core-reflected S – Researchers say a seismic wave reflected off Earth’s core after Japan’s 2011 Tohoku earthquake set faults slipping across the country—an unprecedented case that also implies a new kind of seismic hazard.
For years, seismologists have known that earthquakes can move land. What they didn’t have—until now—was a clear example of an Earth-wide tremor effect tied to a wave that rebounds off the planet’s core.
In the wake of Japan’s March 11, 2011 Tohoku earthquake, the country didn’t just shake. It shifted a few millimeters east. In research published June 18 in Science. investigators report the trigger: a seismic wave that plunged down through Earth’s rocky mantle to the edge of the core and then returned. driving faults to slip. The team describes it as the first recorded case of a core-reflected S wave setting a fault in motion.
Seismic waves bounce through Earth all the time. and S waves are a familiar type—motion that travels through the planet by shearing. But core-reflected S waves have not previously been linked to fault slip. In this account. the wave barrells through the mantle’s full thickness—about 2. 900 kilometers—before reaching the core’s boundary. then returning.
The researchers. led by seismologist Sunyoung Park. say they spotted one such core-reflected wave by combing through archival seismic records and GPS data from the Tohoku disaster. That wave appeared roughly 15 minutes after the magnitude 9.0 mainshock. It didn’t show up alone: it matched a coincident shifting of the ground recorded by hundreds of GPS sensors spread across Japan.
Park, of the University of Chicago, described what the data showed. “We see this permanent offset,” she said.
Permanent offset matters because it suggests the ground didn’t merely vibrate while the wave passed. Caltech seismologist Zachary Ross, who was not involved in the study, said the implication is straightforward. “That implies that there’s some amount of fault slip.”
The reach of the shift is what allowed the team to infer how big the underlying rupture could have been. The movement stretched across Japan itself—from the northern island of Hokkaido to the southern island of Kyushu. Park and her colleagues concluded that a large portion of the plate boundary must have “unzipped.” They determined that two separate plate boundaries. totaling at least 3. 000 kilometers in extent. probably let loose.
Andrea Donnellan. a geophysicist at Purdue University in West Lafayette. Ind. said the basic mechanism fits what seismologists already understand about long-built tectonic stress. “A seismic wave can trigger the release of tectonic stress that’s built up over decades. centuries or even millennia. ” she said. “I think it’s very plausible.”.
Park stressed what makes the finding land in a different place than typical earthquake modeling. “This is the first time that a core-reflected S wave has been shown to trigger a fault to slip. ” she said. She also warned that this type of triggering could be part of seismic hazard planning that has not received enough attention. “That’s a type of seismic hazard that we didn’t think about before.”.
There’s another striking detail: the rupture length suggested by the GPS and seismic evidence is unusually large. Park said it is more than twice the rupture length of the massive 2004 Sumatra earthquake. She noted that. for the Tohoku case. the slippage caused by the core-reflected S wave probably wouldn’t have been perceptible. The energy was distributed over such an enormous area and it occurred relatively slowly—over about three minutes.
But the researchers caution that future events might not be so gentle. If a core-reflected wave can line up with fault slip over thousands of kilometers, then the difference between “unnoticeable” and “dangerous” may come down to how the energy lands, and how quickly it transfers.
In the end, the story is not about a bigger earthquake happening on the same day. It’s about something quieter—and later—coming from deep inside Earth. bouncing off the core. and leaving a subtle but permanent mark on the surface. For Japan, it was a few millimeters east. For seismology, it’s a new reminder that Earth’s internal reverberations can reach farther than anyone expected.
Tohoku earthquake core-reflected S wave seismic waves GPS offsets Earth’s core fault slip seismic hazard Japan plate boundaries June 18 Science
So Japan moved a few millimeters east?? That’s like nothing but also terrifying. Earth really just be doing stuff under our feet.
I don’t get why “core reflected” sounds like a video game glitch. Like was the earthquake caused by the core bouncing the wave back or is that just what they’re calling it now? Either way, more hazard seems scary.
Wait, so it happened 15 minutes after the quake and they found it in GPS records? That’s wild, but also GPS can be off sometimes, right? Unless they’re saying the wave actually triggered more slipping after the main shock, like a delayed effect.
Millimeters east?? That’s what, like moving your whole neighborhood a tiny bit? But the article says “new kind of seismic hazard” which sounds like they’re warning it could happen elsewhere, not just Japan. Also I thought S waves were the slower ones… unless they’re all just the same to me.