Preamble

Our knowledge of the Earth's internal structure would almost certainly be very primitive if there were no earthquakes. Explosive sources of elastic waves would have been recognized and it is possible that exploration seismology, directed to the identification of oil-bearing structures in the crust, would have been well developed, but it is likely that deep-Earth seismology would have been rudimentary. The larger nuclear weapons tests generate waves of sufficient amplitudes to be detected at remote stations, but recognition of the test-monitoring possibilities of seismology depended on the fact that the subject was already well developed when nuclear testing began. Even if the weapons-testing agencies had appreciated the seismic detection possibilities, a shroud of secrecy would have ensured that evidence of deep Earth structure emerged only very slowly.

This hypothetical situation emphasizes how completely our detailed knowledge of the deeper parts of the Earth relies on observations of seismic waves. Quantitative instrumental data on teleseismic waves (from distant earthquakes) were first obtained in the late nineteenth century and, as the theory of elastic waves was then already well established, seismology developed rapidly. The use of earthquake-generated waves to study the Earth's interior is a mature and sophisticated science, to which this chapter is necessarily only a brief introduction. Comprehensive treatments are by Bullen and Bolt (1985), Aki and Richards (2002) and Stein and Wysession (2003).

As we now know, the Earth's internal layers, as well as the surface, are close to oblate ellipsoids, symmetrical about the rotational axis.