An earthquake occurred off the coast of Japan on Thursday night. It was one of the largest earthquakes since measurements began, measuring a 6 on the Richter scale. Earthquakes are caused by a buildup of strain in the earth's crust until the elastic limit is reached, releasing energy in seismic waves. When tectonic plates converge under the sea, it can cause both earthquakes and tsunamis as the sudden movement of the seafloor displaces large volumes of water.