deeper earthquakes, and Rg is readily disrupted by crustal heterogeneity. Rg energy may thus scatter into trapped crustal S-waves near the source region and contribute to low-frequency Lg wave. In this study, a finite-difference modeling plus the slowness analysis are used for investigating the above mentioned Lg-wave excitation mechanism. The method allows us to investigate near source energy partitioning in multiple domains including frequency, slowness and time. The main advantage of this method is that it can be applied at close range, before Lg is actually formed, which allows us to use very fine near source velocity model to simulate the energy partitioning process. We use a layered velocity structure as the background model and add small near source random velocity patches to the model to generate the Rg to Lg coupling. Two types of simulations are conducted, (1) a fixed shallow explosion source vs. randomness at different depths and (2) a fixed shallow randomness vs. explosion sources at different depths. The results show apparent couplings between the Rg and Lg waves at lower frequencies (0.3-1.5 Hz). A shallow source combined with shallow randomness generates the maximum Lg-wave, which is consistent with the Rg energy distribution of a shallow explosion source. The Rg energy and excited Lg energy show a near linear relationship. The numerical simulation and slowness analysis suggest that the Rg to Lg coupling is an effective excitation mechanism for low frequency Lg-waves from a shallow explosion source.
DE: 0902 Computational methods, seismic
DE: 1734 Seismology
SC: Seismology [S]
MN: 2003 Fall Meeting