Japan isn’t quite where it once was. Scientists say a newly recognized seismic event is to blame
With the 9.0 magnitude earthquake that shook Japan on March 11, 2011, the ground also made a more permanent move. According to GPS station measurements, nearly the entire country shifted eastward about 15 minutes after the event began at 14:46 local time.
The tremor was small (5 to 6 millimeters, or 0.20 to 0.24 inches) but persistent and was largely unnoticed at the time or ignored as a data error. But University of Chicago geophysicist Sunyoung Park felt that the recorded signals indicating a change pointed to something concrete. In fact, the ground motion reflected an “extraordinary” and previously undocumented seismic event. to a new study.
“What was unusual about this movement was that basically all of Japan moved in almost the same way at the same time,” said Park, who led the research.
He added that the movement, which affected an area about 1,800 miles (3,000 kilometers) long from mainland Japan Hokkaido to Kyushu, did not match the timing of the initial earthquake and that it occurred before any significant aftershocks.
After years of analyzing GPS and seismic data, Park and his colleagues found that waves from the earthquake propagated into the Earth’s core and then returned to the crust, displacing the four major tectonic plates.
While seismologists knew that waves from large earthquakes could travel across the planet and be reflected from its outer core, which is liquid metal, they thought the energy dissipated before returning to the Earth’s crust.
“This type of deep-diving wave that triggers an event is new, and this event is also very unusual in that it is very large,” Park explained.
Although earthquakes can cause dramatic ground movements—creating cracks in the land and moving larger areas by several centimeters—such movements are generally more local than the nationwide seismic event detected by Park and colleagues.
In the 2011 earthquake, for example, two plates sliding past each other under Japan moved about 10 meters, said geophysicist Goran Ekstrom of Columbia University.
“This rapid movement created the earthquake and tsunami, and also caused the entire island of Honshu to shift eastward by about 20 centimeters,” said Ekstrom, who was not involved in the research, referring to Japan’s largest island.
The displacement Park and colleagues discovered, although smaller, is remarkable because it occurred over such a large area, making it the largest ever recorded and releasing nearly the same amount of energy as a 7.5-magnitude earthquake. newsletter.
People in a floating container were rescued from a building following the earthquake and tsunami in Miyagi Prefecture, northeastern Japan. – Kyodo/Reuters
A new seismic hazard
The earthquake occurred 231 miles (372 kilometers) northeast of Tokyo in March 2011. It was the worst incident to hit Japan It triggered a massive tsunami and nuclear crisis, killing an estimated 20,000 people. Park said policymakers should be aware of this previously unknown source of seismic hazard.
Unlike aftershocks, which cannot be predicted with certainty, the round-trip journey to the Earth’s core (about 3,600 miles) takes about 15 minutes, making it a seismic event that can be predicted and potentially prepared for. However, because the energy of the seismic event was distributed over an extremely wide area, it would be felt less strongly and cause less damage than a typical 7.5 magnitude earthquake, which would concentrate the energy in a smaller area.
“Even if there was any damage, it would probably be very difficult to separate it from damage caused by the main shock and subsequent aftershocks,” Park said.
The 2011 shift, caused by a seismic wave visiting the core, spanned the intersections of the Pacific and Okhotsk tectonic plates and the boundary between the Philippine Sea and Eurasian plates. tectonic plates are pieces of the Earth’s rocky crust that move slowly and steadily.
The strong shaking from the initial quake may have facilitated the wave’s arrival from the core, which reactivated the fault around the main earthquake as well as triggered movement along more distant plate intersections, Park said.
Vedran Lekić, a professor in the department of geology, environmental and planetary sciences at the University of Maryland, said Japan has a “magnificent” network of seismic and satellite monitoring stations that made it possible to record such an event. But it is possible that “such a phenomenon may occur elsewhere in poorly equipped regions where it has not been documented with certainty.”
Lekić, who was not involved in the study, said via email that, to his knowledge, ground motion along a large fault system like the one beneath Japan has never before been associated with the arrival of a seismic wave bouncing off the core.
Park and his colleagues said they considered other explanations for Japan’s eastward drift, including an undersea landslide, but argued that the impact of such an event would be much more local.
Amanda Thomas, a geophysicist at the University of California at Davis who was not involved in the latest research, said the research is “very important” if the interpretation of the data is correct.
“The broader implication of the study is that large earthquakes can continue to affect fault systems in unexpected ways for several minutes after the main rupture, not just through aftershocks but also through the passage of subsequent seismic waves,” he said.
“We still don’t fully understand how bugs work, and this kind of observation gives us another piece of the puzzle.”
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