Researchers at the University of Tokyo used high- resolution radiocarbon analysis of a marine sediment core from the Kumano Sea (off the southeast coast of central Japan) to detect previously invisible “hidden” traces of submarine landslides, likely triggered by earthquakes in the Nankai Trough.
The Nankai Trough is a major subduction zone where Japan’s next megaquake is anticipated (possibly within the next few decades). Conventional methods, visual inspection, X- ray, and CT imaging, showed the ~40 cm core as largely homogeneous fine- grained hemipelagic mud (clayey silt), with no clear event layers.
Using an ultrasensitive radiocarbon (carbon- 14) method on total organic carbon at high resolution (0.5- 1.0 cm intervals), the team identified abrupt shifts: older organic carbon mixed into younger sediment layers.
These were interpreted as redeposited material from seafloor disturbance (submarine landslides or related events possibly linked to earthquakes, floods, or storms). The layers were chemically and structurally indistinguishable from surrounding sediment, which is why they were invisible to prior techniques.
The oldest part of the core dates to approximately 1440 CE (at ~37- 38 cm depth). By converting radiocarbon ages to calendar years (calibrated with foraminifera ages), researchers correlated some events with historical records of submarine earthquakes from the 13th to 20th centuries.
The work is published in Radiocarbon (Omura, Yokoyama, Hirabayashi et al., 2026; DOI: 10.1017/RDC.2026.10255). Lead researcher Professor Yusuke Yokoyama (Atmosphere and Ocean Research Institute) noted that the approach can help reconstruct a fuller history of geophysical disasters, improving hazard assessment not only for Japan but for other seismically active regions. Further sampling across the Kumano Sea and beyond is planned.
This provides a new tool for paleoseismology by revealing “invisible” event deposits that traditional methods miss.
Sequential high- density radiocarbon dating to detect invisible event layers in marine sediments
The authors applied continuous, high- resolution radiocarbon dating of total organic carbon (¹⁴C TOC) at 0.5- 1.0 cm intervals to a 40 cm hemipelagic sediment core (KT- 12- 34- MC01) from the western Kumano Trough (2080 m water depth, off the Kii Peninsula, Nankai Trough region).
The core appears homogeneous (clayey silt) by visual inspection, soft X- ray, and CT imaging, with only subtle brightness variations.
Age reversals, horizons where ¹⁴C TOC ages are older than those of underlying sediment, reveal seven “invisible” event layers caused by remobilization of older seafloor material (submarine slope failures, landslides, likely triggered by earthquakes). These were not detectable by conventional methods.
δ¹³C values (−20 to −22‰) and C/N ratios (~8) indicate a predominantly marine particulate organic carbon source for both normal and event layers, ruling out significant terrestrial (flood- derived) input.
Age model and correlations
- Planktonic foraminifera ¹⁴C ages (small- scale measurements) provided the true depositional chronology.
- TOC ages are systematically older (Δage offset of ~700– 1100 years); these offsets were calibrated using coeval foraminifera- TOC pairs.
- The core spans roughly the last 600 years (oldest material ~1440 CE).
- Calibrated event layers correlate with historical Nankai Trough earthquakes after the 15th century:
- Layers 1– 2: possible links to 1944 Tonankai and 1946 Nankai or 1854 Ansei Tokai events.
- Layer 3: 1707 Hoei earthquake.
- Layers 4– 5: 1707 Hoei or minor events.
- Layer 6: 1605 Keicho earthquake.
Continuous high- density ¹⁴C TOC dating can detect event deposits within otherwise featureless hemipelagic mud, expanding the paleoseismic record beyond classic turbidites.
The method is particularly useful in regions with sparse foraminifera and supports improved hazard assessment for the Nankai Trough megaquake.
The paper was presented as a selected contribution from the 25th International Radiocarbon Conference (Kraków, 2025).
Journal: Radiocarbon (First View), published online 9 November 2026
Provided: University of Tokyo
Authors: Akiko Omura,
Yosuke Miyairi and
Abstract
We present a unique application of radiocarbon measurements on hemipelagic sediments that uses continuous radiocarbon dating of total organic carbon (14CTOC age) to recognize event deposits. The 14CTOC age of sediments is not commonly used for age determination because total organic carbon includes various origins, such as marine, terrestrial, and reworked old fragments. In this study, the 14CTOC ages of some horizons were found to be older than those of the deeper ones through continuous 14CTOC measurements at 0.5- to 1.0-cm intervals. The reversal 14CTOC ages resulted not from the settlement of organic particles from the terrestrial environments, such as floodings, but from the submarine slope failures. Therefore, we can regard the reversal 14CTOC layers as event layers due to geohazards. After reconstruction of the age model using foraminiferal tests obtained from the sediment core used in this study, depositional ages of event layers were identified. Well correlation with historical natural disasters after the 15th century suggest validity of the method proposed by the current study.
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