What Causes Earthquakes? Plate Tectonics, Faults and Magnitude Explained

Earthquakes are caused by the sudden slip of rock along a fault, a fracture in the Earth’s crust. Stress builds up slowly as the giant tectonic plates that make up the Earth’s outer shell grind past, pull apart or push into each other. When that stress overcomes the friction holding the rocks together, they lurch into a new position and release energy as seismic waves that shake the ground. The US Geological Survey (USGS) locates about 20,000 earthquakes a year worldwide, roughly 55 a day. This guide explains the science, why some places shake far more than others, how magnitude works and what you can do to prepare.

What Happens Inside the Earth During an Earthquake?

The Earth’s rigid outer layer, the lithosphere, is broken into about a dozen major tectonic plates and many smaller ones. They move only a few centimetres a year, about as fast as fingernails grow, driven by heat flowing from the planet’s interior.

Where plates meet, the rocks on either side of a fault are often locked together by friction while the plates keep moving. The rocks bend and store energy like a stretched spring. Scientists call what happens next elastic rebound: when the strain becomes too great, the fault suddenly slips, sometimes by several metres, and the rocks snap back to a less strained shape. The theory was developed after the 1906 San Francisco earthquake, when surveyors found that fences and roads crossing the San Andreas Fault had been offset by as much as 6 metres.

The point underground where the rupture starts is the hypocentre (or focus), and the point on the surface directly above it is the epicentre.

What Are the Main Types of Faults?

• Strike-slip faults: the blocks slide horizontally past each other. California’s San Andreas Fault, where the Pacific Plate moves north-west relative to the North American Plate, is the best-known example.
• Normal faults: the crust is being pulled apart, and one block drops down relative to the other. They are common in rift zones and in the Basin and Range region of the western US.
• Reverse or thrust faults: the crust is being squeezed, and one block is pushed up over the other. The largest thrust faults, called megathrusts, occur at subduction zones, where one plate dives beneath another, and produce the world’s biggest earthquakes.

Where Do Most Earthquakes Happen?

Most earthquakes occur along plate boundaries. According to the USGS, about 81% of the world’s largest earthquakes happen around the Pacific “Ring of Fire”, a horseshoe of subduction zones and faults stretching from Chile and the west coast of North America through Alaska, Japan, Taiwan and Indonesia to New Zealand.

The second most active zone is the Alpide belt, which runs from the Mediterranean through Turkey, Iran and the Himalayas to Indonesia. The collision of the Indian and Eurasian plates there built the Himalayas and the Tibetan Plateau, and still causes powerful quakes, such as the magnitude 7.0 earthquake we reported on in Tibet’s Nagqu region.

Earthquakes can also strike in the middle of plates, along old weaknesses in the crust. The New Madrid Seismic Zone in the central US produced a series of large earthquakes in 1811 and 1812.

Can Humans Cause Earthquakes?

Yes, though usually small ones. “Induced” earthquakes can be triggered by injecting fluids deep underground, filling large reservoirs behind dams, mining and geothermal energy projects. The most striking example is Oklahoma, where the disposal of wastewater from oil and gas production caused a dramatic rise in earthquakes in the 2010s, including a magnitude 5.8 quake near Pawnee in 2016. Earthquakes fell after state regulators limited injection volumes.

How Is Earthquake Magnitude Measured?

Seismologists now use the moment magnitude scale, which measures the total energy released, based on the area of the fault that slipped and how far it moved. The older Richter scale is rarely used for large earthquakes today. Key points:

• The scale is logarithmic: each whole-number step means about 10 times more ground motion and roughly 32 times more energy. A magnitude 7 releases about 1,000 times more energy than a magnitude 5.
• Small quakes are very common: the USGS estimates there are about 500,000 detectable earthquakes a year, of which about 100,000 can be felt.
• Big quakes are rare: on average there are about 15 earthquakes of magnitude 7 to 7.9 and one of magnitude 8 or greater each year.
• The largest ever recorded was the magnitude 9.5 earthquake in Chile on 22 May 1960. The largest in the US was the magnitude 9.2 Alaska earthquake of 1964.

