Earthquakes do not occur everywhere throughout the world. Instead, the great majority of quakes take place in or near narrow belts that coincide with the boundaries of the tectonic plates. These plates make up the rocky crust at the Earth's surface and underlie both the continents and the oceans. Oceanic crust is sometimes compared to a conveyor belt: New crust is continually created at midocean ridges and destroyed where it disappears into trenches at the edges, usually where the ocean collides with a continent. Both oceanic ridges and trenches are sites of earthquake activity.

An earthquake consists of the shock waves created when rocks below the surface suddenly slip along a fault plane. Earthquake are classified by their intensity, which is the amount of energy released by the movement and by the depth to the center of the slip zone, or focus.

Although earthquakes occur along all plate boundaries, they are much more common along collision zones that include an oceanic trench than they are at midocean ridges. This difference in frequency is because at midoceanic ridges, the crust is both thin and hot, which reduces the amount of pressure (called strain) that can build up before slip on a fault happens. The rock at oceanic ridges is also somewhat softer because it is hot. At trenches, the crust is thicker and cooler, which allows more strain to accumulate, leading to more earthquakes.