An earthquake is a sudden shaking of the ground caused by movements deep inside the Earth. The Earth's outer layer is made of huge pieces called tectonic plates that fit together like a cracked eggshell. These plates move very slowly, sometimes grinding past each other, pulling apart, or crashing together. When they get stuck and then suddenly slip, the energy is released as waves that travel through the ground, making everything above shake, sometimes very strongly.

Most earthquakes are too small to feel, but powerful ones can knock down buildings, crack roads, and change the landscape. Scientists measure earthquake strength using magnitude scales: a magnitude 3 earthquake might rattle dishes, while a magnitude 7 can devastate entire cities. The epicenter is the point on the Earth's surface directly above where the earthquake starts underground.

Earthquakes happen most often along fault lines, places where tectonic plates meet. California's San Andreas Fault, Japan, and the Pacific Ring of Fire experience frequent earthquakes. Some regions rarely experience them because they sit in the stable middle of tectonic plates.

Engineers design buildings in earthquake zones to sway rather than crack, using flexible materials and deep foundations. People living in these areas practice earthquake drills, learning to “drop, cover, and hold on” by getting under sturdy furniture until the shaking stops.