How does a tsunami start?
Tsunamis have been a source of fascination for centuries with illustrations of ancient tsunamis preserved throughout countless generations of art and stories. Indeed, tsunamis are prevalent in Greek mythology, and they can be seen in the anger of Poseidon, as well as the mythology of the Duwamish people in the form of shape-shifting water spirits, A'yahos.
For something that seems like such a crazy anomaly, it might seem surprising that there are such detailed records of tsunamis dating back to prehistoric times. In fact, these prehistoric tsunamis, which have captivated humans’ imaginations since time immemorial, are identifiable through geological evidence.
But what are tsunamis and how do they start?
How does a tsunami start?
A tsunami is a giant wave usually caused by an underwater earthquake. Tsunamis can also be caused by underwater landslides, volcanic eruptions or anything else that causes the sudden movement of a large amount of water.
In an earthquake, two tectonic plates shift suddenly, causing the earth to shake. Tectonic plates are the large jigsaw puzzle-like pieces that form Earth’s crust. An earthquake in the ocean transfers its energy to the water, causing widespread displacement in its wake. This displaced water then ripples out from the spot of the earthquake, causing a tsunami.
Tsunamis that result from landslides or volcanic eruptions also form in a similar way. Landslides happen underwater in a similar manner as they do on land, as large amounts of sediment slide down steep hills. Underwater, this results in the large displacement of water, which can cause a tsunami.
Tsunamis caused by volcanic eruptions are less common and they can occur in a number of ways. Volcanic eruptions can lead to landslides because part of the volcano may collapse. This landslide, whether underwater or in a coastal area, could cause a tsunami.
Underwater volcanic eruptions can also lead to a steam geyser where hot magma and cold water meet. This steam geyser can cause the disruption of water that leads to a tsunami.
What happens when a tsunami reaches land?
Once an earthquake, landslide, or volcanic eruption causes a sudden movement of the ocean, water ripples out from the point displacement. This water can move in a single direction or ripple out in multiple directions.
As the tsunami moves through open water it may be barely noticeable, but as it moves closer to shore or to shallower water it grows in height. In fact, tsunamis can grow to be a hundred feet or more. In the open water, tsunamis can travel up to speeds around 500 miles per hour, but as they reach shore and become bigger they slow down to a speed around 30 miles per hour.
A person on the beach may see the ocean water pull back or drain away as a tsunami approaches. The tsunami would then most likely be a series of floods though sometimes there can be a large wave called a bore. These floods can be strong enough to destroy nearby infrastructure, pulling everything out to sea on when the wave recedes.
As well as being destructive to nearby buildings, these giant waves can be deadly to people in the vicinity. According to the World Health Organization, tsunamis killed more than 250,000 people between 1998 and 2017. One of the most deadly recent tsunamis, the Indian Ocean Tsunami in 2004, was responsible for 227,000 of these deaths.
The Indian Ocean Tsunami of 2004 was caused by a 9.1 magnitude underwater earthquake off the coast of northern Indonesia. This earthquake, thought to be the third strongest earthquake ever recorded at the time, displaced an estimated 30 cubic kilometers of water and generated a tsunami that impacted 14 countries including Indonesia, India, Thailand, Sri Lanka and even as far away as South Africa.
Tsunami waves of heights of up to 100 feet hit the shores of Indonesia within 15 minutes of the earthquake. Indonesia was the hardest hit with deaths totaling 168,000. Seven hours after the initial quake, the tsunami was still active, propagating across the ocean to places as far away as South Africa.
How do you know a tsunami is coming?
The National Oceanic and Atmospheric Administration (NOAA) and other organizations like the United States Geological Survey have systems in place to monitor possible tsunamis.
These organizations monitor seismic activity in order to watch for any earthquakes that could trigger tsunamis. If there is an earthquake that meets a predetermined set of criteria that makes it likely to cause a tsunami, then scientists start to monitor sea levels to see if a tsunami is imminent.
If these organizations determine that a tsunami was triggered they send out a warning. In the United States there are four levels of warnings: Information Statement, Watch, Advisory, and Warning.
These organizations make models of the incoming tsunami in order to estimate the wave height, impact places, and time of arrival. These models can also help to revise the warnings sent out to be more accurate.
The tsunami advisory warnings are then sent to the news, officials, and the public. Once a tsunami warning has been sent out, government agencies generally advise immediate evacuation in the most tsunami-prone areas.
Where are tsunamis most likely to occur?
While tsunamis can happen anywhere, they are most commonly formed in the Pacific Ocean. In fact, an estimated 80% of tsunamis originate in the Pacific Ocean.
The Pacific Ocean is such a major tsunami hotspot because of something called the “Ring of Fire.” The Ring of Fire contains around 75% of earth’s volcanoes and it is very seismically active. An estimated 90% of earthquakes happen in the Ring of Fire.
The abundance of volcanoes and earthquakes in the region makes the Ring of Fire a place with prime conditions to trigger tsunamis.
What was the biggest tsunami ever?
Although most tsunamis reach only 10 feet or less in height, there are also tsunamis that tower 100s of feet high.
The tallest tsunami on record reached a staggering 1,720 feet. This tsunami took place in Lituya Bay, Alaska in 1958. An 8.3 magnitude earthquake caused a landslide which triggered this tsunami. Luckly, because of the isolated nature of Lituya Bay only 5 people died.
More recently, a tsunami in Japan generated 38 foot waves. This tsunami was caused by an earthquake with a magnitude of 9.0 in the Pacific Ocean and an estimated 20,000 people were killed.
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