Geologists have a major theory of the earth just like biologists have a major theory. In biology its called Evolution, in Geology the major theory is called plate tectonics. The idea is that the earth spherical surface is broken up into about 15 PLATES. These plates are all in relative motion. There are three kinds of plate boundaries: strike slip, where one plate slides past another forming a transform fault such as the San Andreas in California, (2) one plate moves apart from another plate creating new sea floor and (3) where one plate dives BACK into the interior of the earth to create a subduction zone and a topographic trench. The giant magnitude 9 quake in Japan was a subduction zone quake where the pacific plate slipped under the Asian plate; the movement was several meters which is very large for a single event.

The March 11 earthquake in Japan was caused by movement of two of the Earth's tectonic plates against each other.The Pacific plate (underlying the Pacific Ocean) is colliding with the Okhotsk plate (under Japan) at around 9 cm every year. Because the Pacific plate is denser than the Okhotsk plate, it dives down into the Earth along what is called a "subduction zone". As the Pacific plate subducts beneath the Okhotsk Plate, friction between the two causes them to lock against each other. Eventually, enough stress builds up to overcome the friction, and the Pacific plate slides toward and beneath Japan, causing an earthquake. This same process has happened thousands of times before humans were around to record it. Measurements from the 2011 earthquake showed that the ocean floor on the Pacific plate moved up to 100 feet closer to Japan during the earthquake.

The Japan earthquake was rated a 9.0, which ranks it as the fifth strongest quake ever recorded since man started keeping measurements. In addition to the strong ground shaking, which toppled buildings and destroyed roads and utilities, the earthquake caused a large tsunami that devastated cities along the Japan coastline. The tsunami was generated because when the Pacific plate slipped toward Japan, the whole ocean floor jumped up by around 10 feet. This sudden movement caused the ocean water above the plate to rise as well. This upraised water then flowed away from the zone of uplift, causing a tsunami that reached heights of up to 120 feet when it hit the Japan coastline.

The Pacific plate of the Earth's crust, which underlies most of the Pacific ocean, is sliding under the Asian Plate underneath Japan, and then plunging downward into the mantle toward the Earth's core. This oceanic crust is very cold relative to other crustal materials, and thus very sticky and not-well lubricated, so it jams easily on the Asian plate that it is sliding under. While the plates are jammed, the inexorable force of gravity that is pulling the plate downward into the mantle continues to accumulate stress on the descending plate, and eventually the stress is able to overcome the jamming, causing the entire plate to slide rapidly until it jams again - an earthquake. Because the jamming force is so strong, there is plenty of time for the stress to build up, which is why the earthquakes are so huge.

All of the really big (magnitude 9+) earthquakes happen this way: cold ocean plates sliding underneath warmer and lighter continental plates. The 2004 Sumatra earthquake happened because of the Indian Ocean plate sliding underneath the Asian plate beneath Java. The 1960s Anchorage Earthquake is because the Farralon plate (eastern pacific north of about San Francisco) is sliding underneath North America in Alaska. The largest earthquake on record, in Chile, is because the Juan de Fuca plate (eastern pacific south of the Sea of Cortez) is sliding underneath South America. Most of the earthquakes caused by plates simply sliding past each-other, such as nearly all of the faults in California, are much smaller.

Earthquakes are caused by rocks moving against each other. Most of the world's earthquakes occur at the boundaries between tectonic plates. The largest earthquakes are found at plate boundaries where the plates are colliding with each other, and one plate is diving down into the earth. These are called subduction zones, and there is one just off the east coast of Japan. Basically, the bigger the area of the surface where the rocks are sliding against each other (this is referred to as the fault plane), and the larger the distance they slide, the bigger the earthquake. The area of the fault zone that ruptured in Japan's last big earthquake (called the 'Tohoku earthquake', magnitude 9.1) is estimated to have been 230 miles long and 120 miles wide.