What is an earthquake? This is what happens when the ground starts to shake

Deep beneath the surface, our planet is a reactor, and one of the ways you'll feel this activity is when there's an earthquake.

Earthquakes are violent shaking from the planet's surface, according to AccuWeather. They're caused by movement from Earth's outermost layer, the crust.

ABC7 has compiled a list of items that are important to have in your earthquake kit so that you're prepared for "the big one."

The crust is broken up into pieces called tectonic plates that are always on the move, though usually in ways we cannot feel. When the plates hit a roadblock, stress builds and leads to cracks in the Earth called faults.

MORE | 4.7 magnitude earthquake hits NJ, shaking felt across Northeast, including New York, Philly

In time, the energy from within builds to the breaking point. That sudden release leads to an earthquake.

The shaking begins at a central region called the epicenter and spreads far and wide. An earthquake can then be followed by additional shaking known as an aftershock.

Earthquake scale: How they are measured and what the magnitude and intensity scales mean

The movement of sudden tectonic plates causes seismic activity underground.