As plates continue to move and plate boundaries change over long periods of time, weakened boundary regions become part of the interiors of the plates. These zones of weakness within the continents can cause earthquakes in response to stresses that originate at the edges of the plate or in the deeper crust. The point beneath the Earth's surface where the rocks actually break and move is called the focus of the earthquake. The focus is the underground point of origin of an earthquake. Directly above the focus, on the Earth's surface, is the epicenter .
Earthquake waves reach the epicenter first.
During an earthquake, the most violent shaking is found at the epicenter. The lithosphere covers the whole Earth. Therefore, ocean plates are also involved, more particularly in the process of sea-floor spreading. This involves the midocean ridges which are a system of narrow submarine cracks that can be traced down the center of the major oceans. The ocean floor is being continuously pulled apart along these midocean ridges.