About a half-million earthquakes rock Earth every day, but most are too small or too deep underground to be felt. The ones we feel are caused by sudden movement of Earth’s crust at faults. The spot on the surface above where the rock suddenly slips is called the epicenter. The slipping creates vibrations called seismic waves that can be felt thousands of miles away.
The top, stiff layer of the Earth’s crust isn’t all in one piece like an eggshell, but it’s made up of giant puzzle pieces called tectonic plates that drift around on the thick, slow-moving mantle below. This nonstop movement can cause stress on the plates, especially when they collide or slide past each other. This friction builds up until the pressure becomes so great that the rocks along the plate boundaries break and slip. The resulting release of energy causes the earthquake we feel.
Plates can move in many different ways, but most of the world’s earthquakes occur at so-called convergent or divergent boundaries where two plates collide, or one plate is forced under another in a process called subduction. When the plates move, they can also change shape and deform the surface of the Earth. This can produce a wide range of earthquakes, from tiny tremors to the massive 9.1 magnitude earthquake that killed nearly 4,000 people in 1960 in Chile.
Earthquakes are often preceded by foreshocks, which can be just as strong or even stronger than the main shock. These tremors can be dangerous if they are close to the epicenter or if they cause buildings or bridges to collapse or break. Make sure everyone knows what to do during and after an earthquake and is familiar with their local emergency plans, including what to do in the event of an aftershock. Encourage older members of your family to know where the gas, water and electrical shutoffs are so they can turn them off if necessary.