Magnitude is not the same as damage. Shaking intensity, measured on the Modified Mercalli Intensity scale, depends on the depth of the quake, distance from the epicentre, local soil and, above all, building quality.

Why Do Earthquakes Cause Tsunamis and Landslides?

When a megathrust fault under the sea slips, it can lift or drop the seabed by several metres, displacing the water above and sending out a tsunami. The 2004 Indian Ocean earthquake (magnitude 9.1) and the 2011 Tohoku earthquake in Japan (magnitude 9.1) caused devastating tsunamis. Shaking can also trigger landslides and cause water-saturated sand and soil to behave like a liquid, a process called liquefaction, which can topple buildings.

Can Scientists Predict Earthquakes?

No. The USGS states that neither it nor any other scientists have ever predicted a major earthquake, and they do not expect to be able to do so in the foreseeable future. What scientists can do is calculate the probability of significant earthquakes in an area over decades, which guides building codes and insurance. The Uniform California Earthquake Rupture Forecast (UCERF3), published by the USGS and partners in 2015, for example, estimates that California has a greater than 99% chance of a magnitude 6.7 or larger earthquake within 30 years.

Early warning is a different thing from prediction. Systems such as ShakeAlert, operated by the USGS in California, Oregon and Washington, and Japan’s national warning system detect the first, faster seismic waves and send alerts to phones before the stronger shaking arrives, giving people seconds to take cover. Earthquakes are very different from weather hazards, which can be forecast days ahead, as our explainer on hurricanes vs tornadoes shows.

How Can You Prepare for an Earthquake?

Emergency agencies in the US, Canada, Japan and New Zealand give the same core advice:

• During shaking, Drop, Cover and Hold On: drop to your hands and knees, cover your head and neck under a sturdy table, and hold on until the shaking stops. Do not run outside or stand in a doorway.
• Secure your home: fix tall furniture, televisions and water heaters to walls, and store heavy items low down.
• Make a plan: agree where your household will meet and how you will contact each other.
• Build an emergency kit: water, food, medicines, a torch, a radio and a first aid kit for at least three days.
• Enable alerts: turn on emergency alerts on your phone, and in the western US, check that ShakeAlert-powered warnings are enabled.
• Check your building and insurance: older masonry buildings are most at risk, and standard home insurance policies usually exclude earthquake damage.

Earthquake Causes: Key Points

• Earthquakes happen when stress built up by moving tectonic plates causes rock to slip suddenly along a fault
• Most occur at plate boundaries, especially around the Pacific Ring of Fire
• Each step on the magnitude scale is about 32 times more energy
• Earthquakes cannot be predicted, but early warning systems can give seconds of notice
• Drop, Cover and Hold On is the safest response to shaking

Featured image: Rescue workers near the semi-collapsed ten-story Uranus Building on Xuanyuan Road after the 2024 Hualien earthquake, by Shufu Liu / Office of the President (Taiwan) / Wikimedia Commons, CC BY 2.0. Cropped, colour-graded and combined with MediaPavilion branding.

Sources

• US Geological Survey — Earthquake Hazards Program: Earthquake facts and statistics (accessed 1 November 2026).
• US Geological Survey — Can you predict earthquakes? (accessed 1 November 2026).
• US Geological Survey — Where do earthquakes occur? (accessed 1 November 2026).
• US Geological Survey — How much bigger is a magnitude 8.7 earthquake than a magnitude 5.8? (accessed 1 November 2026).
• US Geological Survey — Induced earthquakes (accessed 1 November 2026).
• US Geological Survey — UCERF3: A new earthquake forecast for California’s complex fault system (Fact Sheet 2015-3009) (2015).
• ShakeAlert — ShakeAlert Earthquake Early Warning (accessed 1 November 2026).
• Ready.gov (FEMA) — Earthquakes (accessed 1 November 2026).