Each earthquake radiates many waves through the earth. Seismometers located in many parts of the world receive these waves. A spinning body, such as a figure skater or the earth, spins faster if it suddenly becomes more compact about its spin axis. This skater starts a spin with outstretched arms. Then, as she pulls her arms in near her spin axis, she spins so fast she becomes a blur.
Gravity tries to make the earth as compact and round as possible.
Earthquakes cause the earth to become more compact and spin slightly faster. Knowing the precise time of arrival and the time of an earthquake, each wave's average velocity along a specific path can be calculated. After many earthquakes and knowing the average velocity along tens of thousands of different paths, a computer can estimate the wave speed at every point inside the earth. Higher than normal speed implies either colder or denser rock at that point. Earthquake waves travel faster under continents.