Earthquake

An earthquake (also called a seismic event or temblor) is the sudden release of energy stored in Earth's lithosphere, generating seismic waves that cause the ground to shake. Earthquakes occur across the globe but are concentrated along tectonic plate boundaries, fault zones, and volcanic regions. They are among the most destructive natural hazards known to humanity, responsible for hundreds of thousands of deaths and trillions of dollars in economic losses over recorded history.

Causes and origins

Most earthquakes result from the movement of tectonic plates. Earth's outer shell is divided into roughly a dozen major plates and several minor ones that move continuously, driven by convection currents in the underlying mantle. When stress accumulates along a fault — a fracture or zone of fractures in the crust — and exceeds the frictional strength of the rock, the fault ruptures suddenly, releasing energy as seismic waves.

Tectonic earthquakes

Tectonic earthquakes are by far the most common type. They occur at three principal plate boundary settings:

• Convergent boundaries, where plates collide, producing powerful subduction-zone earthquakes such as the 2011 Tōhoku earthquake (magnitude 9.0–9.1) off the coast of Japan.
• Divergent boundaries, where plates pull apart, generating generally smaller earthquakes along mid-ocean ridges.
• Transform boundaries, where plates slide horizontally past each other, as along the San Andreas Fault in California.

Volcanic and induced earthquakes

Volcanic earthquakes are caused by the movement of magma and associated pressure changes within volcanic systems. Induced earthquakes result from human activities such as reservoir impoundment, deep fluid injection (notably in association with hydraulic fracturing), and mining. Induced seismicity has become a recognised concern in parts of the United States and elsewhere since the early 21st century.

Structure of an earthquake

Focus and epicentre

The focus (or hypocenter) is the point within Earth where rupture initiates. The epicentre is the point on Earth's surface directly above the focus. Earthquakes are classified by focal depth:

• Shallow-focus: 0–70 km — the most destructive category.
• Intermediate-focus: 70–300 km.
• Deep-focus: 300–700 km — less damaging at the surface despite potentially high magnitudes.

Seismic waves

Energy radiates outward from the focus as several types of seismic waves:

• P-waves (primary or compressional waves) travel fastest and pass through solids and liquids.
• S-waves (secondary or shear waves) travel more slowly and move only through solids.
• Surface waves (Love and Rayleigh waves) travel along Earth's surface and are primarily responsible for the damage felt during a major earthquake.

Measurement and magnitude

Magnitude scales

Earthquake size is expressed on several scales. The original Richter scale, developed by Charles F. Richter in 1935, was a local magnitude scale applicable mainly to Southern California. Modern seismology uses the moment magnitude scale (Mw), which more accurately captures the total energy released by large earthquakes. Each whole-number increase on the Mw scale represents approximately 31.6 times more energy released.

Intensity scales

While magnitude measures energy at the source, intensity describes the effects felt at a specific location. The Modified Mercalli Intensity (MMI) scale ranges from I (imperceptible) to XII (total destruction) and varies with distance from the epicentre, local geology, and building construction.

Seismographs and seismic networks

Earthquakes are detected and recorded by seismographs — instruments that measure ground motion. Global networks of seismographs, coordinated by organisations such as the United States Geological Survey (USGS) and the Incorporated Research Institutions for Seismology (IRIS), allow rapid determination of an earthquake's location, depth, and magnitude within minutes of occurrence.

Geographic distribution

Approximately 90 percent of the world's earthquakes, and nearly all of the largest ones, occur along the Ring of Fire — a horseshoe-shaped belt encircling the Pacific Ocean where several major tectonic plates converge. Other seismically active zones include the Alpide Belt, stretching from the Mediterranean through the Middle East and South Asia to Southeast Asia, which produced the 2004 Indian Ocean earthquake (magnitude 9.1–9.3), the deadliest seismic disaster of the modern era.

Intraplate earthquakes, occurring far from plate boundaries, are less frequent but can be destructive because infrastructure in those regions is often not built to seismic standards. The 1811–1812 New Madrid earthquakes in the central United States are a well-known historical example.

Effects and hazards

Ground shaking

The primary hazard of an earthquake is ground shaking, which can cause structural collapse of buildings, bridges, and other infrastructure. The severity depends on magnitude, focal depth, distance, and local soil conditions. Soft sediment amplifies shaking significantly — a phenomenon that contributed to severe damage in Mexico City during the 1985 and 2017 earthquakes.

