The earthquake and resultant tsunami that caused such destruction in Japan earlier this year (2011) is an example of an underwater earthquake. The epicenter of the earthquake was recorded as being 80 miles east of Sendai at depth below sea level of approximately 20 miles. The earthquake was one of the largest recorded to hit Japan, with a magnitude of 9. The more recent earthquake in Christchurch, New Zealand, in comparison had a magnitude of 6.3.
There is no difference between an earthquake that occurs underwater or on the land, the processes involved are the same. The Earth’s crust is formed of many rigid plates which all move in relation to one another. These tectonic plates, as they are known, also form the ocean floors and are in constant motion. Where plates meet, either on land or underwater, earthquakes and volcanic activity are common.
Tectonic plates may be moving towards each other (convergent) or away from each other (divergent). Two plates that meet may also slide laterally against each other and in such cases are said to transform.
This movement is occurring continuously yet major earthquakes are not. This is because generally the movement between two plates is smooth. When two plates contact and the movement is irregular or the plates lock, enormous forces are built up and eventually something has to give. The two surfaces in contact slip, releasing the vast amounts of stored up energy as a shock wave traveling through the crust. This is the earthquake. The shockwaves are known as seismic waves.
Before a large earthquake happens there will often be a series of smaller earthquakes called foreshocks as stress builds up where tectonic plates meet. The main earthquake will also be followed by a series of smaller earthquakes called aftershocks, a process which may go on for years. The Japan earthquake occurred under the ocean where the Pacific tectonic plate meets the plate on which Japan sits. Japan has a long history of earthquakes for this very reason. Because the depth of the earthquake at 20 miles was fairly shallow, a lot of the energy of the earthquake was transferred to the sea pushing it first upwards and then inland causing the devastating tsunami.
Unless the actual epicenter of an underwater earthquake occurs close to the shore, the main damage to human populations occurs from the resultant tsunamis. The tsunami which caused such destruction in Sri Lanka and other parts of South East Asia in 2004 was caused by an underwater earthquake with its epicenter in the Indian Ocean. This earthquake had a magnitude of 9.2 and originated approximately 100 miles from the nearest coastline, at a depth below sea level of 19 miles.
Not all underwater earthquakes produce tsunamis. Those with a magnitude lower than 7 generally do not produce enough energy to initiate a tsunami. Other earthquakes may have a magnitude larger than 7, but their epicenters may be hundreds of miles below the ocean floor and not enough energy is transferred to create a tsunami.