Since 2011, scientists have been puzzled about the force resulting from a gigantic earthquake and tsunami that destroyed, among other things, Japan's Fukushima Nuclear Plant. Now, a Guinness World Record drilling expedition has solved the puzzle.
The catastrophic event, technically known as the Tōhoku earthquake and tsunami, occurred at the Japan Trench, an area off the country's east coast where the Pacific tectonic plate is being pushed under the Okhotsk plate (part of the North American Plate). When those plates slip, the resulting event is called a subduction-zone earthquake. When the slip is particularly large – as it was with Tōhoku – it is called a megathrust earthquake. In fact, the Tōhoku quake is considered the most powerful to ever hit Japan, and the fourth most powerful earthquake ever recorded, since seismology record keeping began in 1900.
Normally, in a megathrust quake, the slip between the plates occurs at a significant depth, and the rupture created is confined by a layer of "locked" rock up near the seafloor that acts like brakes on the tear. However, in the case of Tōhoku, the slip actually grew larger as it propagated upward and the rift reached the trench itself – an occurrence that has puzzled scientists for years.
But now, an international research expedition has uncovered the reason for the strange behavior and resultant power of the quake. A study on the discovery has been published in the journal Science.
Shallow slip
The researchers found that, in the case of Tōhoku, the normal layer of firm rock that usually sits between the plates actually consisted of a 30-meter-thick layer of pelagic clay, a soft, slippery substance that accumulated there over millions of years as microscopic particles settled. As the plates began to slip, this clay acted as a kind of earthquake lubricant, accelerating the movement rather than hindering it.
The result was 50-70 meters of shallow slip that displaced major sections of the seafloor and caused the thunderous shockwaves and gargantuan tsunami. The quake was so consequential that the island of Honshu was moved 2.4 m (8 ft) east, the Earth's axis was shifted an estimated 10-25 cm (4-10 inches), and its rotational speed increased by 1.8 microseconds per day.
"This work helps explain why the 2011 earthquake behaved so differently from what many of our models predicted," said study co-author Patrick Fulton, from Cornell University. "By seeing exactly how the fault zone is constructed, we can better understand where slip is likely to concentrate and how much tsunami potential a given subduction zone might have."
Record-breaking research
To reach their discovery, the scientists embarked on an expedition known as the Japan Trench Fast Drilling Project (JTRACK) in 2024, which you can learn more about in the following video.
That expedition used Japan's ultra-advanced drilling ship Chikyū, and marked the first time scientists had ever drilled directly into the fault zone of a recent megathrust earthquake. They drilled 7,906 m (almost 26,000 ft) beneath the sea surface, earning them a Guinness World Record for the deepest scientific drilling ever recorded.
The findings from the expedition and study, say the researchers, should help them better understand the seismic activity off Japan's coast, where the pelagic clay extends for hundreds of miles, making shallow-slip earthquakes more likely than previously thought.
"At the Japan Trench, the geologic layering basically predetermines where the fault will form," Fulton said. "It becomes an extremely focused, extremely weak surface, which makes it easier for ruptures to propagate all the way to the sea floor."
Fulton also says their new understanding of the mechanics at these types of fault zones could help them better assess earthquake and tsunami risks for other coastal communities.
Source: Cornell University