Secondary hazards

Earthquakes trigger a range of secondary hazards:

• Tsunami — large earthquakes beneath or near the ocean floor can displace vast volumes of water, generating destructive ocean waves.
• Landslides and rockfalls — ground shaking destabilises slopes, sometimes causing mass movements that bury communities.
• Liquefaction — water-saturated, loosely consolidated sediments lose strength and behave like a fluid, causing buildings to sink or tilt.
• Fires — ruptured gas lines and downed electrical infrastructure can ignite fires, as occurred in San Francisco following the 1906 earthquake.

Earthquake prediction and early warning

Long-term forecasting

Seismologists can estimate the probability of earthquakes of specified magnitude occurring in a region over decades, using historical seismicity data, geodetic measurements, and paleoseismology (the study of prehistoric earthquakes). However, reliable short-term prediction — specifying the time, location, and magnitude of an individual earthquake days or hours in advance — remains beyond current scientific capability.

Earthquake early warning systems

Earthquake early warning (EEW) systems detect the first, less-damaging P-waves and issue automated alerts before the more destructive S-waves and surface waves arrive. Japan's nationwide EEW system, operated by the Japan Meteorological Agency, provides seconds to tens of seconds of warning. The ShakeAlert system serves parts of the western United States. While the warning time is short, it is sufficient to slow trains, open fire-station doors, and prompt individuals to take protective action.

Earthquake engineering and preparedness

Earthquake engineering applies principles of structural and geotechnical engineering to design buildings, bridges, and infrastructure capable of withstanding seismic forces. Key strategies include base isolation (mounting structures on flexible bearings that absorb seismic energy), moment-resistant frames, and shear walls. Building codes in seismically active regions — such as Japan, New Zealand, and California — mandate earthquake-resistant design.

Public preparedness measures recommended by civil authorities typically include securing heavy furniture, storing emergency supplies, and practising drop, cover, and hold on procedures.

Notable historical earthquakes

• 1556 Shaanxi earthquake (China) — estimated magnitude 8.0; death toll of approximately 830,000, making it the deadliest earthquake in recorded history.
• 1906 San Francisco earthquake (United States) — magnitude approximately 7.9; fires following the earthquake destroyed much of the city.
• 1960 Valdivia earthquake (Chile) — magnitude 9.4–9.5; the largest earthquake ever instrumentally recorded.
• 2004 Indian Ocean earthquake — magnitude 9.1–9.3; triggered a tsunami killing an estimated 227,000 people across fourteen countries.
• 2010 Haiti earthquake — magnitude 7.0; killed an estimated 100,000–300,000 people (figures remain disputed) and devastated Port-au-Prince.
• 2011 Tōhoku earthquake (Japan) — magnitude 9.0–9.1; generated a massive tsunami and caused the Fukushima Daiichi nuclear disaster.

Frequently asked questions

Is it possible to predict earthquakes accurately?

Long-term probabilistic forecasting of earthquake hazard in a region is well established, but reliable short-term prediction of exactly when, where, and how large a specific earthquake will be remains beyond current scientific capability. No method has demonstrated consistent, verifiable short-term prediction performance.

Is the Ring of Fire the most earthquake-prone region on Earth?

Yes. The Ring of Fire, encircling the Pacific Ocean, is responsible for roughly 90 percent of the world's earthquakes, including nearly all of the highest-magnitude events, because it marks the boundaries of several major converging tectonic plates.

Are human activities capable of causing earthquakes?

Yes. Induced seismicity linked to deep fluid injection, reservoir filling, and mining is well documented. Most induced earthquakes are small, but some have reached magnitudes capable of causing damage. The extent to which hydraulic fracturing directly causes significant earthquakes, as distinct from wastewater disposal, remains an active area of research.

Is a large earthquake always followed by a tsunami?

No. A tsunami requires a large vertical displacement of the ocean floor, which typically occurs during subduction-zone earthquakes. Strike-slip faults, which produce horizontal rather than vertical motion, rarely generate significant tsunamis even at high magnitudes.

Is the moment magnitude scale the same as the Richter scale?

No, although values from the two scales often appear similar for moderate earthquakes. The Richter scale was a local magnitude measure calibrated for Southern California and specific instruments. The moment magnitude scale (Mw) is now the standard used by seismologists worldwide because it accurately measures energy release across all sizes and types of earthquakes.