The tidal wave scare that briefly disrupted one of Japan's most popular holidays Monday was just business as usual for a nation that always lives under threat of the earth - and sea - moving.

Japan's Meteorological Agency estimates nearly 10 percent of the energy released worldwide by earthquakes each year is concentrated in and around the string of islands that make up the nation of Japan.In Monday's scare, a 7.7 Richter Scale earthquake 20 kilometers under the sea off Japan's southernmost Okinawa islands triggered tidal waves that briefly forced authorities to evacuate people from coastal areas.

Japan is unusually prone to earthquakes because it is located at the meeting point of two key tectonic plates, massive slabs that make up the earth's crust.

Here, geologists say, the Philippine Sea plate is trying to force its way under the Eurasian plate.

According to the widely accepted "elastic rebound hypothesis," the Philippine Sea plate is pulling down the lip of the Eurasian, like someone bending a plastic ruler by pulling it down over the edge of a table.

People in Japan know all too well that when it goes too far it will spring back, causing a massive earthquake.

Although the entire nation is vulnerable to quakes, the possibility of a quake that could wipe out the heavily populated Tokyo area causes the most anxiety.