Nankai Trough: Japan's major submarine subduction zone

Geology and tectonics

The Nankai Trough marks a convergent plate boundary where the Philippine Sea Plate descends beneath the overriding plate of the Japanese archipelago. This subduction creates a steep trenchfront, a broad accretionary prism of sediments scraped off the incoming plate, and a complex fault system. The zone of contact between plates is a potential site of large, the so-called megathrust, earthquakes capable of rupturing along many tens to hundreds of kilometers.

Seismic history and segmentation

Earthquakes generated along the Nankai Trough have a long historical record and are often grouped into named segments (Tokai, Tonankai, Nankai) that can rupture separately or together. Major events in the historical and modern record illustrate the system's capacity for repeated large earthquakes and tsunami generation:

• Pre-instrumental and historical accounts show recurring great earthquakes over centuries.
• Notable modern ruptures in the twentieth century demonstrate the hazard to coastal communities.
• Some ruptures have occurred in close succession, emphasizing complex interactions between segments.

Scientific study and monitoring

The Nankai Trough is a primary target for marine geoscience and earthquake research. International and Japanese programs have conducted drilling and deep-sea investigations to sample the accretionary prism, measure stress and temperature conditions, and test models of fault behavior. Dense networks of seafloor and land-based sensors, tsunami modeling, and scenario planning aim to improve early warning and preparedness for a future large rupture.

Importance and notable facts

Beyond its scientific value, the trough has major social and economic relevance: a large megathrust event would threaten ports, coastal infrastructure, and populated plains on Honshū and Shikoku. Studies of the Nankai system contribute to understanding subduction processes worldwide and to developing risk reduction strategies. Ongoing research continues to refine estimates of recurrence intervals and to identify signals that might precede large ruptures.