Numerical Studies of Propagation of Lg Waves Across Ocean Continent Boundaries Using the Representation Theorem

Citation

Regan, Janice (1987) Numerical Studies of Propagation of Lg Waves Across Ocean Continent Boundaries Using the Representation Theorem. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/vc5q-tp40. https://resolver.caltech.edu/CaltechTHESIS:10232019-124415442

Abstract

The methods for Representation Theorem (RT) coupling of finite element (FE) or finite difference calculations and Harkrider's (Harkrider 1964, 1970) propagator matrix method calculations to produce a hybrid method for propagation of SH mode sum seismograms across paths that contain regions of non plane-layered structure are explained and developed. The coupling methods explained in detail use a 2-D Cartesian FE formulation. Analogous methods for the 3-D method follow directly. Extensive tests illustrating the validity and accuracy of the implementation of these coupling methods are discussed. These hybrid techniques are developed to study the propagation of surface waves across regional transition zones or other heterogeneities that exist in part of a longer, mostly plane-layered, path. The effects of a thinning or thickening of the crustal layer on the propagation of L_g mode sum seismograms have been examined in this study. The thinning or thickening of the crustal layer is used as a simple model of ocean continent transitions. The L_g phase is of particular interest since it is used in several important applications such as mapping the extent of continental crust, magnitude determination, and discrimination between explosive and earthquake sources. The understanding of the observations that L_g wave is attenuated completely when the propagation path includes an oceanic portion of length greater than one hundred to two hundred kilometers or a region of complex crustal structure is not complete, and a clear explanation of these phenomena could have important consequences for all these types of studies. The transition model calculations done in this study show that passage through a region of thinning crustal thickness, the model for a continent to ocean transition, increases the amplitude and coda length of the L_g wave at the surface, and allows much of the modal energy trapped in the crust, which forms the L_g phase, to escape in to the subcrustal layers as body waves or other downgoing phases. The magnitude of both these effects increases as the length of the transition increases or the slope of the layer boundaries decrease. The passage of the wavefront exiting the continent to ocean transition region through the oceanic structure allows further energy to escape from the crustal layer, and produces a decrease in L_g amplitude at the surface as the length of the oceanic path increases. The amplitude decrease is maximum near the transition region and decreases with distance from it. Passage through a region of thickening crust, the model of a ocean to continent transition, causes a rapid decrease in the L_g amplitude at the surface of the crust. The energy previously trapped in the oceanic crustal layer spreads throughout the thickening crustal layer, and any amplitude which has been traveling through the subcrustal layer but has not reached depths below the base of the continental crust is transmitted back into the continental crust. The attenuation of L_g at the crustal surface along a partially oceanic path occurs in the oceanic structure and in the ocean to continent transition region . The attenuation at the surface depends in part on the escape of energy at depth through the continent to ocean transition region into the underlaying half-space. The total attenuation of Lg due to propagation through a forward transition followed by a reverse transition is at most a factor of four to six. This is inadequate to explain the observed attenuation of Lg. Thus, additional effects, other than geometry must be considered to provide a complete explanation of the attenuation of Lg.

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