What is an earthquake? 

When the ground starts shaking! This is called an earthquake.It happens because big pieces of Earth move under our feet!

What is happening underground?

Deep under the ground are big pieces of Earth. They move and rub against each other at cracks called a fault. 

The shaking begins underground at the focus this is where the earthquake starts.
The place right above it is the epicenter  this is when the earthquake shaking feels strongest

The shaking travels as seismic waves energy moving through the Earth.

Where and How It Starts