Why Do Earthquakes Happen? Tectonic Plates, Faults and How They Are Measured
An earthquake is an intense shaking of Earth's surface. According to NASA, the shaking is caused by movements in Earth's outermost layer.
Earth is active beneath the surface
Earth looks solid, but NASA describes it as extremely active just below the surface. It has four basic layers: a solid crust, a hot, nearly solid mantle, a liquid outer core and a solid inner core.
Earthquakes are caused by shifts in the outer layers, a region called the lithosphere. The lithosphere is made of the solid crust plus the top, stiff layer of the mantle.
Tectonic plates
NASA explains that the lithosphere is not one continuous piece like an eggshell. It is made of giant puzzle pieces called tectonic plates, which are constantly shifting as they drift on the slowly flowing mantle beneath. They have been moving very slowly over Earth's surface for millions of years.
How stress becomes an earthquake
- The non-stop movement of the plates puts stress on Earth's crust.
- When the stress gets too large, the crust cracks. These cracks are called faults.
- When tectonic plates move, they also cause movement at faults.
- An earthquake is the sudden movement of Earth's crust at a fault line.
NASA gives the San Andreas Fault in California, 750 miles long, as an example of a fault.
Epicentre
The place where an earthquake begins is called the epicentre. The most intense shaking is often felt near it. NASA adds that vibrations can still be felt and detected hundreds or even thousands of miles away.
How earthquakes are measured
The energy from an earthquake travels through Earth as vibrations called seismic waves. Scientists detect them with a seismometer, which records seismic waves as a series of zig-zag lines.
From the record, scientists can work out the time, location and intensity of an earthquake. The record also shows what kinds of rocks the waves travelled through, because different materials vibrate in different ways.
Do earthquakes happen on other worlds?
Yes. NASA says scientists have measured quakes on Earth's Moon and see evidence of seismic activity on Mars, Venus and several moons of Jupiter. NASA's InSight mission placed a seismometer on Mars to study marsquakes. Studying their vibrations helps scientists learn what materials are inside Mars.
What this article does not cover
NASA's explainer does not cover how to prepare for an earthquake or how to predict one. For safety guidance, rely on your local disaster management authority.
Facts checked October 2, 2026.
Sources
- NASA Space Place, "What Is an Earthquake?" (last updated December 5, 2024): https://spaceplace.nasa.gov/earthquakes/en/