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Tony Dahlen and Jeroen Tromp have written a treatise, Theoretical Global Seismology, that is authoritative and comprehensive. The subject is developed from the theory of small oscillations in classical mechanics, in this case the free oscillations of the Earth. In other words, the conceptual pathway chosen by the authors is to represent the seismic transient response of the Earth in terms of free oscillations, or standing waves. This is followed by representation in terms of traveling waves, rays, and pulses. These representations taken together give the reader broad insight into global seismology and the methods used to study the structure of the Earth and of earthquake sources.
Freeman Gilbert (Tue,) studied this question.