Earthquakes are geological events caused by rocks beneath Earth's surface becoming overwhelmed by mechanical stress and moving suddenly.
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At these locations, plates slide past one another, deforming rock through friction as energy builds like a stretched rubber band. Volcanic eruptions, the collapse of subterranean caverns, or underground nuclear explosions can also cause earthquakes.
Because primary waves are faster than secondary waves, seismic stations will detect longer gaps between these waves’ arrivals the farther an earthquake is. Calculating the distance to the earthquake from three stations allows its origin to be triangulated.
Invented in China in 132 CE, the seismoscope was an urn with dragons facing different directions. If an earthquake occurred, the dragon facing the direction of the tremor released its ball. Like modern seismographs, its mechanism may have involved a suspended weight.
In 1960, a magnitude 9.5 earthquake in Chile became the largest in recorded history, leaving about 2 million people homeless. Two of the largest occurred in back-to-back years in Alaska, with 9.2 and 8.7 quakes each generating tsunamis.
The scale, designed to rate the energy of the largest seismic waves released by an earthquake, was calibrated to the region’s crust and mantle properties. Since the 1970s, seismologists have instead used the moment magnitude scale, which considers all waves of released energy.
Since Richter and moment magnitude scales rank earthquake energies logarithmically, theoretical magnitude values can cover an enormous range with no upper or lower mathematical limit. A particle of dust drifting onto a table would have a magnitude of about -15.
The severity of earthquakes depends on factors beyond the energy released in a region, including local architecture and geological conditions, such as water-saturated soil behaving like quicksand. Secondary effects, such as landslides, floods, and fires, may also contribute.
Different rock types can withstand different amounts of stress before breaking and melt at different temperatures within Earth’s interior, preventing uniform modeling. The planet’s uneven mass distribution also creates variations in gravitational force, complicating mantle movement.
Primary (P) and secondary (S) waves are compression and shear waves, respectively, and travel through Earth’s interior, though secondary waves only move through solids. These waves’ energy is converted into rolling Rayleigh or horizontal Love waves at the surface.
Changes in temperature, density, and composition alter the path of primary and secondary waves, impacting where they are detected along Earth’s surface. These deflections create shadow zones 104 degrees to 140 degrees away from the epicenter, where no waves are directly detected.
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