Japan is one of the world's most earthquake-prone countries, with a tremor occurring at least every five minutes.

Located in the "Ring of Fire" arc of volcanoes and oceanic trenches partly encircling the Pacific Basin, the country accounts for about 20 percent of the world's earthquakes of magnitude 6.0 or greater. Tokyo, with a population of 12 million, sits on the junction of four tectonic plates: the Eurasian, North American, Philippine and Pacific.

The sudden bending or breaking of any plate can trigger an earthquake.

Source: Reuters.com