Earthquakes and volcanoes go hand in hand like peanut butter and jelly or rice and beans. Where there are volcanos there are usually tectonic plates beneath them either pushing against or pulling away from one another. Take for instance the Cascade Mountain Range in Northern California and stretches through Oregon and into Vancouver, British Columbia and sits northeast of the San Andreas Fault line that runs 750 miles through California. This fault creates the tectonic boundary between the Northern America and Pacific Plates. The three types of plate boundaries are convergent, meaning basically they are colliding together, divergent meaning they are pulling away from one another and transform which means that the plates slide past each other.
