The word “tsunami” in Japanese means “harbor wave”. Tsunamis are waves on the water surface that are caused by displacement of the ocean floor during earthquakes, landslides and volcanic eruptions. Tsunami waves are observed mainly in seismically active areas of the World Ocean. Tsunamis cause numerous casualties and destruction, hitting the coast as long waves of considerable height.
Tsunami waves often appear as walls of water and can attack the coastline and be dangerous for several hours, arriving at intervals of 5 to 60 minutes. The first tsunami wave is not the biggest. Most often, the 2nd, 3rd or 4th waves are particularly destructive. They overturn boats in harbors, sweep away buildings, break piers, and the returning water flow carries debris and people out to sea. After the waves recede, new water channels and riverbeds can form on land, extending far inland.
Causes of tsunami formation
Tsunami waves arise as a result of vertical movement of the seabed followed by displacement of the water mass. The causes can be the following:
Seismic tremors
Tsunamis are formed by underwater earthquakes with a magnitude greater than 6.0, the epicenter of which is located at a depth of less than 50 km from the seabed. They form in fault zones at the boundaries of tectonic plates. The fault boundary and the characteristics of tsunami waves are determined by the type of movement of tectonic plates:
- spreading, where two plates move apart;
- subduction, where two plates move towards each other and one slides beneath the other;
- transform fault, where two plates slide horizontally past each other.
The most powerful earthquakes occur at subduction zones, where an underwater plate slides beneath a continental or other, younger oceanic tectonic structure.
There are several conditions under which tsunamis occur. For the formation of huge waves, the following conditions are necessary:
- earthquake must occur beneath the ocean floor;
- seismic tremors should have a magnitude of at least 6.0 on the Richter scale;
- earthquake’s epicenter should be no more than 70 km deep from the Earth’s surface;
- seismic activity must cause vertical displacement of the ocean floor (up to several meters).
The amplitude of tsunami waves increases with the strength of the earthquake. Tsunamis caused by seismic tremors of magnitude over 8.0 are considered catastrophic and the most destructive.
Landslides
Underwater landslides are formed by the movement of large masses of clayey, carbonate silt or sedimentary rocks along the sloping seabed. If a landslide forms along the coast, it will push a lot of water and create giant waves. Tsunamis from landslides also form when water and loosened seabed materials move at high speeds.
An example of some of the most destructive tsunamis caused by landslides were waves reaching a height of 524 meters that struck Lituya Bay (southern Alaska) on July 9, 1958. As a result of an earthquake of magnitude 8.3, a landslide descended from the mountains into the bay and 30 million cubic meters of glacier collapsed into the waters of the bay, causing megatsunamis.
Volcanic activity
The eruption of a coastal or underwater volcano causes impulsive disturbances of the ocean floor. Such seismic activity can displace large volumes of water and cause extremely destructive tsunami waves in the immediate source area. The sudden displacement of water caused by a volcanic explosion, slope failure or the opening of magma chambers leads to the formation of tsunami waves.
One of the largest and most destructive tsunamis occurred on August 26, 1883 after the explosion and collapse of the Krakatoa volcano in Indonesia. This explosion generated waves over 40 meters high, devastating coastal cities and villages along the Sunda Strait on the islands of Java and Sumatra, resulting in the death of 36,417 people.
The effects of the tsunami
As a tsunami approaches the coastline, the length of each wave sharply decreases but still exceeds several kilometers. Since the volume of water remains constant, the height of the wave increases. Once the tsunami reaches the shore, it is flooded due to the high rise in the water level. During retreat, destructive waves can carry away people, vehicles, houses and other objects located near the shore into the ocean.
Damage from a tsunami depends on several factors:
- wave heights and speeds, their impact on the environment, buildings and structures;
- shape of the coastline, presence of bays and harbors;
- rapid sea retreat;
- level of coastal soil erosion.
Over the past 20 years, more than 250,000 people have become victims of the tsunami. And the total economic damage from the impact of rogue waves during this period amounted to 280 billion dollars.