eorþcwacung

Eorþcwacung in Old English means exactly what its Modern English descendant means: eorþe (earth) + cwacung (quaking, trembling). The compound is transparent and has been since Anglo-Saxon times. Most languages build their earthquake words the same way: Latin terrae mōtus (earth movement), Japanese jishin (地震, ground shake), Greek seismós (a shaking). The phenomenon is too visceral for euphemism. The ground moves. The word says so.

The 1755 Lisbon earthquake was the event that changed European thinking about natural disasters. On November 1 — All Saints' Day — a massive earthquake struck Lisbon, followed by a tsunami and fires. Between 30,000 and 50,000 people died. The earthquake destroyed churches full of worshippers on the holiest day of the year. Voltaire wrote a poem about it. Rousseau argued it was caused by urbanization. Kant wrote three essays on the earthquake, contributing to the founding of modern seismology. The Lisbon earthquake did to philosophy what the Newtonian apple did to physics.

The scientific explanation came in stages. By the late nineteenth century, seismographs could record ground motion. In 1906, Harry Fielding Reid proposed the elastic rebound theory after studying the San Francisco earthquake: rocks on either side of a fault accumulate strain over decades or centuries, then snap back to their original shape in seconds, releasing the stored energy as seismic waves. Plate tectonics, accepted in the 1960s, explained why faults exist and where earthquakes occur.

The Richter scale, introduced in 1935 by Charles Richter and Beno Gutenberg at Caltech, gave the public a number to attach to an earthquake's size. The scale is logarithmic: a magnitude 7 earthquake releases about 31 times more energy than a magnitude 6. The Tōhoku earthquake of 2011 was magnitude 9.1. The word earthquake has not needed to change in a thousand years. The science behind it has changed entirely.