Mechanism and Occurrence of the Nankai Trough Earthquake

The Nankai Trough mega-earthquake is a subduction zone earthquake that occurs at the boundary between the Philippine Sea Plate and the Eurasian Plate. Its mechanism and occurrence can be divided into the following three stages:

1. Plate Subduction and Strain Accumulation

• The Philippine Sea Plate is subducting beneath the Eurasian Plate at a rate of several centimeters per year.
• This subduction generates strong friction at the plate boundary, causing the Eurasian Plate to be dragged and deformed, leading to the accumulation of strain.
• This strain accumulation progresses slowly over many years.

2. Strain Release and Earthquake Occurrence

• When the accumulated strain at the plate boundary exceeds a certain limit, the locked portion ruptures, and the Eurasian Plate springs upward.
• This sudden movement triggers a massive earthquake, and seismic waves propagate throughout the surrounding area.
• The magnitude of the earthquake is determined by the extent of the ruptured area and the amount of plate displacement.

3. Tsunami Generation

• The uplift or subsidence of the seabed causes significant fluctuations in seawater, generating a tsunami.
• The tsunami propagates as high waves towards coastal areas, causing severe damage.
• The height and reach of the tsunami depend on factors such as the earthquake’s magnitude and the seabed topography.

Several patterns are considered for the occurrence of the Nankai Trough mega-earthquake:

• Single Type: An earthquake occurs almost simultaneously over a wide area from Suruga Bay to off the coast of Shikoku.
• Multiple-Segment Type: Earthquakes occur in multiple areas with time lags, eventually leading to a mega-earthquake.
• Tokai, Tonankai, and Nankai Earthquake Linkage: The Tokai, Tonankai, and Nankai earthquakes occur in conjunction.

These patterns significantly influence the earthquake’s magnitude and the occurrence of tsunamis. Predicting the Nankai Trough mega-earthquake is extremely challenging, but by analyzing past earthquake patterns and current crustal movements, we can assess the likelihood of future earthquakes.

The Nankai Trough mega-earthquake could occur at any time. It is crucial to be well-prepared for earthquakes and to know how to act appropriately in an emergency.