How do tsunami warning systems work?
The 8.8 magnitude quake prompted tsunami alerts across the Pacific, as waves pushed forward towards Russia, Japan, Hawaii and the United States' West Coast, as well as many other nations.
Tsunamis can't be predicted. They don't occur during a specific season, they don't strike in the same place, and they don't follow any pattern that would allow scientists to estimate how strong the next one could be.
They can hit any coast at any time. But they can't be prevented, and for people living in coastal communities, all they can do is prepare for if, or when, one happens.
The only respite is that tsunamis don't occur often, and the majority are not destructive.
What is a Tsunami?
The word Tsunami is Japanese, coming from the characters for harbour ("tsu") and wave ("nami").
A tsunami is a series of extremely long waves, primarily caused by underwater earthquakes, which suddenly displace the ocean's floor.
Other possible causes are landslides, volcanic activity, certain types of weather, and meteorites.
Like a rock that is dropped in a puddle - tsunamis radiate outwards in all directions from the point of origin.
Tsunami waves can move as fast as a jet plane, clocking in at over 500 mph. Some have enough power to cross entire ocean basins.
If looking at the water's surface while far out at sea, tsunamis can go undetected. Boats may not normally notice one as it passes below them.
As the waves reach shallow water near land, they slow down and grow in height - most tsunamis are less than 10 feet high, but some can reach over 100 feet.
They pose an incredibly dangerous threat to coastlines, risking coastal flooding and powerful currents that can last for hours or days.
The History of Tsunami Warning Systems
The creation of large-scale tsunami warning systems began in the 1940s, formed separately by Japan and the US after both experienced destructive tsunamis.
Japan's functioned as a local warning system, while the USA's covered more distance.
In the aftermath of the 1960 Chilean earthquake and tsunami, which killed hundreds of people in total across Chile, Hawaii, Japan, and the Philippines, the nations of the Pacific decided to coordinate efforts.
Most earthquakes and tsunamis occur in the Pacific Ocean, as it holds the largest ocean basin and the "Ring of Fire".
The Ring of Fire is an area laden with volcanoes and sites of seismic activity, covering the edges of the Pacific Ocean.
An estimated 75% of all active volcanoes are dotted along the Ring of Fire, and 90% of all earthquakes occur there.
Unesco says 76% of history's fatal tsunamis have occurred in the Pacific Ocean.
However, the Indian Ocean Tsunami, or the Boxing Day Tsunami, which killed 230,000 people, highlighted the need for tsunami warning systems elsewhere and changed the way they work.
How do Tsunami Warning Systems Work?
The first sign of a potential tsunami is usually an earthquake, which creates seismic waves.
Seismic waves travel around 100 times faster than tsunamis, meaning they can be analysed far earlier than other methods.
Seismic networks, a collection of stations whose job it is to measure ground movement, can provide information about an earthquake's location, depth, and magnitude.
Tsunami warning centres are then able to use this information to determine whether a tsunami could have been formed, and whether it is necessary to issue a tsunami alert.
If the earthquake meets certain criteria, warning centres use information from water-level networks.
These monitor changes in the water using sea-floor censors and surface buoys, which can indicate the existence and size of a tsunami.
Tsunami warning centres will run forecast models to simulate the tsunami's movements across the ocean and estimate its predisposition for damage, such as wave height, arrival times, location and duration.
These forecasts allow warning centres to decide if a tsunami alert should be updated or cancelled.
The system is not foolproof, though.
If a tsunami is triggered by a non-seismic event, like a volcanic eruption or meteorite, water-level networks may not be able to create a detailed forecast with enough time, and seismic networks won't pick it up accurately.
What Types of Tsunami Alerts are there?
There are three levels to tsunami alerts: a tsunami watch, a tsunami advisory, and a tsunami warning.
A tsunami watch is issued when a tsunami may later impact the watch area. It can be upgraded or cancelled depending on updated information.
A tsunami advisory is issued when a potentially dangerous tsunami is imminent, expected or occurring. Severe flooding is not expected in areas under an advisory, but local officials may evacuate immediate coastal areas like beaches, harbours and piers.
A tsunami warning is issued when a tsunami with the potential to cause widespread flooding is imminent, expected or occurring, which could be accompanied by powerful currents and continue for several hours. Officials may evacuate low-lying coastal areas and reposition ships to deep waters if there is time.
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