“Japan sits on the boundaries of four tectonic plates, ... The Nankai Trough is a 700-kilometer long (435-mile) subduction zone, which refers to when tectonic plates slip beneath each other.

This schematic diagram shows the Nankai Trough, which forms a trench-like topography on the seabed where the Philippine Sea Plate and the Eurasian Plate meet.

Japan issued a “megaquake advisory” following a 7.1-magnitude earthquake off its coast. That raised the risk of a larger quake on the Nankai Trough, an underwater subduction zone.

The Nankai Trough, where the Philippine Sea Plate is slipping under the Eurasia Plate at the bottom of the sea off the southwest coast of Japan, produces massive earthquakes about every 100-150 years.

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 Japan Meteorological Agency (JMA) issued its first advisory of a megaquake, along the Nankai Trough, which is a 900-km undersea fault that runs from Kyushu to Shizuoka, in August 2024. The ...

As the Philippine Sea Plate sinks beneath the Eurasian Plate, strain accumulates until it releases in the form of a powerful earthquake, known as a Nankai Trough earthquake.