A giant earthquake can make the whole Earth ring like a bell for weeks
The biggest quakes do more than shake the ground. They set the entire planet vibrating in tones with periods of up to an hour, and that ringing can be detected for as long as a month. Those vibrations have become some of the sharpest tools for probing Earth's deep interior.
Seismology, from the Greek seismos for earthquake, studies quakes and the elastic waves that travel through planets. Curiosity is ancient: Thales, Anaximenes and Aristotle all guessed at natural causes, and in 132 CE Zhang Heng of Han China built the first known seismoscope. Later theories blamed underground fire, as Athanasius Kircher argued in the 17th century, or chemical explosions. The 1755 Lisbon earthquake spurred serious study, and in 1761 John Michell concluded that quakes were waves set off by rock masses shifting miles below the surface.
Instruments and ideas then came quickly. Robert Mallet, often called the father of the field, coined the word seismology and from 1857 ran experiments with explosives. In 1889 a quake in Japan was picked up in Potsdam, Germany, the first long-distance recording. Fusakichi Omori showed in 1894 that aftershocks die away in a regular pattern after the main shock. Harry Fielding Reid's elastic rebound theory, drawn from the 1906 San Francisco earthquake, still underpins tectonic studies, and by the 1960s plate tectonics tied everything together.
Waves exposed Earth's anatomy. Richard Dixon Oldham separated the arrivals of different wave types in 1906 and found the first clear evidence of a core. Andrija Mohorovičić identified the boundary between crust and mantle in 1909, now called the Moho. Harold Jeffreys argued in 1926 that the core is liquid, and in 1937 Inge Lehmann showed that a solid inner core sits inside it.
The key lies in how waves behave. P waves compress and stretch the rock and always arrive first; S waves shear it sideways, travel more slowly and cannot pass through liquid at all. Surface waves, of the Rayleigh and Love types, are slower still, but they lose energy more gradually with distance and usually do the worst shaking. Controlled explosions put the same physics to work mapping oil traps, faults and buried craters, including Chicxulub, the impact scar tied to the dinosaurs' extinction.
Source: Seismology
