What causes earthquakes beneath a volcano? what can scientists infer using data collected from earthquakes beneath a volcano?

Hello.  Happy to help.  

Most earthquakes directly beneath a volcano are caused by the movement of magma.  The magma exerts pressure on the rocks until it cracks the rock.  Then the magma squirts into the crack and starts building pressure again.  Every time the rock cracks it makes a small earthquake.  These earthquakes are usually too weak to be felt but can be detected and recorded by sensitive instruments. 

Hope that helps.