Explain why can tsunamis be triggered by underwater earthquakes?

Link between submarine earthquakes and tectonic movements

The surface of our planet is divided into huge pieces, called tectonic plates, which float on a semi-fluid layer. These plates are constantly moving, slowly but surely, at a rate of a few centimeters per year. Sometimes, under the ocean, two plates collide or slide beneath one another, creating enormous friction. When this friction becomes too strong, the plates suddenly shift: this sudden movement is what a submarine earthquake is. The seabed is then abruptly raised or lowered in a matter of seconds over thousands of square kilometers. And it is precisely this rapid displacement of the ocean floor that sets water masses in motion and can trigger a tsunami.

Physical process of tsunami formation following an earthquake

A tsunami often starts due to a sudden rupture of the seabed, triggered by an underwater earthquake. When a tectonic plate suddenly moves, it will raise or lower abruptly an enormous amount of water just above it. Imagine throwing a large flat stone into a puddle: it creates a wave that spreads across the surface, except here, the stone is the seabed itself moving, and the puddle is the entire ocean above! This sudden disturbance creates gigantic waves that then rush at high speed towards the coasts. As they approach the shore and the waters become shallower, these waves slow down but increase in height, forming huge waves capable of destroying everything in their path.

Factors determining the magnitude of tsunamis generated by underwater earthquakes

The magnitude of the earthquake, meaning the amount of energy released during the tremor, largely determines the potential scale of the tsunami. The stronger it is, the more water it can displace and generate impressive waves. The depth of the earthquake's focus also plays a crucial role: a shallow earthquake near the seabed often causes more significant water movements than a deeper earthquake. The type of tectonic fault involved also matters: vertical faults, which abruptly raise or lower entire sections of the seabed, greatly favor the formation of tsunamis compared to horizontal movements. Another essential factor is the topography of the seabed around the earthquake site. A shallow and extensive area facilitates a rapid rise of waves towards the coast, while a deep trench could partially dampen the phenomenon. Finally, the distance to the coast is certainly important: a tsunami formed close to the shoreline quickly reaches land, often with less warning time, and potentially with greater violence.

Mechanisms of tsunami propagation across the oceans

Tsunamis move very quickly in open sea, reaching speeds of about 700 to 800 km/h, which is comparable to the speed of a commercial airplane. However, in the open ocean, their height is often low, just a few dozen centimeters, with very long waves, sometimes several hundred kilometers apart between two crests. That’s why boats in the open sea usually don’t feel anything passing. As they approach the coasts and enter shallower waters, the tsunami's speed abruptly slows to just a few dozen kilometers per hour. At that moment, the wave increases in height: it compresses, with water being pushed upward. This is when we see those famous huge and destructive waves that devastate coastlines. Tsunamis can travel very long distances across the ocean while retaining a great deal of energy, sometimes even crossing an entire ocean to reach shores thousands of kilometers from the initial earthquake.

Effects of tsunamis on coastlines after an underwater earthquake

When a tsunami hits the coast after an underwater earthquake, the very sudden rise in water often causes massive flooding. The tsunami does not resemble a normal wave; it is more like a sudden rise in sea level whose power tears away everything in its path: boats, cars, lightweight buildings, trees. The very rapid withdrawal of water just before the tsunami arrives is a typical sign: the sea seems "sucked" away from the shore, sometimes revealing several hundred meters of usually hidden seabed. After this alarming withdrawal, a series of destructive waves often follows. The devastating force of these waves can cause human losses, significant damage to coastal infrastructure, and severe disruption of coastal ecosystems. Even once the waters have receded, the shoreline often remains permanently disturbed by significant erosion and the accumulation of debris carried from the sea.