Causes of tsunami¶
One of the most common causes of a tsunami is an earthquake. However, as shown in Fig. 1.1.6, while 81% of tsunamis are caused by earthquakes, they are also caused due to volcanoes, landslides, and even meteoritic impacts. The above events can lead to sudden movement of the ocean floor and displace a large mass of water, leading to a tsunami.
Fig. 1.1.6 Classification of source and mechanism of tsunami events. (Source: NOAA report on tsunami)¶
While wind-generated waves only cause the motion of the water near the surface, tsunami waves cause a motion of the entire water column. The depth of the ocean thus controls the speed of the tsunami. Tsunami waves travel at much higher speeds in deeper water, and the speeds reduce as they get closer to the coast. However, since the total energy available in a wave is constant, the energy is transferred. It leads to increasing the wave amplitude. This process is known as wave shoaling. The characteristics of tsunami waves are as shown in Fig. 1.1.7.
Fig. 1.1.7 General characteristics of tsunami waves (Source: North-Eastern Atlantic, Mediterranean, and connected seas: Tsunami information center.)¶
The tsunami waves can have periods of 5 - 60 minutes. The first wave is not necessarily the most enormous either. When the tide recedes, it significantly exposes the shore. It is replaced by a second wave that also includes debris from the previous destructive wave.
Earthquakes¶
The earthquakes are generated by the movement of the fault zones associate with the plate boundaries. There are different types of plate boundaries like spreading (two plates moving away from each other), subduction (two plates move towards each other with one sliding beneath the other), and transform (two plates sliding horizontally past each other).
Most of the intense earthquakes occur in the subduction zones where the ocean plate slides beneath a continental or another younger ocean plate. However, it is not necessary that every earthquake would produce a tsunami. Some of the necessary conditions include:
The earthquake must occur beneath the ocean or should cause the material to slide into the ocean
The earthquake has a reasonably large magnitude, like 6.0 or more
The earthquake occurs at a reasonably shallow depth - like less than 70 km below the surface of the earth
The earthquake must cause vertical movement of several meters of the seafloor
Landslides¶
When a landslide occurs along the coast, this can force large amounts of water into the sear, creating a disturbance sufficient to generate a tsunami. Similarly, underwater landslides can also cause the material to loosen to violently push the water in front of it.
Volcanic eruptions¶
Tsunamis due to volcanic eruptions are not very common. However, volcanoes can cause: (a) catastrophic collapse of coastal landslides (b) pyroclastic flows that include hot blocks, pumice, ash and can plunge down the volcanic slopes into the ocean resulting in pushing the water outwards (c) caldera volcano that collapses after an eruption causes overlying water to drop suddenly.
One of the most giant and most destructive tsunamis ever recorded was on 26th Aug 1883 after the explosion and collapse of the volcano of Krakatao in Indonesia. This generated tsunami waves of a height of almost 135 feet.
Extraterrestrial¶
Tsunamis can also erupt due to extraterrestrial collisions like asteroids and meteors. Although none have been recorded in recent history, it is believed that if a moderately large asteroid of about 5 - 6km in diameter were to strike in the Atlantic, then it can travel all the way to the upper part of the Appalachian mountains.