The displacement now discovered, although smaller, is notable because it occurred over such a large area, making it the largest ever recorded, and released approximately the same amount of energy as a magnitude 7.5 earthquake.
When a 9.0 magnitude earthquake struck Japan on March 11, 2011, the ground also underwent more lasting movement. About 15 minutes after the start of the event, at 2:46 pm local time, almost the entire country moved eastward, according to measurements from GPS stations.
The displacement was small — 5 to 6 millimeters — but permanent and, at the time, went virtually unnoticed or was attributed to a flaw in the data. However, geophysicist Sunyoung Park of the University of Chicago felt that the recorded signals indicating a shift pointed to something tangible. In fact, the ground motion reflected an “extraordinary” and previously undocumented seismic phenomenon.
“What was unusual about this movement is that basically all of Japan was moving almost uniformly at the same time,” says Park, who led the investigation.
The expert adds that the movement, which affected mainland Japan — from Hokkaido to Kyushu — an area approximately 3,000 kilometers long, did not coincide with the moment of the initial earthquake and occurred before any significant aftershocks.
After years of analyzing GPS and seismic data, Park and his colleagues discovered that waves from the earthquake traveled to the Earth’s core and then bounced off the crust, shifting four major tectonic plates.
Although seismologists knew that waves from large earthquakes can travel through the planet and bounce off its outer core, composed of liquid metal, they thought the energy dissipated before returning to the Earth’s crust.
“This kind of deep immersion wave that triggers some kind of event is new, and this event is very unusual, also in the sense that it is so comprehensive,” explains Park.
Although earthquakes can cause dramatic ground movements — ruptures in the ground and shifting large areas by several centimeters — this movement is typically more localized than the nationwide seismic event detected by Park and his colleagues. Goran Ekstrom, a geophysicist at Columbia University, says that, in the 2011 earthquake, for example, the two tectonic plates that were sliding past each other under Japan moved about 10 meters.
“This rapid movement was what generated the earthquake and tsunami, and also caused the entire island of Honshu to move to the east by about 20 centimeters”, points out Ekstrom, who was not involved in the study, referring to Japan’s largest island.
The displacement discovered by Park and his team, although smaller, is notable because it occurred over such a large area, making it the widest ever recorded, and released about the same amount of energy as a magnitude 7.5 earthquake, according to .
People taking refuge in a floating container are rescued from a building after the 2011 earthquake and tsunami in Miyagi Prefecture, in northeastern Japan. photo Kyodo/Reuters
A new seismic risk
The March 2011 earthquake, which struck an area 372 kilometers northeast of Tokyo, triggered a huge tsunami and a nuclear crisis, as well as killing around 20,000 people. Park says public policy chiefs should be aware of this previously unknown source of seismic risk.
Unlike aftershocks, which cannot be accurately predicted, the round trip to the Earth’s core — about 5,800 kilometers — takes approximately 15 minutes, making 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, it would have been 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 main earthquake and subsequent aftershocks,” says Park.
The 2011 shift caused by the seismic wave that hit the core spanned the intersections of the Pacific and Okhotsk tectonic plates, and the boundary between the Philippine Sea and Eurasian plates. The are portions of the Earth’s rocky crust that move slowly and constantly.
The strong shaking of the initial earthquake may have facilitated the arrival of the core wave, which reactivated the fault around the main earthquake, in addition to triggering movement along more distant plate intersections, says Park.
Japan has a “magnificent” network of seismic and satellite monitoring stations that make recording such an event possible, says Vedran Lekić, professor in the department of geological, environmental and planetary sciences at the University of Maryland. But it is possible that “this type of phenomenon occurs in other places, in regions with few instruments, where it cannot be definitively documented”.
According to Lekić, who was not involved in the study, to the best of his knowledge, ground movement along a vast fault system, like the one beneath Japan, has never been associated with the arrival of a seismic wave that bounces off the core.
Park and his colleagues say they have considered other explanations for Japan’s eastward shift, including an undersea landslide, but the impact of such an event would be much more localized, they argue.
If the interpretation of the data is correct, the research is “very significant,” says 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,” says Thomas.
“We still don’t fully understand how faults work, and this type of observation provides us with another piece of the puzzle.”
