Modeling seismic wave propagation using paraxial extrapolators

[摘要] The development of a paraxial extrapolation system to simulate seismic wavefield propagation in complex three-dimensional (3-D) media results in a practicalapproach to address modeling problems that require large computer memory.The paraxial approach applies to wave propagation problems in which most of theenergy is traveling within a restricted angular cone about a principle axis of theproblem. To set up the paraxial system, the equations of motion are initially castas a first-order extrapolation system. Approximating the exact one-way extrapolationoperator for this system with a truncated series expansion yields a sequenceof paraxial extrapolation operators. Using the second-order operator results in aparaxial system which is accurate for propagation angles out to 60° with respect tothe extrapolation axis. The acoustic formulation of this system produces excellentresults as compared to a full wave field calculation. Formulating an appropriatesystem for the elastic case is more difficult due to the coupling between P andS energy. Specifying media variations as small perturbations to a homogeneousreference medium leads to a useful formulation of the paraxial system for the 2-D elasticcase.Using the acoustic system to model path effects for local earthquakes recordedat two southern California stations indicates the strong influence that the 3-Dcrustal basins of this region have on the propagation of seismic energy. Althoughthe simulation tracks only acoustic waves, the method is capable of modeling effectsdue to focusing, diffraction and the generation of multiple reflections andrefractions. The modeling results show that the 3-D structure of the Los Angeles,San Fernando and San Gabriel basins create strong patterns of focusing and defocusingfor propagation paths coming into the stations located at Pasadena (PAS)and the University of Southern California (USC). These simulations compare wellwith earthquake data recorded at both stations.

[发布机构] University:California Institute of Technology;Department:Geological and Planetary Sciences

[关键词] Geology