Ultrasensitive Radiocarbon Method Reveals Hidden Quake Layers Off Japan

"Because the redeposited sediments are the same as the materials below, the chemical and structural features were indistinguishable, making the event layers invisible," Yokoyama said.

The new approach sidesteps that problem. Instead of looking at structure, the team dated the organic carbon itself and mapped how its age jumped along the core. Where older carbon appears sandwiched between younger layers, something disturbed the seafloor. The researchers converted those radiocarbon ages to calendar years using ages obtained from forams — microscopic marine organisms with shells — and matched some of the detected events to written historical records of submarine earthquakes spanning the 13th to the 20th century.

How does the dating work?

Radiocarbon, or carbon-14, decays at a known, steady rate. All living things absorb it while alive; once they die, the clock starts. Measuring how much remains lets researchers date samples as far back as 50,000 to 60,000 years.

The catch is scarcity. Carbon-14 makes up just one-trillionth of natural carbon on Earth, which is why the team needed what Yokoyama calls a "new, ultrasensitive method" to measure trace amounts at a resolution of 1 centimeter or less — finer than earlier techniques could resolve.

What changes for earthquake forecasting?

Japan already monitors earthquakes, tsunamis, and typhoons with modern warning systems and hardened infrastructure. But prediction depends on knowing how often past events struck, how large they were, and where. Not every earthquake leaves an obvious scar in the geological record — and the invisible ones distort the recurrence count.

That is the practical stake here. By recovering events that visual observation, X-ray, and CT analysis missed, the technique gives seismologists a more complete inventory of Nankai Trough activity. A fuller record sharpens estimates of recurrence intervals in a region where a megaquake is anticipated within the next 30 years or so.

The method is not limited to Japan. Any region with a history of geological activity and access to seafloor sediment cores can apply the same high-density radiocarbon dating to fill gaps in its own disaster record.

What comes next?

Yokoyama plans to widen the survey — not just deeper into the ocean floor but across the Kumano Sea and beyond. The goal is to assemble a broader picture of Japan's hidden disaster history and use it to prepare for future events.

Caveats apply, as with any single-core study: the team interpreted the mixed-carbon layers as evidence of disturbance by floods, storms, or earthquakes, and correlated only some events with historical records. More cores from more sites would firm up the picture.

One measurement per half-centimeter turned an unremarkable clay core into a ledger of forgotten earthquakes.