Japan issues tsunami advisory after 6.9 magnitude earthquake

A strong earthquake with a preliminary magnitude of 6.9 hit the Kyushu region at 9:19 pm local time in southwestern Japan on Monday (January 13), according to the Japan Meteorological Agency. It said the quake was centred at a depth of 30 kilometres.

Tsunami advisories have been issued for waves up to one metre high in the Miyazaki Prefecture in Kyushu, as well as the Kochi Prefecture nearby, the agency said. Residents in coastal Kochi were asked to evacuate as a precautionary measure. There have been no reports of damage so far.

Public broadcaster NHK TV reported that a 20-centimetre-high tsunami had hit Miyazaki city, which has a population of about 400,000. NHK also reported that the agency is investigating whether the current quake is related to the Nankai Trough, for which it had issued a megaquake advisory last August.

Japan’s position along the “Ring of Fire” makes it vulnerable to frequent earthquakes. We explain.

What is the Ring of Fire?

The Ring of Fire is a string of hundreds of volcanoes and earthquake sites which runs along the Pacific Ocean. It is shaped like a semicircle or horseshoe and stretches nearly 40,250 kilometres. The Ring of Fire traces the meeting points of numerous tectonic plates, including the Eurasian, North American, Juan de Fuca, Cocos, Caribbean, Nazca, Antarctic, Indian, Australian, Philippine, and other smaller plates, which all encircle the large Pacific Plate, according to a report by National Geographic.

It runs through 15 more countries including the USA, Indonesia, Mexico, Japan, Canada, Guatemala, Russia, Chile, Peru, and the Philippines.

Why is the Ring of Fire vulnerable to earthquakes?

The Ring of Fire witnesses so many earthquakes due to constant sliding past, colliding into, or moving above or below each other of the tectonic plates. As the edges of these plates are quite rough, they get stuck with one another while the rest of the plate keeps moving. An earthquake occurs when the plate has moved far enough and the edges unstick on one of the faults.

Japan experiences earthquakes due to the interactions of four tectonic plates — the Pacific Plate, the Philippine Sea Plate, the Okhotsk Plate and the Eurasian Plate.

Why are there so many volcanoes in the Ring of Fire?

The existence of volcanoes in the Ring of Fire is also due to the movement of tectonic plates. Many of the volcanoes have been formed through a process known as subduction. It takes place when two plates collide with each other and the heavier plate is shoved under another, creating a deep trench.

“Basically, when a ‘downgoing’ oceanic plate [like the Pacific Plate] is shoved into a hotter mantle plate, it heats up, volatile elements mix, and this produces the magma. The magma then rises up through the overlying plate and spurts out at the surface,” which leads to the formation of volcanoes, according to a report by DW.

Most subduction zones on the planet are located in the Ring of Fire and that’s why it hosts a large number of volcanoes.