On March 11, 2011, an earthquake off the coast of Japan triggered a tsunami that has caused massive destruction. The earth science behind this natural disaster is explained below.
On March 11th, 2011, a magnitude 9.0 earthquake occurred near the northeast coast of Honshu, Japan.
Fast Fact: Magnitude refers to the amount of energy released by an earthquake, and is often described using the logarithmic Richter scale. A magnitude 6 earthquake on the Richter scale has a shaking amplitude 10 times larger than a magnitude 5.0, and 100 times larger than a magnitude 4.0.
Earthquakes occur when rocks break and slide suddenly against one another along faults in the earth. The Tokohu earthquake happened at a subduction zone plate boundary where the Pacific plate is moving westward beneath the North American plate (Japan is on the North American plate). The epicentre of the March 11 earthquake was east of the northeast coast of Japan, in the ocean, between Japan and the Pacific/North American plate boundary.
Fast Fact: Epicentre is the point on the surface of the earth above the focus of an earthquake. The focus is where the energy for the earthquake originated (where the rock breaking and sliding happened).
Plate tectonics and earthquakes
The outermost portion of the earth is composed of relatively brittle tectonic plates, which overlie more ductile mantle. Because of heating and cooling of the earth's interior, the overlying plates are forced to move around. Where plate boundaries occur, stress can build up as plates slide against one another, or one plate is forced beneath another in a subduction zone. Tremendous stress can accumulate in these zones, and when the accumulated stress exceeds the strength of the rock – the rocks can break unexpectedly, releasing large amounts of energy that can cause the ground to shake sometimes hundreds of kilometres away.
Earthquakes and tsunamis
A tsunami happens when the ocean floor is suddenly lifted or dropped (in the case of Tokohu, this was caused by the earthquake in deep water offshore), displacing large volumes of water, resulting in large waves with very long wavelengths. The large waves, originating in the deeper ocean, then travel away from the epicentre. As the waves reach shallower water, the wave height increases dramatically, and can be extremely dangerous upon reaching the shore. Eyewitness accounts of the Japanese tsunami reported wave heights of 13 metres.
Fast Fact: Other large earthquakes in recent history include the 9.5 magnitude earthquake that affected Chile in 1960 and the 9.3 magnitude Sumatran-Andaman earthquake in 2004, both of which had related tsunamis.
The US Geological Survey website
The Intergovernmental Tsunami Programme
The National Oceanic and Atmospheric Administration
Article first published March 28, 2011.
Photo Credit: Karsten N., Germany