They are typically triggered by earthquakes with a magnitude of 6.5 or greater and a focal depth of less than 20-50 kilometers. More destructive events are often caused by earthquakes with a magnitude of 7.8 or greater. They are primarily caused by the dramatic rise and fall of the seafloor crust, which can be categorized as "downwelling" (where the crust sinks, causing seawater to flow back) or "upwelling" (where the crust rises, causing seawater to spread). Typical examples include the 1960 Chilean tsunami and the 1983 Japanese tsunami.
Tsunamis can travel at speeds of up to 800 kilometers per hour in deep waters and have wavelengths of hundreds of kilometers. Nearshore, they are affected by topographical uplift, forming a tens-meter-high "wall of water." The Pacific Ring of Fire is responsible for approximately 80% of the world's tsunamis, with Japan, Hawaii, and other areas frequently affected.
The Fundamental Causes of Tsunamis
- After an earthquake, seismic waves propagate from the epicenter in all directions. The energy of these waves is affected by factors such as topography and geological structure, resulting in varying sensations in different regions. When an earthquake originates from the seabed, the seafloor topography rises and falls due to the earthquake's energy, causing strong disturbances in the ocean water and, in turn, generating tsunamis. Tsunamis caused by earthquakes can be divided into two types:
- Tsunamis occur when an earthquake causes the seafloor crust to rise, leading to localized uplift and accumulation of seawater. Energy is generated as the seawater radiates from the energy center.
- Tsunamis occur when an earthquake causes the seafloor crust to sink, causing a massive surge of seawater. When the seawater encounters resistance, it returns to the surface, forming waves that spread outward.
How to Prepare for a Tsunami
Tsunamis have a wide impact and are highly destructive.
How should you prepare for a tsunami?
1. Before a tsunami arrives, everyone should stay away from beaches and low-lying coastal areas. If you are on the beach and feel strong ground shaking, notice a sudden receding of the seawater, or hear a roaring sound like an approaching train, these are signs of an approaching tsunami. Immediately seek higher ground or the upper floors of a secure building.
2. The destructive power of a tsunami is closely related to coastal topography. Areas with steep slopes or U-shaped harbors can increase the wave amplitude, leading to disasters. Even small tsunamis should not be taken lightly. Stay in safe areas until the relevant authorities issue the all-clear.
3. While people are evacuating immediately, vessels in deep waters should avoid approaching the coast or shallow waters. Tsunamis in deep waters have minimal wave height and are not destructive. Furthermore, vessels in harbors should avoid the risk of attempting to maneuver small boats into deep water. All onboard should immediately disembark and seek higher ground.
Tsunamis are characterized by several key characteristics:
First, the magnitude must be large enough and the energy must be strong enough. Second, the focal depth must be shallow, allowing more energy to be released and causing tsunami motion. There must be sufficient water depth to store the tsunami energy. The focal displacement must be primarily vertical, causing the seabed to rise or fall dramatically. Earthquake tsunamis often occur in areas with significant tectonic differences, such as trenches, island arcs, and young crustal folds.
