After 10 years from the devastating 2011 Tōhoku earthquake and tsunami, scientists are still unlocking the dark secrets of one of the most destructive natural disasters of all time. But now it seems that the big earthquake may have done what was once impossible; it may have permanently moved parts of Japan itself.
The findings were published in the journal Science. And they found that the powerful seismic wave of the magnitude 9.0 earthquake penetrated deep into the Earth, bounced off a layer nearly 2,900 kilometers below the surface and was strong enough to trigger further movement along already stressed fault lines.
What is the result? Parts of Japan seem to have shifted eastward by a few millimetres—and that change may be permanent.
The Tōhoku earthquake struck Japan’s northeastern coast in March 2011 and caused thousands of deaths and the Fukushima nuclear disaster. It is still one of the strongest earthquakes ever recorded and has been studied by scientists worldwide.
But at least after all the years of research, the earthquake is still teaching us new things.
Researchers analyzing data from Japan’s huge GPS monitoring network, GEONET, found something unusual. Some monitoring stations seemed to have moved around 5 to 6 millimetres eastward after the quake.
For the most part, scientists regarded the readings as just an error. Because the ground usually settles down after an earthquake and such small shifts can sometimes be caused by technical issues. But repeated analyses kept finding the same result. The team scrupulously ruled out other possible explanations such as underwater landslides and separate seismic events.
What remained was a fascinating possibility: the movement was real. A seismic signal called the ScS wave was discovered by the researchers. The wave moved across the Earth until it reached the rocky mantle and liquid core. It then reflected back to the surface.
By the time it returned to Japan, the fault systems in the area had already been under enormous stress from the main earthquake. Scientists believe the returning wave acted as a last push, triggering a synchronized slip along the boundary where tectonic plates meet.
The theory was backed by computer simulations. The reflected wave produced enough stress to release energy comparable to a magnitude 7.5 earthquake, according to the models.If confirmed, it would be the first known case of a fault-slip event being triggered by a seismic wave that traveled all the way to the Earth's core-mantle boundary and back.
The discovery shows how interrelated Earth’s interior is. A wave from one earthquake travels thousands of kilometres through the planet, returns to the surface and can change the position of a whole nation forever.
And for scientists it’s a reminder that the story of the 2011 Tōhoku disaster is still being written. And for the rest of us, this is a humbling reminder of those huge forces running through our lives, powerful enough to move mountains, reshape coastlines and maybe even shift a country itself.
