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1. Structure
Monitoring of seismic waves has given insights into the Earth's internal structure
The main evidence for the layered internal structure of the Earth comes from the seismic waves generated by earthquakes. Two types of wave are generated: 'compressional waves' (which move back and forth in the direction of travel); and 'shear waves' (which move at right angles to the direction of travel). Laboratory experiments show that shear waves cannot pass through liquids, in contrast to compressional waves. Seismic stations on the Earth's surface can record the waves generated from earthquakes anywhere in the world. Analysis of these waves shows that those which originate from the opposite side of the Earth and travel through it, are compressional (not shear) waves. Hence at least part of the Earth's core must be liquid, probably molten metal. The mantle, which surrounds the core, transmits both compressional and shear waves and is therefore solid.
What are the characteristics of the two types of seismic waves generated by earthquakes?