Earthquakes are the most devastating and destructive natural disasters that can happen on the planet. They can result in enormous loss of life and damage to buildings and infrastructure. An underwater earthquake is caused in the same way as any other earthquake but, depending on the magnitude and length, they can also cause tsunamis and huge tidal waves. Underwater earthquakes can destroy communication cables that have been buried under the seabed, disrupting telephone networks and the Internet in the affected areas.
Human activities, such as injection of fluids into deep wells during waste disposal, can cause earthquakes, but the tremors caused are not as frequent or as destructive as natural occurring earthquakes.
Tectonic Plate Movement
The main cause of earthquakes is tectonic plate movement. Tectonic plates are pieces of the Earth’s lithosphere, the rigid outer shell of the planet. Tectonic plate movement also causes volcanoes to form, which is the reason we have impressive geographical features like the Mariana Trench and Mount Everest.
Tectonic plates can move in three directions when they meet, convergent (towards each other), divergent (away from each other) and transform (side-to-side). They are enormous, the size of continents, and mostly can move along smoothly unless they lock. When locking occurs, a loss of stress builds up at the edges because they try to move in opposite directions.
The stress builds until it is stronger than the thing that was stopping the boundaries from moving and the two surfaces slip. When two tectonic plates slip, it is called the fault or fault plane. The locking and slipping is known as stick-stick behaviour. A sudden release of the stored energy is caused by this release, which spreads through shock waves that are called seismic waves.
Scientists can detect seismic waves by using a seismograph, they use the results to predict the possibility of an earthquake. Seismic waves spread out from the focus, which is where the earthquake starts. This is where most damage is done when the earthquake happens on land. The area immediately above the focus is called the epicentre.
Sometimes there are smaller earthquakes before the main earthquake that are called foreshocks. Until big earthquakes occur, scientists don’t know that these are foreshocks. When an earthquake occurs, it is called the mainshock and after the mainshock, aftershocks always follow. Aftershocks are smaller earthquakes that happen in the same location. A very big mainshock can be followed by aftershocks for weeks, months and years.
Japanese Earthquake and Tsunami 2011
The earthquake that hit Japan in 2011 had a magnitude of 8.9 and was the most powerful ever recorded in Japan. It was an underwater earthquake that started 80 miles off the coast of Sendai, which is where the Pacific tectonic plate slides underneath the plate Japan sits on. The trench where the plates meet suffered ten miles of ruptured crust along it. As the water was only 15 miles deep where the earthquake occurred, most of its energy went along the seabed. This energy caused waves measuring 7 metres high in the north-east of Japan. Some of the waves were estimated to be 10 metres high. The waves spreaded around the coast of Japan and moved across the Pacific where the height of the waves reduced. The devastation along the Japanese coast was horrific, the cost estimated is around $300 billion. Hundreds of people were killed.
http://www.soest.hawaii.edu/GG/ASK/earthquakes.html
http://www.buzzle.com/articles/what-causes-earthquake.html
http://en.wikipedia.org/wiki/Submarine_earthquake
http://www.mirror.co.uk/news/top-stories/2011/03/11/japan-earthquake-and-tsunami-hundreds-killed-in-one-of-largest-quakes-on-record-115875-22983080/
