A giant seismic wave bounced off the Earth’s core and may have traveled to Japan: ScienceAlert

When the 9.0-magnitude Tohoku earthquake struck off the coast of Japan in 2011, its seismic tremors did more than just ripple across the planet.

At least one wave traveled 2,900 kilometers (1,800 mi) to the boundary between the Earth’s mantle and the liquid outer core, where it reflected back to the surface.

There, according to a new analysis of seismic data from across Japan, it may have done something that scientists have never identified before.

GPS observations from the time of the quake showed that parts of Japan moved eastward by as much as 5 to 6 mm.

A team led by seismologist Sunyoung Park of the University of Chicago says the reflected wave may be what gave Japan an eastward push.

Earthquakes are among the most devastating natural disasters our planet can be exposed to, and the Tohoku earthquake, which generated the tsunami that triggered the earthquake Fukushima Daiichi nuclear disasterwas one of The most powerful thing ever recorded.

It happened when the Pacific plate suddenly slid beneath the plate bearing northern Japan, generating a devastating tsunami and sending seismic waves racing across the planet.

The Tohoku earthquake remains one of the most closely studied natural disasters in history. Scientists are still combing through the observations they generate, looking for clues about how major earthquakes appear and what happens in their wake.

The event was so massive that it produced an unusually clear ScS signal in Japan’s GNSS Earth Observing Network system (GeoNet). This name means a Shear wave (S) which is reflected at the core-mantle boundary (c), and returns as another shear wave (S).

The amplitude of this ScS wave was so large that it could have been detected in China.

This is interesting in itself because GNSS measures ground motion, not seismic waves directly. It is not a traditional seismometer.

So, researchers were looking at this signal to see what it could tell us about the earthquake itself. That’s when they noticed something strange.

After the seismic wave passes, the Earth is expected to return to its starting position. However, the researchers noted that some GPS stations in Japan appeared to have been shifted slightly to the east compared to their starting locations.

The obvious explanation is that this is a glitch, possibly the result of the data processing process.

But when the researchers took this into account and tried to correct it, the shift persisted, suggesting that it was real and permanent. This cannot be easily explained by other possibilities, such as a large underwater landslide or known main shock rupture.

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What’s more, the obvious shift occurred just as the ScS wave arrived in Japan after its journey to the core-mantle boundary and back.

This suggested to the scientists that they were looking for something new, so they set about modeling to try to determine which process could reproduce the observed signal.

One of their models fits the observational data better than the others.

In this case, the returning shear wave caused a broad pulse of fault slip at the interface between tectonic plates – not a large rupture, but a micro-rupture across a large area.

Imagine that you are pushing two rough surfaces at an angle against each other. Friction will prevent them from moving until the point where the force overcomes them, and they suddenly vibrate against each other.

This, on a large scale, is what happens at the boundary between two tectonic plates during a powerful earthquake like the one in Tohoku, where the edge of one plate buckles under its neighbour.

Gliding is very similar, except that the motion is much smaller. In this case, the researchers infer millimeter-to-centimeter movement across a large portion of the plate boundary, resulting in GPS-detectable shifts of just a few millimeters at the surface.

The researchers believe that the returning wave may have been a gentle push applied to faults that were already under tremendous pressure from the main Tohoku earthquake.

Although the ScS wave was much weaker than the original tremor, it reached most of Japan at about the same time. The team argues that this synchronized pulsation may have been enough to stimulate a small amount of movement along the already stressed plate boundaries.

The inferred event was surprisingly large on the one hand and small on the other.

Researchers estimate that it released a total of energy equivalent to that of a 7.5-magnitude earthquake.

But instead of occurring as a single rupture, the movement spread across a wide area of ​​the plate boundary, resulting in just millimeters to centimeters of sliding and little additional shaking.

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According to the researchers, if this interpretation is correct, this would be the first known example of a fault-slip event caused by a seismic wave reflecting from the core-mantle boundary.

Future observations of large earthquakes may be needed to confirm this – but this confirmation is worth seeking out, because the results suggest that earthquakes may contain hidden complexities that we may have overlooked.

“The observation underscores the importance of considering this previously unrecognized source of seismic hazard resulting from the potential (re)activation of the mainshock zone and its surrounding area, even tens of minutes after the mainshock.” The researchers write.

The results have been published in sciences.