It all depends on where you are and the tectonic plates. Most earthquakes happen becuase of plates sliding next to each other. This creates a earthquake. So to answer the other part of ur question volcanic eruptions are caused by preasure of the playes pushing up and coloding into eachother. Only certina mountains do this. Magma forms. And there i have it a volcano. I mogjt be wrong only becuase half of this was tought by some sub :X but it sould be right