A planet-shaking megathrust
On May 22, 1960, at 3:11 PM local time, the ground in southern Chile began to shake. It did not stop for approximately 10 minutes. This was the Great Chilean Earthquake, the most powerful seismic event ever instrumentally recorded. Registering 9.4–9.6 on the moment magnitude scale, its epicenter was located off the coast, with the city of Valdivia being the most affected. The event was a megathrust earthquake, the result of the Nazca Plate releasing immense stress as it subducts beneath the South American Plate.
The rupture zone was enormous, stretching for about 800 to 1,000 kilometers along the Chilean coast, from Arauco to the Chiloé Archipelago. The fault slipped an average of 11 meters, with some sections displacing as much as 30 meters. This immense movement of the seafloor caused extreme ground shaking and permanently altered the landscape. Large areas of the coast, including parts of Valdivia, subsided by up to 2 meters, turning dry land into new wetlands and estuaries overnight. Conversely, some areas, like Isla Guafo, experienced uplift of as much as 3 meters. The raw energy released was so great that it triggered Earth's free oscillations, vibrations of the entire world that were detectable by sensitive instruments for weeks after the main shock, a phenomenon never definitively observed before.