The Day the Nankai Coast Broke: Remembering the 1946 Earthquake and Tsunami

On December 21, 1946, a powerful Nankaidō earthquake struck southern Japan along the Nankai Trough, triggering a devastating tsunami. The disaster killed at least 1,000 people and destroyed approximately 36,000 homes, leaving coastal communities in ruins. This event became a cornerstone case for Japan’s modern tsunami preparedness.

Key Facts Table

Attribute	Detail
Date	December 21, 1946
Magnitude	~8.1
Epicenter	Nankai Trough, Japan
Secondary Hazard	Tsunami
Fatalities	≥ 1,000
Homes Destroyed	~36,000

Impact & Significance

• Massive coastal flooding from tsunami waves
• Severe infrastructure loss in Shikoku and Honshu
• Accelerated research into subduction-zone earthquakes

Simple Seismic Relation

A commonly used approximation links seismic energy $E$E to magnitude $M$M:$$\log_{10}(E) \approx 1.5M + 4.8$$log10​(E)≈1.5M+4.8

This illustrates why M8+ events release enormous energy capable of generating tsunamis.

• Nankai Trough: A subduction zone where the Philippine Sea Plate dives beneath Japan.
• Tsunami: Long-wavelength sea waves caused by sudden seafloor displacement.
• Subduction Earthquake: A quake occurring where one tectonic plate moves under another.

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