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Earthquakes and Seismic Waves Explained

Every year the Earth experiences roughly half a million detectable earthquakes. Most are too faint to feel, but the largest reshape coastlines, collapse cities, and generate tsunamis that cross entire ocean basins. Understanding how earthquakes work — and how scientists measure and locate them — is fundamental to both Earth science and disaster preparedness.

Why Earthquakes Happen: Faults and Elastic Rebound

Earthquakes are caused by the sudden release of energy stored in Earth's crust. That energy accumulates because tectonic plates are in slow, continuous motion. Where plates meet, the rocks on either side of a fault — a fracture in the crust — are locked together by friction. Stress builds as the plates attempt to move past each other but cannot.

Eventually the stress exceeds the strength of the rock, and the fault slips abruptly. The stored elastic energy is released in seconds, radiating outward as seismic waves in all directions. This is elastic rebound theory, developed by geologist Harry Fielding Reid after studying the San Andreas Fault following the 1906 San Francisco earthquake. Rock on either side of the fault snaps back toward the shape it had before the stress accumulated — analogous to a stretched rubber band being released.

Most earthquakes occur at plate boundaries, but they can also happen far from boundaries — intraplate earthquakes — where ancient faults deep in the crust are reactivated by regional stress. The 1811–1812 New Madrid earthquakes in the central United States, which temporarily reversed the flow of the Mississippi River, are a dramatic historical example.

Focus, Epicentre, and Depth

The point inside the Earth where the fault first ruptures — where the energy release originates — is called the focus (or hypocentre). Directly above it on the Earth's surface is the epicentre, which is not where the earthquake "starts" underground but is the surface point closest to the focus and typically where shaking is most intense.

Earthquake depth is significant. Shallow earthquakes (focus within 70 km of the surface) typically cause the most surface damage because the seismic energy has less distance to travel and spread out before reaching buildings and people. Most damaging earthquakes are shallow. Intermediate-depth (70–300 km) and deep earthquakes (over 300 km) occur in subduction zones where cold oceanic crust descends into the mantle; they can be felt over enormous areas but generally cause less damage per unit of energy than shallow quakes.

Seismic Waves: P-waves, S-waves, and Surface Waves

Earthquakes release energy in the form of seismic waves, which travel outward from the focus through the Earth's interior and along its surface. There are three main types, each with distinct properties.

Primary waves (P-waves) are compressional: they push and pull rock particles in the same direction the wave travels, alternately compressing and expanding the material. P-waves are the fastest seismic waves and travel through solids, liquids, and gases. They are typically the first waves recorded on a seismograph after a distant earthquake.

Secondary waves (S-waves) are shear waves: they move rock particles perpendicular to the direction of travel, like a wave along a shaken rope. S-waves travel at roughly 60% of the speed of P-waves and are generally more destructive because the shearing motion is harder for buildings to withstand. Crucially, S-waves cannot travel through liquids — a property that seismologists used in the early twentieth century to infer that Earth's outer core is molten, because S-waves disappear when they reach it.

Surface waves travel along Earth's surface rather than through its interior. They arrive after both P- and S-waves and are the slowest of the three types, but they typically cause the greatest damage in large earthquakes. Two kinds exist: Love waves (side-to-side shaking) and Rayleigh waves (elliptical rolling motion similar to ocean waves). The rolling motion of a Rayleigh wave is what gives large earthquakes their nauseating character.

Measuring Earthquake Size: Magnitude and Intensity

Two different concepts describe how "big" an earthquake is.

Magnitude measures the energy released at the source. The original Richter scale, developed by Charles Richter in 1935, calculated local magnitude from the amplitude of seismic waves recorded on a specific type of seismograph at distances up to about 600 km. It is a logarithmic scale: each whole number increase represents roughly 31.6 times more energy released. A magnitude 7.0 earthquake releases about 31.6 times as much energy as a 6.0, and about 1,000 times as much as a 5.0.

The Richter scale has been largely replaced for large or distant earthquakes by the moment magnitude scale (Mw), which directly measures the physical parameters of the fault rupture — the area of the fault surface that moved, the average displacement, and the rigidity of the rock. The two scales give similar numbers in the middle range but Mw is more reliable for very large earthquakes. The 2011 Tohoku earthquake that caused the Fukushima nuclear disaster measured Mw 9.0.

Intensity, by contrast, measures the effects at a specific location — how much shaking was felt, what damage occurred. The Modified Mercalli Intensity scale runs from I (not felt) through XII (total destruction). The same earthquake produces different intensities at different distances from the epicentre, and local geology matters: soft, water-saturated sediments amplify shaking far more than solid bedrock.

Locating an Earthquake: Triangulation with Seismographs

Because P-waves travel faster than S-waves, the gap in arrival times between the two wave types at a seismograph station reveals how far the station is from the epicentre. The greater the P-S time gap, the more distant the earthquake.

A single station can determine the distance to the epicentre but not the direction. To locate the epicentre precisely, seismologists use data from at least three separate stations. Each station defines a circle on a map (the epicentre is somewhere on that circle). The point where the three circles intersect is the epicentre — this method is called trilateration. With dozens of global seismic monitoring stations now in operation, the epicentre of any significant earthquake anywhere on Earth can be determined within minutes.

Summary

Earthquakes result from the sudden release of elastic strain energy stored along faults in Earth's crust. The focus is the underground rupture point; the epicentre is the point directly above it on the surface. Energy radiates outward as P-waves (compressional, fastest, through any material), S-waves (shear, slower, through solids only), and surface waves (slowest, most destructive at distance). Magnitude scales — the Richter and the modern moment magnitude scale — measure energy release logarithmically. Earthquake location is determined by comparing P-wave and S-wave arrival times at three or more seismograph stations, whose distance circles intersect at the epicentre.