Single wave as intense as 7-magnitude earthquake bounced off Earth's core and shifted Japan

The Tōhoku earthquake that struck off the coast of Japan in 2011 significantly shifted some parts of the country from their current place, a study has found. This is the first time that an earthquake jolted a country into changing its position. A new analysis shows that at least one seismic wave from the massive earthquake was so intense that it travelled 2,900 kilometres inside Earth, to the boundary between Earth's mantle and liquid outer core, and shot upwards. GPS observations showed that Japan had shifted to the east by up to 5 to 6 millimetres. Seismologists think that this reflected wave was responsible for the movement. The findings have been published in Science.

The Tōhoku earthquake triggered a massive tsunami and killed thousands. It remains one of the most studied natural disasters. The researchers studied the ScS signal, or the wave that returned from deep inside Earth, and was recorded by Japan's GNSS Earth Observation Network System (GEONET).

GPS stations shifted after 2011 earthquake

They found that some GPS stations in Japan were not in their original position and had shifted slightly eastward. Ideally, after an earthquake, the ground returns to its starting position. So the researchers believed it to be a glitch and tried correcting the data. Also Read: 100 foot tsunami, vigorous shaking: This is what a megaquake would look like in Japan, according to official projections

Surprisingly, it still showed a shift in the position of the stations. It could not be explained by any other event either, such as a large underwater landslide. This led them to conclude that the shift was real and permanent. It had occurred when the ScS wave arrived back in Japan after travelling to the core-mantle boundary. Was this wave responsible for the change?

Models confirmed movement in Japan

To know, the researchers created models recreating the wave. One of them showed that the shear wave that returned triggered a broad pulse of fault slip where the two tectonic plates met. This is what happens at the boundary between two tectonic plates during a megathrust earthquake, such as the one in Tōhoku. Also Read: Can a magnitude 9 earthquake hit Earth? This fault has a history of terror tremors

When the faults were already under tremendous pressure from the main Tōhoku earthquake, the returning wave added to the stress. Despite being much weaker, it was able to push the faults, leading to a permanent shift. The ScS wave arrived across Japan at the same time, and so this synchronised pulse had the force to move the stressed plate boundaries. The researchers estimated that this event released energy almost equivalent to a magnitude 7.5 earthquake. If their findings are correct, it would be the first known example of a fault-slip event caused by a seismic wave reflected from the core-mantle boundary.