Withstanding a Megaquake

Basic Knowledge About Earthquakes

Types of Earthquakes and Their Mechanisms

Tectonic Plates Around the Japanese Archipelago

In the area surrounding the Japanese archipelago, two oceanic plates, the Pacific Ocean Plate and the Philippine Sea Plate, are subducting the continental plate on which the archipelago rests.

The earthquakes that occur in the Japanese archipelago and the surrounding area can be broadly divided into ocean trench earthquakes, which occur at the boundaries between plates in ocean trenches or close to these boundaries, and active fault earthquakes, which occur in active faults on land and in coastal area. Ocean trench earthquakes can be divided into interplate earthquakes, which occur at the boundaries between plates, and oceanic plate earthquakes, which occur within a single oceanic plate.

These earthquakes occur when the strain energy that accumulates due to the movement of the plates exceeds a certain limit and the energy is released.

Tectonic Plates Around the Japanese Archipelago

Types of Earthquakes Occurring Around the Japanese Archipelago

The Nankai Trough Megaquake is an ocean trench earthquake (a interplate earthquake) that occur due to the release of strain accumulated in tectonic plates, and is not treated as active fault earthquakes. The Earthquake Research Committee and Regulatory Guide for Reviewing Seismic Design also distinguish between ocean trench earthquakes and active fault earthquakes (land-based crustal earthquakes).

Source: Materials formulated by the Earthquake Research Committee (image)

Earthquake Terminology


Magnitude is a measure of the energy released by an earthquake.


The Gal is a unit of acceleration expressing the extent to which an earthquake's tremors shake the ground and structures. The buildings of Hamaoka Nuclear Power Station are designed with resistance to seismic forces in consideration of tremor acceleration, building weight and other factors.

  • The Gal indicates vibration velocity acceleration rather than velocity itself.
  • This value usually increases with greater magnitude if the seismic source and observation point are the same.

Seismic intensity is a measure of the intensity of the ground motion at a certain observation point, and varies by location even for the same earthquake.

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