Tsunamis are tidal waves, also known as seismic waves, that are primarily caused by earthquakes. Tsunamis are a type of long-periodic ocean wave that are enormous and destructive in their shape and magnitude. Although it has been known for hundreds of years that earthquakes generate tidal waves, not all undersea earthquakes produce tsunamis.
The effects of tsunamis can be felt at considerable distances from the epicenter, and the material losses and loss of life caused by waves that sometimes reach wavelengths of more than 6 meters are enormous. For example, the tsunami caused by the Great East Japan Earthquake, an undersea megathrust earthquake that occurred in the Pacific Ocean off the coast of Sanriku, Japan at 14:46 (05:46 UTC) on March 11, 2011, killed 15,894 people and left 2,562 missing.
Causes of Tsunamis
Most tsunamis are directly related to tectonic movements caused by undersea earthquakes, submarine volcanic eruptions, submarine faulting, submarine landslides, and undersea nuclear tests. Natural earthquakes, which cause major tectonic movements of the ocean floor, are the cause of tsunamis. A stronger earthquake of the same magnitude will not generate a tsunami if it does not cause seafloor movement, but a strong deep earthquake can generate an avalanche, which can trigger a tsunami.
The motion of the ocean floor creates surface waves that travel in many directions, but they do not spread equally in all directions, as they would if a stone were thrown into calm water. Depending on the specific shape of the tectonic motion, the initial wave propagates more energy in some directions than in others. Therefore, even if we know that there is an epicenter and tectonic motion, it is not easy to predict the expected damage area.
Characteristics of tsunamis
Because tsunamis are ocean waves, their arrival time can be predicted. Knowing the depth distribution between the epicenter and the predicted point, which is known within minutes by the global seismic detection network, it is easy to estimate the arrival time of the wave.
The average depth of the open ocean is about 4,000 meters, so if a tsunami has a speed of , it will be traveling at about 200 meters per second. A tsunami generated in the active seismic zone of the Aleutian Islands in the Gulf of Alaska would reach Hawaii in less than five hours. The tsunami slows down as it travels through the open ocean. At a depth of 100 meters, the speed is 30 meters per second, and at 50 meters, it slows to 22 meters per second.
It is not yet clear why tsunamis cause more damage in certain areas. Distance from the epicenter, local refraction effects, and the focus of the initial shock wave are all important. However, it is known that large continental shelves not only reflect waves, but also absorb wave energy from tsunamis through friction on the bottom. Continental shelves in the open ocean can also absorb wave energy. Whatever the reason, it is rare for a tsunami to cause significant damage over a large area with a large continental shelf.
Tsunami evacuation tips
Despite all the research into earthquake prediction, it is very difficult to know in advance that an earthquake is going to happen so that you can reduce the damage. However, tsunamis are a phenomenon that occurs after an earthquake, so you can minimize the damage if you prepare in advance. In fact, the Pacific Ocean has a “Pacific Tsunami Warning System” (PTWS) that provides early warning of earthquake-induced tsunamis, allowing countries like the United States, Japan, Russia, and South Korea to act hours in advance.
Just as important as the warning system is educating coastal residents about the dangers of tsunamis so that they can evacuate quickly when a tsunami strikes. If people understand the characteristics of a tsunami, they can make their own decisions about evacuating. To summarize how to evacuate based on the characteristics of a tsunami, here are the steps.
First, if the base of the tsunami reaches the shore first, the wave height will suddenly increase from several meters to tens of meters, so it is necessary to quickly move away from the shoreline to higher ground.
Second, if the crest of the tsunami reaches the shore first, there will be an initial drop in sea level of 3-4 meters, resulting in a sudden outflow of seawater, so it is still necessary to move quickly away from the shoreline and to higher ground.
Third, the tsunami will have a lower wave height in the open ocean and a higher wave height at the coast, so it is safer to be on the open ocean side, so ships should move quickly to the open ocean side if there is time.
Fourth, a tsunami can travel as fast as an airplane, so if an earthquake is detected on the coast, people should be alert and move quickly to higher ground.
Fifth, the period of a tsunami is long (10-30 minutes) and the maximum wave height may be reached in the third or fourth wave, so people should not go to the shore after the first wave has passed.