7.7-magnitude earthquake jolts Japan. What's happening beneath the surface?

Japan is one of the most earthquake-prone places on Earth, accounting for 10% of all tremors worldwide despite making up just 0.3% of the planet's total land mass.

In Short

• Powerful 7.4 magnitude earthquake hits off Japan's northeastern coast
• At least 30 people injured and 90,000 residents evacuated as precaution
• Japan's location at tectonic plate boundaries causes frequent earthquakes

Japan was shaken to its core once again on Monday, April 20, when a powerful earthquake struck off its northeastern coast, sending tsunami warnings flashing across television screens and prompting mass evacuations.

The jolt also reignited a familiar dread that has long haunted this seismically restless nation.

The quake struck at 1:23 pm IST at a depth of 19 kilometres, registering a magnitude of 7.7, according to the Japan Meteorological Agency.

The epicentre was located approximately 100 kilometres east of Miyako in Iwate Prefecture, along the Sanriku coast, which is one of the most tsunami-vulnerable stretches of coastline on earth.

A tsunami warning with the threat of waves up to 3 metres was issued for Iwate, Aomori, and Hokkaido prefectures.

Japan's Prime Minister Sanae Takaichi, and the local media urged residents to move to higher ground, away from rivers and coastlines. The quake triggered tsunamis, injured at least 30 people, and prompted evacuation orders for some 90,000 residents.

WHY DOES JAPAN KEEP HAVING EARTHQUAKES?

Japan is one of the most earthquake-prone places on Earth, accounting for 10% of all tremors worldwide despite making up just 0.3% of the planet's total land mass.

The reason is geology.

Japan sits wedged among four major tectonic plates, including the Pacific, Philippine Sea, Eurasian and North American.

The Pacific Plate constantly grinds beneath the Okhotsk Plate at the Japan Trench, while the Philippine Sea Plate pushes under central and southwest Japan at the Nankai Trough.

As these oceanic plates burrow beneath continental ones, pressure builds over decades until the crust buckles and snaps, releasing energy as an earthquake. When this happens under the ocean, it also displaces seawater vertically, causing tsunamis.

Japan, sitting at the crossroads of all this geological violence, has little choice but to live with the consequences.

The Sanriku coast, where the earthquake struck today, is no stranger to catastrophe.

The 2011 Tohoku earthquake and tsunami, which killed nearly 20,000 people and triggered the Fukushima Daiichi nuclear disaster, originated from precisely this same seismic zone.

Today's quake struck the same boundary where the Pacific plate grinds beneath the North American plate along the Japan Trench, the deep underwater scar off Japan's eastern coast.

WHAT IS A MEGAQUAKE?

Monday's tremor naturally led to many wondering whether a megaquake could strike Japan as a follow-up to the quake. The frenzy behind a potential megaquake is often what follows whenever a strong quake hits the region.

According to Japan's Headquarters for Earthquake Research Promotion, any earthquake measuring magnitude 8.0 or higher qualifies as a mega-quake. These typically occur when one tectonic plate slides beneath another in subduction zones.

Today's 7.7 does not meet that threshold.

Japanese warning guidelines, however, say that the probability a large earthquake will follow a magnitude-7 within a week is roughly "once per a few hundred times", according to a 2023 study.

News agency AFP has reported in the past that Japan's central government had predicted in 2022 that an earthquake of that scale would hit in the next 30 years with a rough 70 per cent probability.

That slim but real possibility is what officials, and millions of anxious Japanese citizens, are watching for.

- Ends