Earthquakes and Seismic Waves
Sudden release of stress along faults generates seismic waves that travel through Earth, measured by seismographs and magnitude scales.
An earthquake is the shaking of the ground caused by the sudden release of accumulated stress in Earth's crust. According to the elastic rebound theory, stress builds up as tectonic plates move past each other along faults; when the stress exceeds the strength of the rock, the fault slips abruptly, releasing stored elastic energy as seismic waves. Most earthquakes occur along plate boundaries, where plates converge, diverge, or slide — the circum-Pacific belt alone hosts about 80% of the world's largest quakes.
The point where slip begins underground is the hypocenter (focus); the point directly above it on the surface is the epicenter. The released energy propagates as seismic waves of three types. P waves (primary) are compressional — they travel fastest, through solids and liquids alike, and arrive first. S waves (secondary) are shear waves that cannot pass through liquids, which is how seismologists know Earth's outer core is molten. Surface waves — Love and Rayleigh waves — travel along the ground, arrive last, and cause most of the damage.
Seismographs record the waves, and the difference in arrival times between P and S waves gives the distance to the epicenter; triangulating from three stations locates it. Magnitude measures the energy released: the Richter local magnitude (1935) was designed for Southern California, while the moment magnitude scale (Mw) is now standard and is open-ended — the 1960 Valdivia earthquake, the largest ever recorded, was Mw 9.5. Intensity scales instead measure shaking and damage at a location, which is why a large distant earthquake can cause little intensity.
Earthquake hazards include ground shaking, surface rupture, liquefaction of waterlogged soils, landslides, and tsunamis when quakes displace the seafloor. Building codes, seismic design — the loads and ductility treated in structural loads and beam bending — and early warning systems reduce risk, but short-term prediction remains impossible; what is known is the long-term probability of quakes on mapped faults.
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earth science earthquakes faults seismology
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