Following the violent earthquake of magnitude 8.8 recorded in the Russian peninsula of Kamchatka, one of the most popular doubts is that relating to Time of arrival of a tsunami on the mainland. In fact, someone might think that these waves are able to reach the coasts within a few moments, when in reality for the countries most distant from the epicenter it is possible that they pass several hours. But how is it possible? Let’s see the formula to obtain this value.
The first thing to keep in mind is that the tsunami in most cases they form following earthquakes: When a fault is activated on a seabed, it can create a “step”, the height of which is linked to the violence of the earthquake. You can then switch from a step of a few millimeters to other several meters – as has probably happened in today’s case.
The formation of a underwater step causes the rapid movement of the whole column of water that dominates it: This is how the wave train that characterizes a tsunami starts.
At this point the waves begin to spread in every direction, reaching a huge amount of coasts even hundreds of km away. Just to give an example, here is a video showing the propagation of the 2011 tsunami in Japan:
https://www.youtube.com/watch?v=Jh3- HQJTGDQ
Once the waves are traveling, their speed can vary depending on several factors, even if the most important one is undoubtedly theheight of the water column: The higher this, the greater the speed of the tsunami. Do you think that in the middle of the ocean, where the water column is higher, the tsunami can exceed 800 km/h: let’s talk about values similar to those of a scheduled plane! At the same time it is right to clarify that, once the lowest waters are reached close to the coasts, theirs speed cala drastically and is normally between 20 and 50 km/h. These values may seem low, but if we consider that the height of the water wall can also be several meters, the destructive power of these phenomena is clear.
The Noaa also provides a formula that allows us to calculate this value with greater precision. This is achieved by making the product between the water depth el ‘acceleration of gravitythen making the square root of the result obtained. Always following their example, in a water column from 4.5 kmthe tsunami will move at a speed of about 756 km/h. This means that a hypothetical tsunami to go from the coasts of Portugal to the United States would use something like Seven and a half hours.
By applying this formula to today’s case and assuming an average depth identical to that proposed in the previous example, the tsunami will use about 2 hours To go from Kamchatka on the island of Thoku, in northern Japan. Obviously this formula is an approximation, mainly linked to the fact that the depth of the ocean is not constant throughout the path, but it can help us have a broad idea of the travel times of the earthquake.
ATTENTION: These data give us information about the arrival time of a tsunami but not regarding its height. Usually the greater the distance from the epicenter, the lower the height with which these waves break down on the coasts.
