A seismic event bounced off Earth’s core and shifted an island country
A seismic event bounced off Earth’s core and shifted an island country
When massive 9.0 magnitude earthquake shook Japan on March 11, 2011, the ground also made a more lasting move. About 15 minutes after the event began at 2:46 p.m. local time, nearly the entire country shifted eastward, according to GPS station measurements.
The lurch was small — 5 to 6 millimeters, or 0.20 to 0.24 inches — but permanent and at the time went largely unnoticed or was passed off as a data glitch. However, University of Chicago geophysicist Sunyoung Park felt the recorded signals that indicated a shift pointed to something tangible. In fact, the ground movement reflected an “extraordinary” and previously undocumented seismic phenomenon, according to a new study.
“What was unusual about this movement is basically the whole of Japan was moving nearly uniformly at the same time,” said Park, who led the research.
She added that the movement, which affected mainland Japan — Hokkaido to Kyushu — an area approximately 1,800 miles (3,000 kilometers) in length, did not match the timing of the initial earthquake, and it happened before any significant aftershocks.
After years of analyzing GPS and seismic data, Park and her colleagues found that waves from the earthquake had traveled down to Earth’s core and then rebounded to the crust, displacing four major tectonic plates.
While seismologists knew that waves from large earthquakes can travel down through the planet and bounce off its outer core, which is liquid metal, they thought that the energy dissipated before returning to Earth’s crust.
“That type of deep-diving wave triggering some kind of event is new, and this event is very unusual, also in the sense that it’s so broad,” Park explained.
Though earthquakes can cause dramatic ground movement — tearing ruptures in land and moving larger areas by several inches — such movement is typically more localized than the country-long seismic event detected by Park and her colleagues.
Goran Ekstrom, a geophysicist at the Columbia University, said that In the 2011 earthquake, for example, the two plates sliding past each other under Japan moved by some 10 meters, said Goran Ekstrom, a geophysicist at the Columbia University.
“This rapid movement is what generated the ground shaking and the tsunami, and it also made the whole island of Honshu shift towards the East by 20 centimeters or so,” Ekstrom, who wasn’t involved in the study, said, referring to Japan’s largest island.
The displacement Park and her colleagues discovered, while smaller, is noteworthy because it occurred over such a large area, making it the broadest ever recorded, and it released about the same amount of energy as a 7.5 magnitude earthquake, according to a news release.
A new seismic hazard
The March 2011 earthquake, which struck 231 miles (372 kilometers) northeast of Tokyo, was the worst to ever hit Japan, triggering a huge tsunami and a nuclear crisis and killing an estimated 20,000 people. Park said policymakers should be aware of this previously unknown source of seismic hazard.
Unlike aftershocks, which can’t be precisely predicted, the round-trip journey to Earth’s core and back — about 3,600 miles — takes about 15 minutes, making it a seismic event that could be anticipated and potentially prepared for. However, because the energy of the seismic event was distributed over an extremely broad area, it would have been felt less strongly and caused less damage than a typical magnitude 7.5 earthquake, which would concentrate energy in a smaller area.
“Even if there was any damage, it would likely be very difficult to distinguish it from damage caused by the mainshock and the subsequent aftershocks,” Park said.
The 2011 shift caused by the core-visiting seismic wave encompassed the intersections of the Pacific and Okhotsk tectonic plates, and the boundary between the Philippine Sea and Eurasian plates. Tectonic plates are pieces of Earth’s rocky crust that are slowly and steadily moving.
The powerful shaking from the initial earthquake may have facilitated the arrival of the wave from the core, which reactivated the fault around the main quake as well as triggering movement along more distant plate intersections, Park said.
Japan has a “magnificent” network of seismic and satellite monitoring stations that make recording such an event possible, said Vedran Lekić, a professor in the department of geological, environmental and planetary sciences at the University of Maryland. But it is possible “this kind of phenomenon occurs elsewhere in poorly instrumented regions where it cannot be definitively documented.”
To the best of his knowledge, ground movement across a vast fault system, like the one that lies underneath Japan, has never previously been associated with the arrival of a seismic wave that bounces off the core, Lekić, who was not involved in the study, said via email.
Park and her colleagues said they considered other explanations for Japan’s shift eastward, including an undersea landslide, but the impact of such an event would be far more localized, they argued.
If their interpretation of the data is correct, the research is “very significant,” said Amanda Thomas, a geophysicist at University of California, Davis who also did not take part in the latest research.
“The study’s broader implication is that large earthquakes may continue influencing fault systems in unexpected ways for many minutes after the main rupture, not just through aftershocks but through the passage of later-arriving seismic waves,” she said.
“We still don’t fully understand how faults work and this sort of observation gives us another piece of the puzzle.”
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