The magnitude 9.0 Tohoku-Oki earthquake shifted Japan eastward in 2011. What nobody noticed at the time was that about 13 minutes after the mainshock, monitoring stations across the country recorded another nearly simultaneous eastward shift of up to 6 millimeters, with no aftershock to explain it. It took years before anyone could say why.
A study published in Science traced the movement to seismic energy that had traveled thousands of miles into the Earth, bounced back from within the planet, and returned to trigger additional movement along fault lines beneath Japan. Seismologists had known these deep-traveling waves existed, but this is the first time one has been shown to cause tectonic plates to slip near the surface.
How the 2011 Tohoku-Oki Earthquake Sent a Wave to Earth’s Core
The March 11, 2011 earthquake struck off northeastern Japan and generated a tsunami that, together with the quake, killed about 20,000 people. It also produced one of the most comprehensive earthquake records ever gathered, thanks to Japan's network of seismic and GPS stations.
Stations throughout Japan detected a wave followed by a permanent eastward offset, even though no aftershock occurred at the right time or place to account for it. The study authors considered several possibilities. An underwater landslide or slow slip along a single fault would have been too localized to produce a nearly simultaneous shift across stations throughout the country.
The signal instead lined up with the arrival of an ScS wave, a type of wave that travels through the Earth's mantle, reflects at the boundary with the liquid outer core, and returns toward the surface. Think of it as a ball dropped through the Earth, bouncing off the core, and coming back up with enough force to move the ground above it. The round trip of roughly 3,600 miles (5,800 kilometers), about the distance from New York to London and back, took about 15 minutes.
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Why Japan’s Second Eastward Shift Went Undetected
The signal went undetected partly because seismic instruments are calibrated to pick up the sharp, fast-moving signals that conventional earthquakes produce. The slower signal returned from within the Earth was easier to miss, particularly amid the noise that followed a magnitude 9.0 event.
By comparing GPS measurements with seismic recordings across Japan, the researchers separated the permanent displacement from the passing wave. The event they identified is now the broadest seismic event on record, stretching across roughly 1,800 miles (3,000 kilometers) and the first known to involve multiple major tectonic plate boundaries simultaneously. Its energy output was comparable to a magnitude 7.5 earthquake, though it produced none of the surface shaking that would typically accompany an event of that size.
The Returning Wave Reactivated Two Tectonic Plate Boundaries
One boundary lies between the Pacific and Okhotsk plates near the mainshock zone, and another between the Philippine Sea and Eurasian plates farther south. The original earthquake appears to have weakened both, leaving them susceptible to the wave when it returned minutes later.
The finding doesn't establish that every major earthquake will produce the same effect, but it identifies a mechanism that hazard models have overlooked.
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Article Sources
Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:
- This article references information from a recent study published in Science: ScS-triggered slip on megathrust interfaces after the 2011 MW 9.0 Tohoku-Oki earthquake
