# Tokyo team finds seven hidden event layers in a 40 cm Kumano Trough sediment core using radiocarbon

- Published: 2026-11-13 21:34 UTC
- Section: Planet › Earth sciences
- Tags: radiocarbon dating, total organic carbon, Kumano Trough, Nankai Trough earthquakes, sediment core, submarine slope failure, turbidity current, foraminifera, Hoei earthquake
- URL: https://realtech.media/news/rt-19aeb8c5cfae83641dcf1dd7_tokyo_team_finds/

**Radiocarbon dating of total organic carbon in mud from off central Japan revealed seven disturbed layers; several match Nankai earthquakes since 1605 in a single core.**

![Tokyo team finds seven hidden event layers in a 40 cm Kumano Trough sediment core using radiocarbon](https://realtech.media/media/2026/11/0358a07d2cece887.jpg)
*Earth.com*

A team from the Atmosphere and Ocean Research Institute at the University of Tokyo found seven disturbed layers in a 40 cm sediment core from the Kumano Trough off central Japan, according to a study in Radiocarbon [1]. The core was taken in 2012 from the research vessel Tansei-maru at a depth of about 2,080 m and had been logged as homogeneous. Lead author Akiko Omura, Yusuke Yokoyama and colleagues measured radiocarbon in total organic carbon (14C TOC) at intervals of 0.5 to 1.0 cm [1].

## Radiocarbon reversals as a marker

Undisturbed seafloor mud gets older with depth. Some horizons in this core broke that rule: their carbon was up to 700 years older than the mud beneath them, against an average measurement error of about 100 years [1]. The authors attribute these reversals to submarine slope failures rather than to river floods, and treat the reversed layers as event deposits left by geohazards [1].

Carbon-to-nitrogen ratios near 8 and carbon isotope values between -20 and -22 per mil, in both normal and disturbed layers, indicate marine organic matter. The paper uses this as evidence against a flood origin.

Organic carbon in this basin tests older than the surrounding mud, so the team built the age model from planktonic foraminifera, using samples of 0.75 to 2.35 mg. At seven paired depths, shell and carbon dates differed by 700 to 1,109 years. The corrected timeline puts the whole core at roughly 600 years of deposition, with the oldest dated part, at 37-38 cm, near 1440 C.E. [1]

## Match with historical earthquakes

The authors compared the layer ages with documented disasters after the 15th century and report good correlation [1]. Layers 1 and 2 may date to the 1944-1946 earthquakes and the Ansei Tokai earthquake of 1854; the paper does not say which layer goes with which. Layer 3 is tied to the Hoei earthquake of 1707. Layers 4 and 5 may record either the Hoei event or a minor submarine earthquake, and layer 6 could come from the Keicho earthquake of 1605. The deepest layer is marked with a question mark and is not assigned to any earthquake.

Only two layers show up in X-ray CT scans: layer 3 matches a brighter band, and layer 5 matches a pale strip of fine laminae. All matches come from this one core, and the paper gives no estimate of the size or location of any event.

The work was presented at the 25th International Radiocarbon Conference in Krakow in 2025. According to the University of Tokyo, Yokoyama plans to collect samples from across the Kumano Sea and beyond.

## Sources

1. [Radiocarbon](https://doi.org/10.1017/RDC.2026.10255)
2. [Earth.com](https://www.earth.com/earth-science/nankai-hidden-earthquakes-sediment-core-radiocarbon)

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Source: RealTech, realtech.media — https://realtech.media/news/rt-19aeb8c5cfae83641dcf1dd7_tokyo_team_finds/