The earthquake was classified as a megathrust quake and occurred along a subduction zone, an area where the Pacific plate is sinking below the North America plate in the Japan Trench. Studies after the event indicate the seabed between the earthquake epicenter and the Japan Trench moved approximately 50 m (164 ft) east-southeast and rose about 7 m (23 ft). Many seismologists were surprised by the strength of the earthquake, as past modeling studies indicated subduction zone quakes in that are would not exceed magnitude 8.4. However, models were generated using data from short historical records. New work is being done to revisit models and include paleoseismic records, although even those events are limited. Several other subduction zone areas in the world may need to be studied and revised as potential 9.0 magnitude zones.
The tsunami generated by the earthquake was far more destructive than the initial quake. A tsunami occurs when the seafloor abruptly deforms, causing a vertical displacement in the water. A series of long-wavelength waves, or wave train, is formed as the water attempts to regain equilibrium. Although the government of Japan issued tsunami warnings after the earthquake, the loss of life was severe due to the unprecedented height of the waves. The tsunami waves inundated approximately 561 km2 (217 mi2) at heights over 9m (29 ft) in many coastal cities.
Photo: Wave crashing over a street in Miyako City, Japan, courtesy of Mainichi Shimbun, Reuters
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