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.

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Around 66 million years ago, an asteroid wiped out 75% of the species on Earth. This event caused the extinction of all dinosaurs except birds, referred to as non-avian dinosaurs. However, an open question in paleontology is whether the dinosaurs were already declining before the asteroid hit. Many scientists think that around 5 to 10 million years before the extinction event, dinosaurs became less biologically diverse across America, making them more prone to extinction. 

To investigate this hypothesis, scientists recently revisited fossils in a rock layer from the end of the dinosaur era that previous researchers had incorrectly dated. The bulk of the fossils from this period had previously been collected from a different section of rocks called the Hell Creek Formation in Montana. These researchers studied a rock layer in New Mexico that is exposed to the surface, known as the Naashoibito Member. It’s approximately 30 meters (about 100 feet) thick and formed from ancient river sediments. 

To determine when the Naashoibito Member formed, the scientists measured the decay of radioactive elements within its mineral grains over time, a technique called radiometric dating. The scientist reasoned that these mineral grains must have crystallized before being transported and incorporated into the Naashoibito Member, so their youngest ages should provide a maximum depositional age. 

They determined the ages of 1,046 crystals of the mineral feldspar from 2 dinosaur-bearing rock layers in the Naashoibito Member. Based on these ages, the researchers found that a sandstone within 5 meters (about 16 feet) of the lowest boundary of the Naashoibito Member must be younger than 66.9 million years old. They found that the other layer, which is around 3.5 meters (11 feet) from the base of the lowest boundary, must be younger than 66.4 million years old. These age ranges indicated that the sandstone contains dinosaur fossils formed 400,000 years before their extinction.

Next, the researchers measured the direction of magnetic minerals inside the rocks, using a practice called magnetostratigraphy. The direction of these minerals indicates where Earth’s magnetic field was pointing at that time. Scientists have shown that the Earth’s magnetic field has flipped multiple times over Earth’s history, so they can use these magnetic signals to create a timescale for when the sediments formed. 

The researchers wanted to determine whether the magnetic minerals were lined up in a direction like today, called normal polarity, or the opposite, called reverse polarity. The researchers found one rock layer with normal polarity at the base of the Naashoibito Member, and one with reverse polarity in the upper part of the Naashoibito Member. These magnetic field directions lined up with 2 known polarity intervals over a timeline that matched the feldspar dates. Together, these data demonstrated that the Naashoibito Member formed at the end of the dinosaur era, around the same time as the Hell Creek Formation.

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The team concluded that a diverse population of Naashoibito dinosaurs lived at around the same time as the Hell Creek dinosaurs. Earlier scientists surmised that dinosaurs were already on the decline because they studied the colder northern regions, where diversity was lower. However, these results suggest that warmer conditions in the south still supported diverse dinosaur populations. Together, this evidence suggests that dinosaurs were still diverse and thriving 66 million years ago, and that their extinction was indeed a result of the asteroid impact.

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