The survivors and residents in the Tohoku region face two impending fears: the radiation hike from the instability of the crippled Fukushima No. 1 nuclear power plant and the threat of more strong aftershocks after the magnitude 8.9 Great East Japan Earthquake struck off the coast on March 11, 2011.
The world's fifth largest quake - 1,000 times more powerful than the 1995 Great Hanshin Earthquake in 1995 - was caused by a rift near the boundary between the Pacific and North American tectonic plates. Massive tsunami ranging from 10 meters to 20 meters swallowed the coastline as a result of the Pacific plate slipping under Japan at the Japan Trench. This huge quake not only caused a displacement of 20 meters and a fault a few hundred kilometers long, but also shifted the Earth's axis by about 10 cm.
An earthquake of this scale could trigger other earthquakes at faults. What's unusual is to see so many aftershocks within a month, almost on a daily basis. The record number of aftershocks, not including those with a magnitude of 5.0 or less, is reported to be over 400 with a magnitude of 5.0 and higher, about 70 registering 6.0 and above and more than 5 at the 7.0 level or higher.
Japan has an average 300 earthquakes throughout Japan everyday.