On March 11, 2011, a powerful 9.0 magnitude earthquake shook Japan, triggering an additional ground-moving phenomenon. Approximately 15 minutes after the initial tremor, recorded at 2:46 pm local time, most of Japanese territory moved towards the east, according to data collected by GPS stations.
This movement, although subtle – between 5 and 6 millimeters (0.20 to 0.24 inches) – was permanent in nature and was initially underestimated or dismissed as a measurement error. However, geophysicist Sunyoung Park, a researcher at the University of Chicago, interpreted the signals differently, seeing in them evidence of a real change. A recent study confirmed that the ground displacement represented an “extraordinary” and unprecedented seismic event.
Park, who led the investigation, highlighted the uniqueness of this event. “What was unusual about this movement is that basically all of Japan was moving almost uniformly and simultaneously,” said the geophysicist.
The researcher further explained that this displacement, which covered the mainland of Japan, stretching from Hokkaido to Kyushu for around 3,000 kilometers (1,800 miles), did not occur together with the main earthquake and preceded any relevant aftershock.
After extensive analysis of seismic and GPS data, Park and his team revealed that the tremor’s waves propagated all the way to the Earth’s core. These waves then returned to the crust, causing the displacement of four important tectonic plates.
Seismologists were already aware that waves generated by large earthquakes could cross the planet and reflect in the outer core, which is a layer of liquid metal. However, the prevailing belief was that this energy dispersed before reaching the Earth’s surface again.
Park emphasized the unprecedented nature of the find. “This kind of deep-diving wave that triggers some kind of event is something new, and this event is very unusual, also in the sense that it is so comprehensive,” said the geophysicist.
Earthquakes are known to cause significant ground movements, resulting in fissures and displacement of larger areas by many centimeters. However, these movements are often more geographically restricted than the overarching phenomenon that affected all of Japan, as identified by Park’s team.
Goran Ekstrom, a geophysicist at Columbia University, pointed out that in the 2011 earthquake, the two interacting plates beneath Japan moved approximately 10 meters.
Ekstrom, who was not involved in the study, explained that “this rapid movement is what generated the earthquake and tsunami, and also caused the entire island of Honshu to shift eastward by about 20 centimeters,” referring to the largest island in the Japanese archipelago.
The displacement identified by Park and his team, although smaller in magnitude, is notable for its geographic scope. It represents the most extensive movement ever recorded and released an amount of energy comparable to that of a 7.5 magnitude earthquake, according to information released in a statement.
Identification of a new seismic risk category
The March 2011 earthquake, whose epicenter was 372 kilometers (231 miles) northeast of Tokyo, is considered the most devastating in Japanese history. It caused a massive tsunami, a nuclear crisis and the loss of approximately 20,000 lives. Park stressed the importance of decision makers being informed about this previously ignored source of seismic hazard.
Unlike aftershocks, whose prediction is imprecise, the waves’ journey to the Earth’s core and return – a journey of around 5,800 kilometers – takes approximately 15 minutes. This interval provides a window to anticipate the event and potentially prepare. However, due to the wide distribution of seismic energy over a vast area, the perception and damage would be less than that of a conventional 7.5 magnitude earthquake, which concentrates its energy in a more restricted region.
Park mentioned that “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.”
The displacement observed in 2011, caused by the seismic wave that reached the Earth’s core, affected the junctions of the Pacific and Okhotsk tectonic plates, as well as the boundary between the Philippine Sea and Eurasian plates. Tectonic plates are segments of the planet’s rocky crust that move continuously and gradually.
Park suggested that the intense impact of the main earthquake may have facilitated the arrival of the wave from the core. This phenomenon would have reactivated the fault close to the epicenter of the main tremor and would also have activated movements at plate intersections located at greater distances.
Vedran Lekić, professor in the department of geological, environmental and planetary sciences at the University of Maryland, highlighted that Japan has a “magnificent” network of seismic and satellite monitoring stations, which made it possible to record an event of this nature. However, he warned that it is possible that “this type of phenomenon occurs in other regions with few monitoring instruments, where it cannot be definitively documented.”
Lekić, who was not involved in the research, added that, to his knowledge, ground movement on an extensive fault system, similar to that in Japan, had never been connected to the arrival of a seismic wave that reflects off the Earth’s core.
Park and his team analyzed other hypotheses for Japan’s eastward displacement, such as a possible submarine landslide. However, they argued that the effects of such an event would be considerably more localized.
Amanda Thomas, a geophysicist at the University of California, Davis, who was also not involved in the recent study, called the research “very significant” if the interpretation of the data is confirmed.
She explained that “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.”
He concluded: “We still don’t fully understand how faults work, and this type of observation gives us another piece of the puzzle.”