A new study discovers seismic phenomenon

When a massive 9.0-magnitude earthquake struck Japan on March 11, 2011, the ground also moved more sustainably. About 15 minutes after the event began at 2:46 pm local time, almost the entire country turned eastward, according to GPS station measurements.

The swing was small — five to six millimeters, or 0.20 to 0.24 inches — but it was permanent and at the time went largely unnoticed or dismissed as a data glitch. However, University of Chicago geophysicist Sunyoung Park felt that recorded signals indicating a shift were pointing to something tangible. In fact, the ground motion reflected an “unusual” and previously undocumented seismic phenomenon, according to a new study.

“What’s unusual about this movement is that all of Japan was moving almost uniformly at the same time,” said Park, who led the research.

She added that the movement, which affected mainland Japan — from Hokkaido to Kyushu — an area about 1,800 miles (3,000 kilometers) long, did not match the timing of the initial quake, and occurred before any major aftershocks.

After years of analyzing GPS and seismic data, Park and her colleagues found that waves from the quake traveled to the Earth’s core and then bounced back into the crust, displacing four major tectonic plates.

While seismologists knew that waves from large earthquakes could travel through the planet and bounce off its outer core, which is liquid metal, they believed that the energy was dissipated before it returned to the Earth’s crust.

“This kind of deep wave triggering some kind of event is something new, and this event is very unusual, and also in the sense that it’s very broad,” Park explained.

Although earthquakes can cause dramatic ground motion — rupturing cracks in the ground, moving larger areas by several inches — this motion is usually more localized than the nationwide seismic event that Park and her colleagues discovered.

In the 2011 earthquake, for example, plates sliding close to each other under Japan moved about 10 meters, said Goran Ekstrom, a geophysicist at Columbia University.

“This rapid movement is what caused the ground to shake and cause the tsunami, and it also caused the entire island of Honshu to shift eastward by 20 centimeters or so,” said Ekström, who was not involved in the study, referring to Japan’s largest island.

The displacement detected by Park and her colleagues, although smaller in scale, is noteworthy because it occurred over a large area, making it the largest scale ever recorded, and released the same amount of energy as a 7.5-magnitude earthquake, according to a press release.

New seismic danger

The March 2011 earthquake, which struck an area 231 miles (372 kilometers) northeast of Tokyo, was the worst ever to hit Japan, triggering a massive tsunami and nuclear crisis and killing an estimated 20,000 people. Park said decision makers should be aware of this previously unknown source of earthquake risk.

Unlike aftershocks, which cannot be accurately predicted, the round trip to the Earth’s core and back — about 3,600 miles — takes about 15 minutes, making it a seismic event that can be anticipated and prepared for. However, because the energy of the seismic event was distributed over a very wide area, it may 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 likely be very difficult to distinguish it from damage caused by the mainshock and subsequent aftershocks,” Park said.

The 2011 shift caused by a seismic wave visiting the core included the junctions of the Pacific and Okhotsk tectonic plates, and the boundary between the Philippine Sea and the Eurasian plates. Tectonic plates are pieces of the Earth’s rocky crust that move slowly and steadily.

Park said the strong shaking from the initial quake may have facilitated the arrival of the wave from the quake’s core, reactivating the fault around the main quake as well as stimulating movement along distant plate intersections.

Vedran Lekic, a professor in the Department of Geological, Environmental and Planetary Sciences at the University of Maryland, said Japan has a “remarkable” network of earthquake monitoring stations and satellites that make recording such an event possible. But it is possible that “this type of phenomenon occurs elsewhere in poorly equipped areas where it cannot be definitively documented.”

To his knowledge, ground motion across an extensive fault system, such as the one that lies beneath Japan, had not previously been linked to the arrival of a seismic wave bouncing off the Earth’s core, Lekic, who was not involved in the study, said by email.

Park and her colleagues have considered other explanations for Japan’s eastward shift, including an undersea landslide, but the impact of such an event would be more local, they said.

If their interpretation of the data is correct, the research is “extremely important,” said Amanda Thomas, a geophysicist at the University of California, Davis, who was also not involved in the latest research.

“The broader implication of the study is that large earthquakes may continue to affect fault systems in unpredictable ways for several minutes after the main rupture, not only through aftershocks but through the passage of subsequent seismic waves,” she said.

“We still don’t fully understand how faults work, and this kind of monitoring gives us another piece of the puzzle.”

Katie Hunt, CNN

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