How does a tsunami originate? From the earthquake to the coast, what factors can aggravate it?

Not all earthquakes at sea cause tsunamis, nor do all tsunamis originate from an earthquake. An earthquake like the one that generated alerts across the Pacific Ocean this Wednesday only occurs when certain factors coincide to generate this massive movement of marine waters. Typically, it must be a major earthquake whose epicenter (the projection on the surface of its underground origin) is on the seafloor or not far from the coast. Furthermore, the earthquake mechanism must generally cause this gigantic surge of water, because there is a rupture of the seabed, with a sudden vertical thrust from the ocean floor. That is, the earthquake must occur underwater, releasing significant energy in a vertical movement, so that the crust pushes the column of water above it up or down.

How do you know when to sound an alert?

The initial surge generates waves that propagate at high speeds through the ocean, up to 800 kilometers per hour in open water. But seismic waves travel much faster than tsunamis, so once a major earthquake like the one that struck Wednesday on Russia's Kamchatka Peninsula has occurred, warning systems can predict the behavior, direction, and arrival of the phenomenon with enough time to alert the population. Specifically, in the Pacific region, many countries have extensive experience dealing with tsunamis, such as Japan, the United States, and Chile, allowing them to react very quickly. The Pacific Tsunami Warning Center advanced its projections shortly after the earthquake, predicting waves of up to three meters on the coasts of Russia, Ecuador, and the Hawaiian Islands. In the latter, the first waves were measured at over one meter in height on Hilo Island, after the tsunami had traveled some 5,000 kilometers from its origin.

What factors aggravate the risk?

There are several factors that can aggravate the risk of an earthquake leading to a tsunami, such as when it occurs underwater or very close to the coast. The risk increases especially when the earthquake is of large magnitude (7 or greater), is shallow, and causes a vertical displacement of the seafloor, either lifting or sinking the ground. This type of movement mobilizes enormous volumes of water. Furthermore, earthquakes can also trigger underwater landslides that push up water and generate very large waves. Joanna Faure Walker, professor of Earthquake Geology and Disaster Risk Reduction at University College London, explained in a statement from the Science Media Centre (SMC) that seafloor displacement, the depth and shape of the coastline, and the existence of marine barriers or physical protection measures also play a role. This is also true for land-based obstacles, whether natural or human-made. "Large areas of low-lying terrain are particularly vulnerable, as they can be largely devastated by a tsunami," she explained.

How long do you have to be alert?

The time between the earthquake and the arrival of the waves depends on the distance. If the epicenter is very close to the coast, the waves can arrive in a few minutes, sometimes in less than 10 or 20. If the epicenter is farther away, the waves take longer, even hours, and warning systems can provide early warning, as happened in this case in Japan and the United States. Monitoring usually lasts at least three to six hours because a tsunami doesn't always consist of a single wave; several can arrive, and the first isn't necessarily the largest.

What to do in case of a tsunami?

In the event of a tsunami, the key is to act quickly and without waiting for official instructions when there are clear signs of danger . If you are on the coast and feel a strong, prolonged earthquake, the most sensible thing to do is assume that a tsunami may follow. You should also react if you notice the sea suddenly receding, leaving the beach dry, or if you hear a strange, loud roaring noise coming from the sea, produced by the movement of the water.

If you see any of these signs, the safest thing to do is to immediately move away from the coast and head for higher ground or inland, at least two kilometers inland or to elevated locations, such as hills or multi-story buildings. It's important not to stare out to sea or try to get closer to see what's happening.

During evacuation, if there is traffic congestion, it is preferable to move on foot to avoid being trapped. Do not return to the beach until instructed by authorities, as waves can occur over the course of several hours. After a tsunami, follow the instructions of the Civil Guard and avoid entering flooded areas or touching downed power lines. Helping others should always be done as long as it does not put your personal safety at risk.

Is there danger on the high seas?

In the deep ocean, tsunamis have lengths of hundreds of kilometers and heights of only a few meters, making them imperceptible. Therefore, it is often said that mariners on the high seas will normally not notice a tsunami when it passes beneath the hulls of their ships. However, as it approaches the coast, the speed decreases and the wave height increases, sometimes exceeding 30 meters. Therefore, the real danger is decided in the last few kilometers, when the tsunami generated by an earthquake leaves the deep ocean and reaches the coast, where the geomorphology of the coastline is crucial in dissipating its force or aggravating its intensity.

How strong was the earthquake?

The 8.8 magnitude Kamchatka earthquake is one of the 10 largest recorded worldwide since 1900, according to the U.S. Geological Survey. It is the worst since the tragic 9.0 magnitude earthquake in Tohoku, Japan, in 2011, which triggered the Fukushima tsunami that devastated its nuclear plant.

Was such an earthquake expected?

Wednesday's quake is the latest in a 10-day sequence of earthquakes off the coast of the Kamchatka Peninsula. Before Wednesday's earthquake, 50 other aftershocks greater than magnitude 5.0 were recorded, including one measuring 7.4 on July 20, according to the U.S. Geological Survey. On July 30, 24 aftershocks greater than magnitude 5.0 were detected, including one measuring 6.9.

Why is Kamchatka moving?

The Earth's outer layer is divided into enormous tectonic plates that fit together like puzzle pieces. The earthquake occurred in a highly seismically active region, where the Pacific Plate slides beneath the North American Plate. In the Kamchatka Peninsula region, the Pacific Plate is moving northwest at a rate of about eight centimeters per year, making it one of the fastest converging boundaries in the world, according to the U.S. Geological Survey. A magnitude 9 earthquake already occurred there in 1952, with an epicenter located just 30 kilometers from Wednesday's. Since then, the plate has moved almost six meters. In 1923, another major earthquake, measuring 8.4, struck the Russian peninsula. The U.S. agency believes that Wednesday's 8.8 quake filled "any small gap" between the rupture of 1923 and that of 1952.