Japan's Nankai Trough earthquake advisory: 5 things to know

TOKYO -- Japan is on alert for the heightened possibility of a huge earthquake that could cause immense loss of life and property across a wide stretch of the central and western parts of the country.

For the first time, the Japan Meteorological Agency on Thursday issued a special bulletin and advisory saying that the odds of such a catastrophe occurring increased after a magnitude 7.1 quake struck off Miyazaki prefecture on the southern island of Kyushu earlier in the day, generating several small tsunami.

Situated on the "Pacific Ring of Fire" -- an arc of volcanoes and fault lines encircling the Pacific Basin -- Japan is one of the most earthquake-prone nations in the world. Experts have warned for decades that it is just a matter of time before a massive Nankai Trough earthquake occurs.

Here are five things to know about it and what might come next:

What is the Nankai Trough?

A trough is a long hollow in the seabed, like a trench. The Nankai Trough runs along the seafloor off Shizuoka prefecture, southwest of Tokyo, to Miyazaki prefecture on Kyushu. It is the meeting point of the Philippine Sea Plate and the Eurasian Plate, which are two of about 10 rock-hard tectonic plates that cover the Earth's surface.

The Philippine Sea Plate is consistently sinking beneath the Eurasian Plate at a rate of several centimeters per year, creating strain along the plates' boundaries in the process. When too much pressure accumulates, the Eurasian Plate bounces up and causes an earthquake.

Throughout history, the Nankai Trough has repeatedly caused earthquakes about once every 100 to 150 years. The last series of them was observed about 80 years ago, and experts have been warning that the next one has a 70% chance of occurring within 30