The Science of Tsunamis: Causes and Effects
Tsunamis are gravity waves, similar to common surface waves, but their wavelengths can reach thousands of kilometers.
Therefore, tsunami waves experience minimal attenuation while traveling in deep water, allowing them to travel across oceans and reach locations thousands of kilometers away. Tsunami wave height is related to the depth of the water. In the deep ocean, tsunami waves are very low, but as they approach shore, the accumulation effect rapidly increases, forming the familiar "wall of water" that wreaks havoc.
Tsunami Causes Analysis
- Main Causes
Undersea Earthquakes (Most Common): When a strong earthquake occurs at a plate boundary, the seafloor suddenly rises or falls, displacing large amounts of water.
Volcanic Eruptions: Undersea volcanic eruptions can cause significant disturbances in the water.
Undersea Landslides: Large-scale sliding of submarine sediments can push water into large waves.
- Wave Characteristics
Tsunamis are characterized by extremely long wavelengths (up to hundreds of kilometers) and rapid propagation speeds (approximately 3 s in deep ocean, where is the acceleration due to gravity and is the water depth). As they approach the coast, wave heights increase dramatically due to the shallowing of the water.
Tsunamis can be divided into three categories based on their causes:
earthquake tsunamis, volcanic tsunamis, and landslide tsunamis. Earthquake tsunamis occur when an earthquake causes a sudden rise and fall of the seafloor topography, causing a strong disturbance of the ocean water.
There are two mechanisms: "downwelling" tsunamis and "upwelling" tsunamis.
- "Downwelling" tsunamis: Certain tectonic earthquakes cause a large, rapid subsidence of the seafloor crust. Seawater initially surges into the sudden subsidence, causing a large accumulation of seawater above it. When the influx of seawater encounters resistance on the seafloor, it returns to the surface, generating compression waves, forming long, large waves that propagate and diffuse. Tsunamis caused by this type of downwelling crustal movement initially manifest as abnormally low tides on the coast. The 1960 Chile earthquake tsunami belongs to this type.
- "Uplift" tsunamis: Certain tectonic earthquakes cause a large, rapid rise in the seafloor crust, lifting the seawater along with the uplift. A large amount of seawater accumulates above the uplift. Under the influence of gravity, the seawater must maintain an equipotential surface to achieve relative equilibrium, causing it to spread outward from the source, forming a massive wave. This type of uplift-type seafloor crust movement generates tsunami waves that first manifest as abnormally high tides on the coast. The tsunami caused by the magnitude 7.7 earthquake in the Central Sea of Japan on May 26, 1983, belongs to this type.
For earthquake tsunamis to generate sufficiently strong tsunami waves, there are generally two main conditions:
- The earthquake's "elastic" energy must be sufficiently strong.
- The two determining factors: the earthquake must be of high intensity, meaning its magnitude is high enough; and the earthquake must occur at a shallow depth. For example, the focal depth of the 2011 East Japan tsunami was only 10 kilometers. Therefore, a large amount of seismic energy is directly released into the seawater.
- The water above the epicenter is deep enough:
Because the "rebounded" seawater vibrates from the surface to the bottom of the ocean, the deeper the water, the greater its potential energy, and the stronger the tsunami energy.
