Even lying down can get you shot! An earthquake hit Chile in South America, but Japan, thousands of miles away, was hit by a tsunami!

On May 22, 1960, the coast of Chile in South America experienced the most violent earthquake in human history ever recorded - a magnitude 9.5 earthquake.

Starting at 3:11 p.m. local time, the earthquake shook the coastal country like crazy. The 1,000-kilometer fault even made the earth tremble. Ten minutes later, a tsunami over 25 meters high engulfed towns and ports near the Chilean coastline. About 3 kilometers inland, the wreckage of wooden houses swept by the tsunami could be seen everywhere. The earthquake and tsunami caused the death of thousands of people in Chile and left 2 million people homeless.

Chile after the disaster

Source | International Tsunami Information Centre (ITIC)

Although the tsunami brought such serious consequences, the end of this disaster was not Chile...

After the tsunami swept through Chile, it quickly spread throughout the Pacific Ocean, threatening almost all countries scattered in the Pacific Ocean, such as the Hawaiian Islands and other island countries scattered in the Pacific Ocean.

Although Hawaii had issued a small-scale early warning, many people were "deceived" by the first small tsunami and failed to avoid the real huge waves. Even 22 hours after the Chilean earthquake, Japan, thousands of miles away, was still hit by a 6-meter-high tsunami, killing hundreds of people.

1

Why is it almost inevitable that a large undersea earthquake will trigger a tsunami?

In our impression, tsunamis are often caused by earthquakes, so we subconsciously think that tsunamis are not "bigger" than earthquakes, but in fact, the scope of damage caused by tsunamis is wider.

Chile is located in the seismic zone where the Nazca Plate subducts the South American Plate. As the Nazca Plate gradually expands eastward and wedges under the South American Plate, stress accumulates. When the stress value reaches the limit that the plate can withstand, the plate suddenly breaks and an earthquake occurs.

The magnitude of the Chilean earthquake was as high as Mw9.5-9.6, the focal depth was 33 kilometers, the width of the fault rupture exceeded 150 kilometers, and the length was as high as 1,000 kilometers.

Source: Find Elements

The subduction earthquake rupture process based on the elastic deformation theory causes the seabed in the distant sea area to rise, while the coastline direction sinks. The rise and fall is like a seesaw that disturbs the overlying sea water from the seabed, which is the main cause of tsunamis.

However, an undersea earthquake does not necessarily trigger a tsunami. The generation of an earthquake tsunami is also affected by the undersea earthquake source fault, the depth of the source area, the magnitude, the depth of the source, and other factors.

Statistics provided by the China Earthquake Administration show that among 15,000 submarine tectonic earthquakes, only about 100 have caused tsunamis. It is generally believed that earthquakes that trigger tsunamis are generally above Mw6.5, with a focal depth of no more than 33km (average thickness of the earth's crust), and the focal area needs to have a relatively deep ocean depth and other conditions.

Still, any time you feel a large undersea earthquake in a coastal city, you should be prepared for a tsunami.

2

Why did Japan, tens of thousands of kilometers away, still suffer serious tsunami damage?

Chile is located in South America, and is separated from Japan in East Asia by the entire Pacific Ocean, a distance of 17,342 kilometers. Why can tsunamis cross the ocean and still cause such huge damage?

Source | Ministry of Natural Resources

The disaster-causing mechanisms of tsunamis and earthquakes are completely different.

Generally speaking, the disaster range of an earthquake is limited, usually no more than 1,000 kilometers. However, the disaster range of a tsunami can sometimes even exceed 20,000 kilometers.

The sea water movement caused by a tsunami is very different from the waves we usually see (wind waves and swell waves). The latter only fluctuate near the sea surface and the depth involved is not large. However, the sea water movement caused by an earthquake is the fluctuation of the entire layer of water from the seabed to the sea surface, with amazing energy.

The most important reason why tsunamis can travel across oceans is that the wavelength of tsunami waves is extremely long, even reaching 150 to 200 kilometers. The longer the wavelength, the less energy is consumed to propagate a certain distance.

Unlike wind waves and surge waves with relatively small wavelengths, for shallow water waves such as tsunamis, the amplitude of water fluctuations changes little as the propagation distance increases. In other words, the energy attenuation of tsunamis is very slow during the propagation process, so it can maintain its energy and propagate to a wider range.

Secondly, the propagation of tsunamis in space is not uniform in all directions. Its main energy has an obvious directionality, which is usually perpendicular to the direction of the fault during the earthquake. The direction of the fault can be approximately considered to be parallel to the overall direction of the coastline.

Therefore, the main energy of this tsunami process is directed towards the coast of Chile on the one hand and towards Japan in the northwest on the other.

In addition, the average water depth of the sea area where the earthquake occurred is about 5,000 to 6,000 meters, and the mass of the seawater above it is very large, and the attenuation of tsunami energy needs to work through seabed friction. During the propagation of a tsunami, if it encounters large seabed undulations such as islands or seamounts, its own reflected waves can offset part of the energy; similarly, if it encounters complex near-shore topography, it can also accelerate the attenuation of tsunami waves. However , in the ocean, the energy attenuation factors that can affect ultra-long waves such as tsunamis are not significant.

Once a tsunami wave enters the continental shelf, the wave height increases suddenly due to the rapid decrease in depth, so that when looking from land to sea, the tsunami is more like a wall of water suddenly pressing towards the land, thus bringing devastating disasters. Therefore, a tsunami is not a disaster in the deep waters of the ocean, but it is a disaster when it reaches the nearshore area.

In summary, a tsunami is a huge wave that has a wide impact range, is not easily attenuated over long distances, and is significantly destructive to coastal areas. Coupled with its unpredictability, scientists regard it as one of the most terrifying natural disasters.

Looking back at history, the tsunami caused by the earthquake in Chile was not accidental. This shows that even if we are at the two ends of the ocean, we still need to be vigilant and prevent the risk of large tsunamis.

Audit expert: Wang Zongchen

National Marine Environmental Forecast Center (Tsunami Warning Center, Ministry of Natural Resources) Assistant Researcher