Science Study suggests 2011 quake shifted Japan more than previously thought

When the magnitude 9.0 Tōhoku earthquake struck off Japan's northeast coast in 2011, the effects were felt far beyond the immediate disaster zone. Researchers say at least one powerful seismic wave travelled some 2,900 kilometres down to the boundary between Earth's mantle and its liquid outer core before bouncing back toward the surface. According to a new analysis of earthquake data collected across Japan, that returning wave may have produced an effect scientists had not previously identified.


GPS measurements taken after the earthquake showed that parts of Japan shifted eastward by about 5-6 millimetres. A team led by seismologist Sunyoung Park of the University of Chicago suggests the reflected seismic wave may have been responsible for that movement. The Tōhoku earthquake was one of the most powerful ever recorded. It triggered the devastating tsunami that struck northeastern Japan and led to the Fukushima Daiichi nuclear disaster. More than a decade later, researchers are still uncovering new details about how the quake affected Japan and the planet.

One of their models fit the observational data better than the others. In it, the returning shear wave triggered a broad pulse of fault slip at the interface between the two tectonic plates – not a major rupture, but a subtle one across a large area. Imagine you're pushing two rough surfaces at an angle against each other. Friction will keep them from moving until the point at which force overcomes it, and they suddenly jerk past each other. That, on a large scale, is what happens at the boundary between two tectonic plates during a megathrust earthquake like the one in Tōhoku, with the edge of one plate lurching beneath its neighbor. A slip is very similar, except the movement is much smaller. In this case, the researchers infer millimeters-to-centimeters of motion across a vast section of the plate boundary, producing GPS-detectable shifts of just a few millimeters at the surface. The researchers think the returning wave may have acted like a gentle shove applied to faults that were already under enormous stress from the main Tōhoku earthquake.

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