Earthquakes occur because the plates get locked together, and the magma pushes so hard they unlock, sliding against each other. Scientists study earthquakes by using instruments that measure the seismic waves in the ground.
Scientists measure earthquakes by using an instrument called a seismograph.
On the seismograph you will see many wiggly lines. The bigger t he wiggle the bigger the shaking. The size of the wiggle is measured on the Richter Scale. The biggest earthquakes have a measurement of 8.0 on the Richter Scale. There are hundreds of seismograph stations all over the world.