The most destructive tsunamis are generated from large, shallow earthquakes with an epicenter or fault line near or on the ocean floor.
Tsunamis are giant waves caused by earthquakes or volcanic eruptions under the sea. However, it should be noted that not all earthquakes generate tsunamis. Usually, it takes an earthquake with a Richter magnitude exceeding 7.5 to produce a destructive tsunami.
The most destructive tsunamis are generated from large, shallow earthquakes with an epicenter or fault line near or on the ocean floor.
The high seismicity of such regions is caused by the collision of tectonic plates. When these plates move past each other, they cause large earthquakes, which tilt, offset, or displace large areas of the ocean floor from a few kilometers to as much as 1,000 km or more.
However, in the depths of the ocean, tsunami waves do not dramatically increase in height. These waves travel inland, they build up to higher and higher heights as the depth of the ocean decreases.
The speed of tsunami waves depends on ocean depth rather than the distance from the source of the wave.
Tsunami waves may travel as fast as jet planes over deep waters, only slowing down when reaching shallow waters.
More than 80% of the world's tsunamis occur in the Pacific along its Ring of Fire subduction zones.
When a great earthquake ruptures, the faulting can cause a vertical slip that is large enough to disturb the overlying ocean, thus generating a tsunami that will travel outwards in all directions.
How earthquakes and tsunami are related?
Although earthquake magnitude is one factor that affects tsunami generation, there are other important factors to consider.
The earthquake must be a shallow marine event that displaces the seafloor.
Thrust earthquakes are far more likely to generate tsunamis. During the rupture, one side of the fault is pushed upwards relative to the other, and it is this type of movement is known as thrust.
Earthquakes of magnitude below 6.5 are very unlikely to trigger a tsunami.
Earthquakes of magnitudes between 6.5 and 7.5 do not usually produce destructive tsunamis. However, small sea level changes might be observed in the vicinity of the epicenter. They can only occur due to secondary effects such as landslides or submarine slumps.
Earthquakes of magnitudes between 7.6 and 7.8 might produce destructive tsunamis, especially near the epicenter. At greater distances, small sea level changes might be observed.
Destructive local tsunamis are possible near the epicenter in case of earthquakes of magnitude 7.9 and greater. Significant sea-level changes and damage might occur in a broader region.
With a magnitude 9.0 earthquake, there is a possibility of an aftershock of magnitude 7.5 or greater.
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