The CRUEL Physics behind Tsunamis!

Tsunami waves that form in the open ocean travel at high speeds but are barely noticeable. Now the wave is approaching the shore where the water depth decreases. Surprisingly, the wave that initially seemed harmless suddenly gains a huge amplitude. A devastating tsunami. The slow-moving ocean beds sometimes interact like this. One tectonic plate sliding under another. Notice the bulging upper layer. How long will this motion continue? These stuck wooden pieces demonstrate what happens. Let's keep on tightening them. But after some point of time... Boom! Similar to the experiment, the ocean beds release all the accumulated energy in a short period of time. This is how tsunami waves are formed. If by magic you could drain the entire ocean, you would clearly see different pieces of the Earth's surface. They are called tectonic plates. The Earth's surface is made up of different tectonic plates moving relative to each other. Precisely seven major tectonic plates. The molten lava in the Earth's core circulates continuously due to convective heat transfer. The tectonic plates float on the molten core like ice floats on water. Due to the continuous motion of lava, the tectonic plates are constantly moving. For example, in this animation, two tectonic plates are diverging away and a rift valley is being formed. The East African Rift Valley is a great example of this kind of motion. Now look at this tectonic plate movement. Here the plates are converging. This kind of interaction can result in the formation of mountains, trenches and sometimes active volcanoes. The best example of this kind of converging motion is Cascade Range, northwestern USA. The movement of tectonic plates is very slow, just a few inches per year. Now an interesting observation. Mark dots on all the earthquake-prone areas of the globe. You will find that they all lie in regions where two tectonic plates meet. Yes, the relative motion between two tectonic plates and associated energy release results in earthquakes. When an earthquake happens deep in the ocean, it may lead to a tsunami. The tectonic plate movement that causes the tsunami is very interesting. It's called subduction zone movement. We've already seen this kind of converging movement, but this time it happens underwater. In this movement, strain energy accumulates in the top tectonic plate over time. Here the continental crust bulges upward and the oceanic plate sinks beneath it. This happens because the oceanic plate is much denser than the continental crust. You may observe one interesting feature of the continental crust. It bends inward, resulting in the formation of huge trenches along the tectonic plate boundary. Look at the beautiful trenches formed on the ocean floor of the Pacific Ocean. But how long does this energy accumulation continue? Some subduction zones store energy for centuries. In other subduction zones, the strain energy is released gradually over time. This slow-slip event does not result in a tsunami. However, in some regions, like the Japan Trench and the Chile-Peru Trench, the energy is released in a fraction of a second. Such earthquakes definitely result in a tsunami wave. The wave so generated will have a speed of more than 200 km per hour, but is barely noticeable due to its low amplitude. Tsunami waves typically have an amplitude of less than half a meter, but a wavelength measuring hundreds of kilometers. However, when the wave approaches the shoreline, the story changes. What's the impact of the decreasing water depth on the wave? Let's generate a wave to find out answer for this question. Note down the time taken by the wave to reach the other end of the tank. Is 1.9 seconds. But when the water depth is half, the wave takes more time to reach the other end. Let's compare it together. This means when the water depth decreases, the wave speed decreases. Similar to the experiment, as the wave approaches the shore, its speed decreases significantly. The frequency of the wave remains constant throughout this process. This means the wave has to decrease its wavelength in the shallow region. However, since the overall energy remains constant, the only option to maintain the constant energy is to increase the amplitude of the wave. This phenomenon is called wave shoaling. This is the reason why the tsunami wave becomes so huge near the shore. A slow-moving wave with an unbelievably high amplitude. Finally, this wave crashes onto the shore, wreaking destruction. It should be noted that in a wave there is no horizontal movement of matter. If you place some balls along the wave path, they will just oscillate in their own positions. The formation of a tsunami is truly the cruelty of physics. No water particle is moving forward. But due to the circumstances, the wave amplitude becomes so high that it crashes onto the shore. There is a popular belief that if you see the shoreline receding dramatically at a beach, you can expect a tsunami soon. There is truth in this belief. Sometimes the trough of a tsunami reaches the shore first. This means you will suddenly see the shoreline receding. After a few seconds, the crest of the tsunami will reach the shore. However, it should be noted that not every tsunami is preceded by a receding shoreline. Sometimes the crest of the tsunami arrives first. The Indian Ocean tsunami of 2004 was the most devastating tsunami mankind has ever witnessed. It rose more than 30 meters and killed over 340,000 people. The tsunami was caused by a massive undersea earthquake near Sumatra, Indonesia. This earthquake was extremely strong, measuring about 9.1 to 9.3 on the Richter scale. In Sumatra alone, more than 100,000 people were killed. The earthquake occurred because the Indian plate was forced underneath the Burma plate. Which suddenly moved and pushed the seafloor upward. It lasted almost 10 minutes, making it one of the longest earthquakes ever recorded. The energy released was equivalent to 23,000 Hiroshima atomic bombs. This sudden movement displaced a huge amount of water, creating enormous waves that spread across the ocean. In some places, the waves were as high as 30 meters. These waves moved very fast, traveling up to 800 kilometers per hour. Coastal areas near the epicenter, like Indonesia, were hit within minutes. While places farther away, like India and Africa, were struck hours later. A size comparison animation of all the major tsunamis mankind has ever witnessed is shown here. Underwater earthquakes are the major cause of tsunamis. However, it should be noted that there are three other causes that can initiate tsunamis. Volcanic eruptions under the sea are another major cause that can initiate a tsunami. When an underwater volcano erupts, it can blow apart or collapse, pushing water outward and forming large waves. Eventually creating a tsunami. Sometimes, landslides under the ocean or near the coast can also cause tsunamis. If a large amount of rock, mud, or ice suddenly falls into the sea, it pushes the water and creates waves. These waves can be dangerous if they reach land. Although very rare, when a large mass falls into the ocean, it can also cause a tsunami. For example, a meteorite. The tsunami in the Vajont Dam was formed in a similar way. A huge mass from a landslide hit the water body. When a big mass hits the water, it creates a splash that can turn into huge waves. Luckily, this kind of tsunami doesn't happen often. Back in 2004, there was no tsunami early warning system in the Indian Ocean. The 2004 tragedy was a wake-up call for all nations. They launched buoys to detect deep ocean changes as early as possible. Pressure sensors on the ocean floor detect any change in water level height. This pressure data is sent to the buoy, which is fitted with an antenna and then relayed to a satellite. Remember, in the 2004 tsunami, the wave took approximately 20 minutes to reach Sumatra, 1 to 2 hours to reach Thailand, and 2 to 3 hours to reach Sri Lanka and India. If the DART buoys can transmit information faster than the tsunami wave travels, authorities would be able to evacuate people in tsunami-prone areas. Here comes the big question. Is it possible to prevent tsunamis? Japan thinks so. After the devastating earthquake and tsunami of 2011, Japan decided to build a seawall 400 kilometers long with a maximum height of 15 meters to reduce tsunami impact. Before 2011, the height of this wall varied from 5 to 10 meters, and the tsunami, which had a height of 15 meters, easily overwhelmed it. The 2011 tsunami also destroyed many seawalls along Japan's northeastern coast, including the famous double seawalls in the Taro district. It should be noted that Japan's early warning tsunami system initially predicted and people assumed that it wouldn't cross a seawall. This raises a valid question. If a technologically advanced country like Japan can misjudge a tsunami height, what about the early warning systems installed in other parts of the world? I hope this video about tsunami was quite informative to you. To support my educational activities, please check out our Patreon page and pledge your support today. Take care, bye-bye.