When a massive 9.0 magnitude earthquake shook Japan on March 11, 2011, the ground also suffered a more lasting shift. About 15 minutes after the start of the event, at 2:46 pm (local time), practically the entire country moved to the east, according to measurements from GPS stations.
The displacement was small, 5 to 6 millimeters, but permanent; at the time, it went unnoticed by most or was attributed to a glitch in the data. However, geophysicist Sunyoung Park, from the University of Chicago, believed that the recorded signals, which indicated such a displacement, pointed to something concrete.
In fact, the ground motion reflected an “extraordinary” and previously undocumented seismic phenomenon, according to a new study.
“What was unusual about this movement is that basically all of Japan moved almost uniformly and simultaneously,” said Park, who led the research.
She added that the movement, which affected Japan’s mainland from Hokkaido to Kyushu and spanned approximately 3,000 kilometers, did not coincide with the timing of the initial earthquake and occurred before any significant aftershocks.
After years of analyzing GPS data and seismic records, Park and his colleagues discovered that the waves generated by the earthquake propagated to the Earth’s core and then ricocheted back to the crust, shifting four large tectonic plates.
Although seismologists already knew that waves from large earthquakes can travel through the planet’s interior and reflect off the outer core, composed of liquid metal, it was believed that the energy dissipated before returning to the Earth’s crust.
“This kind of wave that penetrates deeply and triggers some event is something new; moreover, this event is very unusual because of its vast scope,” Park explained.
Although earthquakes can cause drastic ground movements—causing ruptures in the earth and shifting large areas by several centimeters—such movements are often more localized than the nationwide seismic event detected by Park and his team.
Goran Ekstrom, a geophysicist at Columbia University, observed that in , for example, the two plates sliding past each other under Japan moved about 10 meters.
“This rapid movement was what generated the ground shaking and the tsunami, in addition to causing the entire island of Honshu to move about 20 centimeters to the east,” said Ekstrom — who was not involved in the study — referring to .
The displacement discovered by Park and his colleagues, although smaller, is notable for occurring over such a vast area — being the largest ever recorded — and for releasing an amount of energy equivalent to that of a , according to a press release.
A new seismic risk
The March 2011 earthquake, which hit a region 372 kilometers northeast of Tokyo, was the worst ever to occur in Japan; it triggered a gigantic tsunami and a nuclear crisis, in addition to causing the deaths of around 20,000 people. Park said authorities should be aware of this previously unknown source of seismic risk.
Unlike aftershocks (aftershocks), which cannot be predicted accurately, the round trip to the Earth’s core — a distance of about 5,800 kilometers — takes approximately 15 minutes.
This makes it a seismic event that can be predicted and prepared for. However, because the energy from the seismic event was distributed over an extremely wide area, the shaking was felt less intensely and caused less damage than a typical magnitude 7.5 earthquake, which would concentrate the energy in a smaller area.
“Even if there was some damage, it would probably be very difficult to distinguish it from the damage caused by the mainshock and subsequent aftershocks,” Park said.
The 2011 displacement, caused by the seismic wave that traveled its way to the core, encompassed the areas where the Pacific and Okhotsk tectonic plates meet, as well as the boundary between the Philippine Sea and Eurasian plates.
Tectonic plates are fragments of the Earth’s rocky crust that move slowly and steadily. The strong shaking of the initial earthquake may have facilitated the arrival of the wave from the core, which reactivated the fault in the vicinity of the main earthquake and triggered movement along more distant plate encounter zones, Park said.
Japan has a “magnificent” network of seismic and satellite monitoring stations that makes it possible to record such an event, said Vedran Lekić, a professor in the Department of Geological, Environmental and Planetary Sciences at the University of Maryland. However, it is possible that “this type of phenomenon occurs elsewhere, in regions with little instrumentation, where it cannot be definitively documented.”
To our knowledge, ground movement in a vast fault system, like the one that runs beneath Japan, has never been associated with the arrival of a seismic wave that is reflected in the core, Lekić, who was not involved in the study, said in an email.
Park and his colleagues said they had considered other explanations for Japan’s eastward shift, including an underwater landslide, but argued that the impact of such an event would be much more localized.
If their interpretation of the data is correct, the research is “very significant,” said Amanda Thomas, a geophysicist at the University of California, Davis, who also was not involved in the latest study.
“The broader implication of the study is that large earthquakes can continue to influence fault systems in unexpected ways for many minutes after the main rupture, not only through aftershocks but also through the passage of later-arriving seismic waves,” she said.
“We still don’t fully understand how faults work, and this type of observation provides us with another piece of the puzzle,” he concluded.
