Earthquakes are the sudden release of stress energy in Earth’s crust. This energy creates vibrations called seismic waves. These waves can shake buildings and cause other damage. The strength of an earthquake depends on how much shaking it causes and whether or not the building was built to withstand that amount of shaking.
There are many kinds of earthquakes. Some are very small and only feel like a shudder. Others are much bigger and can cause serious damage. The biggest earthquakes happen along faults, the lines of cracks that run through Earth’s crust. Earthquakes can also happen farther from a fault zone when the plates are stretched or squeezed.
Scientists used to measure earthquakes using a scale called the Richter magnitude. This scale was based on measurements of the amplitude of a waveform recording from a seismograph. Today, scientists use a more precise physical measurement called the moment magnitude to describe an earthquake. This number describes how much energy the earthquake has and how big the movement of rock is.
The outer skin of the Earth isn’t a single piece, it’s made up of giant puzzle pieces that wrap around it like an egg shell. These plates slide past each other and bump into each other on the viscous, or slowly flowing, layer of Earth’s mantle beneath. This nonstop sliding causes enormous amounts of stress to build up in the rocks and other materials that make up the lithosphere, or crust. If these stresses get too large, then some of the plates will unstick at a fault line and an earthquake will occur.