How Earthquakes Work: Faults, Seismic Waves, Magnitude, and Earthquake Preparedness

What Causes Earthquakes?

An earthquake is the sudden release of energy stored in Earth's crust — accumulated as tectonic plates slowly grind against each other over years, decades, or centuries. Where plates meet, friction prevents their surfaces from sliding freely; stress builds like a compressed spring until it overcomes friction, producing rapid slip along a fracture called a fault. This slip releases elastic energy as seismic waves that radiate outward through Earth's rock, shaking the surface.

The point within the Earth where rupture begins is the hypocenter (or focus); the point on the surface directly above is the epicenter. Shallow earthquakes (hypocenter less than 70 km deep) generally cause the most surface damage; deep-focus earthquakes (down to ~700 km) are also associated with subduction zones but produce less surface shaking because the energy attenuates through more rock.

Types of Faults

Faults are classified by the direction of relative movement:

The largest earthquakes occur on megathrust faults — the shallow, gently dipping interfaces between subducting and overriding plates. The 2011 Tōhoku earthquake (M9.1) occurred on the Japan Trench megathrust; the 1960 Valdivia earthquake (M9.5 — the largest recorded earthquake in history) occurred on the Chile subduction zone.

Seismic Waves

Earthquakes generate several types of seismic waves, each traveling differently through Earth's interior:

• P-waves (primary/compressional): Push-pull waves; fastest (6–8 km/s in the crust); travel through solids and liquids; detected first by seismometers; the "P" arrives seconds before the destructive shaking
• S-waves (secondary/shear): Side-to-side waves; slower (~3.5–4.5 km/s); travel only through solids (cannot pass through Earth's liquid outer core — this was the evidence that proved the outer core is liquid); the primary cause of structural damage
• Surface waves (Love and Rayleigh): Travel along Earth's surface; the slowest but largest amplitude; responsible for most damage in major earthquakes; Rayleigh waves produce the rolling ground motion felt in large earthquakes

The time difference between P-wave and S-wave arrival at a seismograph station allows calculation of the earthquake's distance. Three or more stations can triangulate the epicenter. This technique was developed in the late 19th century and remains fundamental to seismology.

The fact that P-waves travel through Earth's interior but S-waves cannot pass through the outer core allowed geophysicists to determine that Earth has a liquid outer core — one of the most important discoveries in Earth science.

Measuring Earthquake Strength

Two scales are commonly used to describe earthquake strength:

The Richter Scale (1935)

Charles Richter developed the local magnitude (Mₗ) scale in 1935 as a logarithmic scale based on the maximum amplitude of seismic waves recorded by a Wood-Anderson seismometer at a standardized distance. Each whole number increase represents a 10-fold increase in wave amplitude and roughly 32-fold increase in energy released. The Richter scale is most accurate for small to moderate earthquakes in California — for which it was designed.

Moment Magnitude Scale (Mw)

The modern standard is the moment magnitude scale (Mw), developed by Hiroo Kanamori and Thomas Hanks in 1979. It measures the total seismic moment — directly calculated from the area of the fault that ruptured, the amount of slip, and the rock rigidity. It provides accurate magnitude across all sizes, avoids the saturation problems of the Richter scale for very large earthquakes, and is the scale used by professional seismologists today. (Media reports often say "Richter scale" when they mean Mw.)

Major Earthquakes and Their Impacts

Earthquake Early Warning Systems

Earthquake early warning (EEW) uses the fact that seismic waves travel faster than destructive shaking can cause widespread damage — and far faster than the affected areas. By detecting P-waves (fast, less destructive) seconds before S-waves and surface waves arrive, EEW systems can automatically alert people, stop trains, open fire station doors, and pause surgeries.

Japan's J-Alert system, developed after the 2011 earthquake, transmits warnings via mobile phones, TV, and radio within seconds of detection. The ShakeAlert system became operational across California, Oregon, and Washington in 2021. Lead time ranges from a few seconds (for those near the epicenter) to over a minute (for those farther away) — enough to drop and take cover, evacuate elevators, and protect critical infrastructure.