Solve the mystery of the Japan earthquake through a drilling mission

Since 2011, scientists have been puzzling over the force generated by the massive earthquake and tsunami that destroyed, among other things, the Fukushima nuclear plant in Japan. Now, a Guinness World Records excavation mission has solved the mystery.

The catastrophic event, technically known as the Tohoku earthquake and tsunami, occurred in the Japan Trench, an area off the country's east coast where the Pacific tectonic plate is being pushed beneath the Okhotsk plate (part of the North American plate). When those plates slide, the resulting event is called a subduction zone earthquake. When the slip is particularly large — as was the case with the Tohoku region — it is called a strong earthquake. In fact, the Tohoku earthquake is considered the most powerful earthquake ever to hit Japan, and the fourth most powerful earthquake ever recorded, since earthquake records began in 1900.

Normally, in a strong earthquake, slip occurs between plates at a great depth, and the resulting rupture is confined to a layer of “locked” rock near the seafloor that acts like a rupture brake. However, in the case of Tohoku, the size of the slip increased as it propagated upward and the fault reached the trench itself – an event that has puzzled scientists for years.

But now, an international research mission has revealed the cause of the strange behavior and the resulting force of the earthquake. A study on this discovery was published in Science magazine.

Shallow slip

The researchers found that in the case of Tohoku, the natural layer of solid rock that normally lies between the plates actually consists of a 30-meter-thick layer of surface clay, a soft, slippery material that has accumulated there over millions of years as microscopic particles settle. When the plates began to slide, this clay acted as a kind of earthquake lubricant, accelerating the movement rather than hindering it.

The result was a shallow slide 50 to 70 meters high that dislodged large portions of the seafloor and generated resounding shock waves and a massive tsunami. The earthquake had such great consequences that the island of Honshu moved 2.4 meters (8 ft) east, the Earth's axis shifted by 10-25 cm (4-10 in), and its rotation speed increased by 1.8 microseconds per day.

“This work helps explain why the 2011 earthquake behaved very differently than many of our models predicted,” said study co-author Patrick Fulton from Cornell University. “By seeing exactly how a fault zone is created, we can better understand where slip is likely to be concentrated and how likely a tsunami is to occur in a given subduction zone.”

Record-breaking research

To make their discovery, scientists set off on an expedition known as the Japan Trench Rapid Excavation (JTRACK) project in 2024, which you can learn more about in the following video.

JTRACK TRIP

That expedition used the advanced Japanese drill ship Chikyo, and it was the first time scientists had drilled directly into the fault zone of a recent powerful earthquake. They drilled 7,906 meters (about 26,000 feet) under the sea's surface, earning them the Guinness World Record for the deepest scientific drilling ever recorded.

The researchers say the findings from the mission and study should help them better understand seismic activity off the coast of Japan, where surface mud extends for hundreds of miles, making shallow earthquakes more likely than previously thought.

“In the Japan Trench, the geological strata essentially determine where the fault will form,” Fulton said. “It becomes a very concentrated and very weak surface, which makes it easier for cracks to propagate all the way to the seafloor.”

Fulton also says their new understanding of the mechanics in these types of fault zones could help them better assess earthquake and tsunami risks for other coastal communities.

Source: Cornell University

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