Where do the majority of earthquakes in the US occur Explain?

Most earthquakes occur along fault lines, caused by plate tectonics.

The idea behind plate tectonics is that there are plates at the surface of the Earth that are constantly moving, due to the flow of rock within the Earth. They are constantly moving up against each other - towards and apart, up and down, etc. Earthquakes are a direct result of this. When the shifting isn't smooth, an earthquake can happen. However, that is not the only effect of plate tectonics, though that is the only cause of earthquakes. Plate tectonics cause tsunamis, mountain forms, oceanic trenches, among other things.

So if you want to know where earthquakes occur, you should look at a map of the Earth's tectonic plates. Along the fault lines where the plates separate is where earthquakes most often occur. Although earthquakes do occur in some places more than others, these fault lines are where they would happen.