A seismic shockwave traveling within the Earth moved the entire Japanese archipelago by 5 to 6 millimeters, posing a previously unknown Earthquake danger, according to a University of Chicago researcher.

Revealed in a recent paper published in the journal Science, Sunyoung Park and colleagues identified a shear wave bouncing off the Earth’s core after the Tohoku-Oki earthquake in 2011. According to the researchers, this specific type of rebound effect in the mainshock area after an earthquake has never been recognized before, offering new insights into the residual dangers of seismic events and how the layers of our planet interact.

Japan Shifts

As the best-recorded megathrust event in history, the 2011 Tohoku-Oki earthquake has already been well characterized in past studies. However, the team decided to take another look at the event, as monitoring stations throughout Japan detected the ScS shear wave’s return after hitting the core, as well as the archipelago’s eastward movement.

“When we examined the GPS time series in detail, we noticed a small, coherent eastward motion across much of Japan that followed seismic waves reflected from the core (the ScS phase) return to Japan,” Park told The Debrief. “This motion was puzzling because there was no known earthquake (aftershocks) associated with it at that timing, and so we wanted to understand what was really happening.”

According to the team, a previously unknown phenomenon produced the notable ground shift. In just those 13 minutes, a shear wave from the Tohoku-Oki earthquake raced through the planet, bounced off the core, and reactivated the tectonic plate beneath Japan. 

Analyzing the Earthquake Data

In their analysis, they found the rebound shear wave aligned with the tectonic plate boundaries, which, combined with weakened frictional forces after the mainshock, produced the shear wave reflection.

“We only gradually became convinced that this returning wave could actually trigger slip on multiple plate boundaries,” Park said. “It was particularly surprising that such a long-traveling wave (traveling ~5800 km, comparable to the radius of the Earth) could both trigger an event and do so over such a broad region, comparable to the length of Japan, involving several different plate interfaces.”

The movement is described as “steplike,” due to the small but sudden nature of the shift. In the data, the movement resembles a staircase: a flat position, followed by a sudden shift, then another flattening at a different level. Resolving that tiny step proved a challenge.

“In this case, the steplike motion is only a few to several millimeters, so it is relatively small compared to the noise and various processing artifacts that can appear in GPS records,” Park explained. “We did not process the GPS data ourselves; instead, we relied on a carefully processed data product. Even so, we had to be very cautious, because subtle signals can sometimes be introduced by the way the data are processed.”

The research reexamined the main shock source for any evidence of large underwater landslides to rule out other potential causes and found no other correlations.

A New Type of Earthquake Events

According to the team, it is no surprise that such events have previously gone unrecognized. For this rebound effect to occur, an extremely powerful earthquake is required to produce a noticeable effect, and dense, high-precision monitoring networks are needed to detect tiny motions over a large area. Additionally, most seismic studies focus on localized effects rather than viewing the aftermath on such a broad scale.

The slip is six to seven times that of the main shock rupture, making it the broadest single event rupture area ever recorded, doubling that of the great Sumatra Earthquake in 2004. According to the researchers, the Sumatra earthquake may be another example of this behavior, although no work has yet been done to investigate this.

From their work, the team says they cannot predict the exact amount of slip from such an event in the future, similar to the inability to predict normal earthquakes; yet, they can predict when the rebound effect will occur after the main thrust.

“I want to understand more generally how processes deep inside the Earth interact with shallow parts of the Earth,” Park concluded. “I am interested in why deep earthquakes happen and how they may be linked to the whole subduction system and the dynamics, from shallow to deep depth ranges.”

Park added that he is also finding “that these deep events can produce measurable deformation at the Earth’s surface, just like the step-like motions we see in this study,” all of which is helping researchers like him “better understand both these earthquakes and the structure of the Earth.”

The paper, “ScS-Triggered Slip on Megathrust Interfaces after the 2011 Mw 9.0 Tohoku-Oki Earthquake,” appeared in Science on June 18, 2026.

Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.

California could be due for a major earthquake in the near future, according to concerning new models based on data comprising 1,000 years of quake activity on the West Coast.

