Earthquakes are caused by stress: massive slabs of Earth’s crust push against each other for years or centuries, and the ground shakes when that built-up stress finally overcomes friction and the rock breaks free in a fraction of a second.
The plates behind the pressure
Earth’s outer shell isn’t one solid piece; it’s broken into roughly a dozen major tectonic plates, huge slabs of rock that fit together like an enormous, slow-motion puzzle covering the entire planet. These plates are constantly in motion, driven by heat and convection deep in the mantle, typically creeping along at only a few centimeters a year, roughly the speed a fingernail grows. Most earthquakes happen along the cracks where plates meet, called faults, where two blocks of crust are grinding, pulling apart, or sliding past one another, and it’s the friction at those cracks, not the motion itself, that ultimately produces the shaking.
How stress builds before the ground ever moves
If tectonic plates simply glided past each other smoothly, there would be no earthquakes at all, just constant, imperceptible motion. But rock is rough, and friction along a fault causes the edges to catch and lock together even as the rest of the plate keeps moving. That locked section resists the motion happening all around it, and mechanical stress steadily accumulates at the stuck point, sometimes for decades or centuries. This buildup is known as elastic rebound: rock on either side of the fault bends and stores energy the way a stretched spring does, waiting for the moment the strain finally exceeds what friction can hold.
The moment a fault finally slips
Eventually the built-up stress overwhelms the friction holding the fault in place, and the locked section suddenly breaks free, letting both sides snap into a new position in a fraction of a second. All the energy stored during that slow buildup releases almost instantly, radiating outward from the point of rupture as seismic waves that travel through the surrounding rock and reach the surface as the shaking people feel as an earthquake. The point underground where the rupture starts is called the focus, or hypocenter, and the point on the surface directly above it is the epicenter, usually where shaking is felt most strongly.
How scientists measure the size of the shaking
Seismographs record the shaking as wiggly lines, small squiggles for minor earthquakes and long, dramatic swings for major ones, and scientists convert those recordings into a magnitude. Most large earthquakes today are measured on the moment magnitude scale, which has largely replaced the older Richter scale because it stays accurate across a much wider range of sizes, including massive events the Richter scale couldn’t measure reliably. Because the scale is logarithmic, each whole-number increase represents roughly ten times more ground motion and about 32 times more energy released, so a magnitude 7 earthquake unleashes vastly more destructive power than a magnitude 5, not just a bit more.
Why some regions shake far more than others
Earthquakes cluster heavily around plate boundaries, which is why places like California, Japan, Indonesia, and the Andes see frequent seismic activity while regions in the middle of stable continental plates see relatively few. Even so, earthquakes can still occur far from plate edges, along older, buried faults left over from ancient geological stress that occasionally reactivates. Pinning down exactly where and how faults are loaded with stress remains one of the central open challenges in seismology, since it’s the key to better early-warning systems and building codes in earthquake-prone regions worldwide.