Aerial view of damage in Wakuya, Japan, following the 9.0 magnitude earthquake and subsequent tsunami in 2011
An extremely unusual tectonic movement occurred 15 minutes after the 2011 Tohoku earthquake, causing almost all of Japan to move five millimeters to the east.
About 15 minutes after the earthquake magnitude 9 that hit Tohoku on March 11, 2011, almost all of Japan advanced half a centimeter to the east.
This small shift resulted from a extremely powerful seismic wavewhich traveled 5,800 kilometers to the Earth’s core and was then reflected back towards the surface.
In the context of the devastation caused by the earthquake, including local ground displacements of several meters and 40 meter tsunami waveswhich led to the meltdown of the core of three reactors at the nuclear power plant, five millimeters may seem insignificant.
But this movement occurred over a distance of 3000 kilometersalmost seven times greater than the length of the main rupture line of the earthquake and greater than any landslide ever recorded.
What also makes this case unusual is the moment in which it occurred and the pattern observed, he says. Sunyoung Parkfrom the University of Chicago.
“We see a small jump of five millimeters to the east that happens almost simultaneously and with similar size in practically all of Japanwithout any common earthquake having occurred exactly at that moment”, says the researcher, quoted by .
The displacement was not only immense in its north-south extension: its width covered all of Japan and extended beyond the archipelagotowards the ocean.
“It wasn’t just a narrow ‘margin’ that moved“, says Park. “The eastward jump extends at least throughout Japan where there are GPS stations. If we had equally dense instruments on the seabed, we could say with greater precision how far this movement continued offshore; but on land, displacement is observed practically throughout Japan”.
By analyzing large volumes of GPS data and seismic data recorded during the catastrophe, Park and his colleagues were able to see how such a vast and why the displacement occurred only 15 minutes after the main earthquake.
Earthquakes often generatenda that travel to deep areas from the Earth’s interior and are reflected by the core, but these tend to arrive quite weakened after traveling their way to the center of the planet and returning to the surface.
No caso de Tohoku, the main shock was so intense that the original wave, although weakened, maintained sufficient energy on return to the surface to cause displacement across the country as four adjacent tectonic plates moved together.
“We think that the violent shaking caused by the original Tohoku earthquake may have already weakened the boundaries between platesmaking them more susceptible to moving when the wave reflected by the core arrives,” says Park.
The episode demonstrates that there are as yet unrecognized destruction mechanisms that can follow earthquakes, argues Park. “This shows that after a big earthquake, we may also have to be attentive to potential seismic risks associated with arrival of waves that have traveled to great depth and that can trigger new events, even at very large distances.”
“This shows that large earthquakes can trigger, minutes laterwidespread and delayed fault movements, in regions much larger than expected”, says Lee.