According to recent findings, tectonic stress has quietly been building in California to levels not seen since the last major earthquake in the region, which bore a magnitude of 7.9 and struck the West Coast nearly 170 years ago.

That event, known as the Fort Tejon earthquake, affected the greater Los Angeles area, and since the time of those historic rumblings, researchers have grown increasingly concerned about a period of seismic silence that could point to the likelihood of an event of similar magnitude at some point in the future.

The potentially concerning new forecast was detailed in a recent study by an international team of researchers led by Dr. Liliane Burkhard of the Division of Space Research and Planetary Sciences (WP) at the Physics Institute of the University of Bern, which focused on historic earthquake data as a means of gauging current stress loading at Cajon Pass.

The study, published in the Journal of Geophysical Research: Solid Earth, indicates tectonic stress levels that are now believed to exceed the highest seen during the years the team surveyed.

The “Earthquake Gate” at Cajon Pass

The researchers aimed to investigate stress that has slowly built along the San Andreas and San Jacinto faults and incorporated what they likened to an “earthquake gate” at Cajon Pass, which, in their models, functioned like a point of control that governed the size of earthquakes and whether they could spread to both systems.

For their research, they used a four-dimensional earthquake cycle model that allowed them to replicate certain processes both in spatial dimensions and across time.

Using a millennium of reconstructed earthquake data from the region based on geological information such as radiocarbon dates, unusual events recorded in dendrochronological data (i.e., information obtained from tree rings), and historical information sources, the team gave this to the model they devised.

Dr. Burkhard said the team’s simulation offered them a unique opportunity to better understand the ways stresses in the fault system build up over long periods of several centuries.

“By running the earthquake history of Southern California as a simulation, we can estimate the extent to which the fault system is already under stress today,” Dr. Burkhard said. The results of those simulations weren’t particularly promising either: according to Burkhard, current stress levels in the region are the highest they have been in 1,000 years, based on the data.

Concerning New Earthquake Data

The team also identified significant new information regarding the function of the so-called “earthquake gate” at Cajon Pass.

“The earthquake gate concept captures something important about how fault junctions work,” Burkhard said in a recent statement. “Cajon Pass doesn’t simply block or channel earthquakes: It responds to stress conditions, and those conditions change over centuries.”

Not only this, but Burkhard and her team also determined that a key element to these quakes involves not only the level of stress that accumulates at a single fault, but perhaps more significantly, the way stress between two systems can align. This is because, as the stress across a pair of faults rises together over longer periods, this appears to be most conducive to a large joint rupture comparable to the 1857 Fort Tejon earthquake.

Conversely, if stress levels are not in concert as they evolve, the team determined that this causes quakes that probably will not propagate beyond the junction.

In short, a more severe earthquake event today would entail a joint rupture that crosses the Cajon Pass, as opposed to one constrained either to the San Andreas or the San Jacinto Fault alone. Should such an event occur, it would likely impact one of the largest population centers, as well as a crucial infrastructure system that supplies energy and transportation to surrounding areas.

For Burkhard, “The question of when and how the next major earthquake will occur in this region is one of the most pressing problems in applied geoscience.” Based on the team’s findings, they say that their new modeling hopes to provide a much more complete picture of the science behind the factors that could lead to a major quake—conditions that could apply equally to other parts of the world as they do to California.

Burkhard adds that her team’s study is not intended merely as a prediction for when the next major earthquake could potentially occur, but also as a means of preparing for future quakes and mitigating their effects.

“What we can say is that the system is critically stressed and that physics-based models like ours give a clearer picture of the range of scenarios we should be prepared for,” Burkhard says. “This information is important for hazard assessment, infrastructure planning, and emergency preparedness.”

The team’s study, “Cajon Pass and the Southern San Andreas Fault System: Earthquake Cycle Stress Accumulation and Present‐Day Loading,” appeared in the Journal of Geophysical Research: Solid Earth.

Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. A longtime reporter on science, defense, and technology with a focus on space and astronomy, he can be reached at micah@thedebrief.org. Follow him on X @MicahHanks, and at micahhanks.com.

For more than four and a half millennia, the Khufu Pyramid has stood on the Giza plateau, enduring dozens of earthquakes without serious structural damage.

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