Plates

 Lets travel to the west coast, where the dense crust of the ocean sub ducts, or in other words when one edge of a crustal plate becomes forced sideways and down into the mantle and pushed under another plate, under continental crust or a crust that is less dense and the geological process continues thus an earthquake can result in addition to magma generation. Because oceanic crust is denser and thinner that continental crust it gets bent and pulled under forming the subduction zone. Subduction zones have a lot of earthquakes and the sub ducting plate causes melting in the mantle above.