Earthquakes shake the surface of the Earth millions of times a year. Most are too small to feel, but some cause serious damage and death. They happen when pieces of the Earth’s outer layer, its crust, move suddenly. The shaking happens when there is a sudden release of strain energy. The point at the surface where the earthquake starts is called the epicenter. Ripples, called seismic waves, spread out from there. They can be felt thousands of miles away.
The Earth’s skin is like a big puzzle, with pieces that slide past each other and bump into each other. These are called tectonic plates, and they constantly move around, slowing down as they get farther apart and speeding up as they come closer together. Sometimes, these edges get stuck on each other. Then they can start to slip slowly. This builds up stress that causes an earthquake.
When the stress finally overcomes the friction, a part of the plate moves down along the fault. That creates a rupture that sends out ripples, like ripples in a pond, that make the ground shake.
These pulsing waves are called P waves and S waves. The P waves travel fastest and shake the ground where you are first, then the S waves follow. The time it takes for these waves to reach you depends on how far away you are from the epicenter. Early warning systems like ours will give you a few to tens of seconds of warning before the shaking starts.