NankaiTrough TokaraIslands TsunamiHazard JapanSeismicRisk

Seismic Swarms in Tokara: A Reminder of Japan’s Tectonic Complexity and the Projection of the Mega Earthquake

*Over 500 tremors rattle southwestern Japan island chain. Source: NHK Newsline*

Over 500 earthquakes shook Japan’s Tokara Islands in June 2025, with the strongest tremors reaching magnitude 5.1. While no major damage was reported, this seismic swarm has prompted renewed analysis by engineers and seismologists regarding the region’s tectonic activity. Located along the Pacific “Ring of Fire,” Japan routinely experiences around 1,500 earthquakes annually. The Tokara swarm, with tremors occurring near the surface and in rapid succession, mirrors similar episodes recorded in 2021 and 2023.

This localised uptick in activity is compounded by national-level concerns. Japan's government recently announced that there is now an 80% probability of a magnitude 9.0 earthquake occurring in the Nankai Trough within the next 30 years. This trench, lying off the southeastern coast, marks a subduction zone between the Philippine Sea Plate and the Eurasian Plate. a geological boundary associated with historically recurring megathrust earthquakes approximately every 100 to 200 years.

If this scenario occurs, the projected impact would be among the most severe in Japan’s recorded history. Up to 300,000 lives are at risk, with tsunami waves projected to reach 34 meters in some areas. Structural failures could impact over 2.35 million buildings, and financial losses may surpass $1.82 trillion. Additionally, more than 12 million people might face long-term displacement.

Both the Tokara swarm and Nankai forecasts underscore the urgent need for integrated seismic hazard assessments. Engineers must proactively evaluate liquefaction potential, strengthen foundation designs in at-risk regions, and ensure lifeline infrastructure can remain operational post-quake. Importantly, coastal cities must be reassessed for slope stability and soil performance under extreme loading conditions.

The lessons of the 2011 Tohoku disaster remain etched in Japan’s engineering history. With the looming Nankai event and persistent Tokara activity, performance-based engineering and resilient geotechnical design are increasingly central to current national preparedness strategies.

The following video provides additional context on the recent increased seismic activity in the Jolt Remote Islands.