11th March 2011 is a date that Japan will remember forever.

That’s because, in the space of six minutes, the island nation experienced an earthquake so severe that it caused a tsunami of unexpected size and destruction.

In total, the 9.1 magnitude Tōhoku earthquake and resulting tsunami caused the deaths of almost 20,000 people and a major accident at the Fukushima Daiichi nuclear power plant.

And now, after fifteen long years, we finally understand why.

Thanks to an international team of researchers, a recent study published in the journal Science has proven that the sheer magnitude of the earthquake was a result of a slippery layer of clay-rich mud, just a few meters thick, in the Japan Trench.

To gather information on the Tōhoku-oki fault, the researchers undertook a significant drilling project – setting a new world record in the process – drilling to 7,906 metres deep below the sea surface.

And what they discovered in the core samples they retrieved left them astounded. The muddy layer, a result of ancient sedimentary deposits, revealed for the first time how and why the 2011 earthquake was so devastating, as Australian National University’s Associate Professor Ron Hackney explained in a statement:

“This clay-rich ancient mud formed from microscopic particles that slowly settled on the seafloor beneath the Pacific Ocean over time – a process that took place over 130 million years – as the Pacific tectonic plate slowly moved west to ultimately be forced under Japan. The fault zone formed in that weak layer of clay as those sediments slowly slid under Japan, moving roughly 10 centimetres a year. Given that the weak clay layer is sandwiched between stronger layers of rock above and below, the clay acted like a natural ‘tear line’ that caused the fault to form within that layer of clay.”

So the slippery nature of the muddy clay layer caused the fault plane rupture to behave very differently to how scientists – and the Japanese government – would’ve ever expected.

That’s because, after the pressure had built up in the plates over hundreds of years, when they finally came together the slippery clay didn’t give the same resistance that hardier rock would, for example, meaning that the rupture continued along the fault, causing the sea floor to rise significantly – which in turn, led to Japan’s biggest ever tsunami, with 130 foot high waves.

This valuable research will help scientists and officials to understand how similar earthquakes may affect Japan in the future – as well as other countries with similar sedimentary layers, as Hackney continued:

“The rupture plane was just a centimetre or so thick, yet it allowed between 50 and 70 metres of movement on the fault and caused the seafloor off Japan to rise abruptly by several metres during the earthquake. There are indications that the sediments being drawn towards and under Sumatra may also contain a weak clay layer, which suggests that the giant 2004 Boxing Day tsunami may be linked to similar fault characteristics. Although we can’t be sure without extracting and analysing core samples directly from that fault.”

The more we know, the better we can prepare.

If you thought that was interesting, you might like to read about the mysterious “pyramids” discovered in Antarctica. What are they?