5-foot-high tsunami hits Hawaii, waves in Japan, Russia: When does a tsunami strike?

In what was one of the strongest recorded earthquakes in the world, a massive 8.8-magnitude quake struck Russia’s Far East early on Wednesday.

The quake led to tsunami waves reaching Japan, and Alaska and Hawaii, and warnings for North and Central America, and Pacific islands stretching south to New Zealand.

Read latest updates on the tsunami warnings here.

A video shared on Russian social media appeared to show parts of a town underwater.

In Japan, residents evacuated to higher ground, either on foot or by car. This included people in Hokkaido, where a wave measuring 30 centimetres was first recorded.

In the United States, tsunami sirens sounded in Honolulu as waves up to 5 feet high reached Hawaii.

Notably, there are four levels of tsunami alerts, which help inform emergency officials, the public, and other agencies about the risk of a tsunami.

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But how do tsunamis form? What do the different alert levels mean?

We answer these questions and look at the latest developments in Russia, Japan and the US below:

How do tsunamis occur?

Tsunamis are a series of waves caused by a sudden and powerful shift in the ocean.

They are most often triggered by large earthquakes that occur beneath or near the sea floor. However, tsunamis can also result from underwater landslides, volcanic eruptions, extreme weather, or even meteorite impacts.

According to the American National Oceanic and Atmospheric Administration (Noaa), earthquakes are responsible for around 80 per cent of all recorded tsunamis.

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These seismic events shake the Earth’s surface, shifting the water above and sending waves racing across the ocean in every direction.

But not every earthquake leads to a tsunami. For a tsunami to form, the quake must be strong enough and close enough to the seabed to cause the vertical movement of the ocean floor.

When the sea floor lifts or sinks, the water above it moves too. As the water tries to settle, it sends out a series of waves that spread out across the ocean.

The strength and size of a tsunami depend on several factors: how much the sea floor shifts, how wide the affected area is (which is often linked to how long the quake lasts), and how deep the water is at the point where the disturbance happens.

In some cases, the earthquake may also cause landslides, which can lead to a tsunami as well.

What are the different types of tsunami alerts?

There are four main types of tsunami alerts, each indicating a different level of risk. These alerts are issued by the Pacific Tsunami Warning Center and the American National Weather Service.

Understanding what each one means can help you stay safe. Here’s a look:

Tsunami Warning

This is the highest level of alert. It means a tsunami is either already occurring or is expected to hit coastal areas soon.

Dangerous flooding and strong currents are likely, and these conditions may last for hours or even days after the first wave.

Follow directions from local authorities. Evacuate if advised. Move to higher ground or go inland, away from the water.

Tsunami Advisory

This alert means that waves and strong currents are expected, which could be a risk to swimmers, boaters, and anyone near the shore.

Major flooding is not expected, but beach and harbour areas may still see some flooding.

Stay out of the water and away from beaches and inlets. Follow orders from local officials.

Tsunami Watch

A watch means a tsunami is possible, but there is still uncertainty. More information is needed to confirm the threat.

This alert often follows a distant earthquake somewhere in the Pacific. Use this time to prepare.

Go over your evacuation plan, check your emergency supplies, and monitor updates regularly. Do not evacuate unless the alert is upgraded.

Tsunami Information Statement

This alert is issued when an earthquake has occurred, but there is no threat of a tsunami or the threat is still being assessed.

It is shared to keep the public informed.

No action is needed, but stay alert in case the situation changes.

Tsunami alerts in Japan and the US

The 8.8-magnitude earthquake struck early Wednesday off the coast of Petropavlovsk, located on Russia’s remote Kamchatka peninsula. According to the US Geological Survey (USGS), it ranks among the ten most powerful earthquakes ever recorded.

Russian officials confirmed that a tsunami hit and flooded the port town of Severo-Kurilsk. Local media reported that waves measuring between three and four metres were observed in Kamchatka’s Elizovsky district.

Authorities said about 2,000 people in the area were evacuated.

Countries along the Pacific coast in North and South America, including the United States, Mexico and Ecuador, issued alerts warning residents to stay away from beaches likely to be affected.

Waves under 30 centimetres above normal tide levels were recorded in Amchitka and Adak, according to Dave Snider from the National Tsunami Warning Center in Alaska.

Thousands of miles from the epicentre, tsunami waves up to 5 feet high have reached Hawaii. Emergency shelters have been opened, and residents in Honolulu have been urged to move to higher ground.

Data from the National Oceanic and Atmospheric Administration showed waves over 4 feet were recorded in Haleiwa, on the north shore of Oahu Island in Hawaii.

Much of the US West Coast, including Canada’s British Columbia, Washington state and California, remains under a tsunami advisory.

In Japan, tsunami waves are hitting several areas along the eastern coast, ranging from the northern island of Hokkaido to large parts of Honshu, including areas near the Tokyo Metropolitan Region.

So far, waves up to 60 centimetres have been recorded, far lower than the earlier prediction of 3 metres.

Over 1.9 million people across 21 prefectures in Japan are currently under local evacuation advisories. In Hokkaido, some residents were seen taking shelter on the rooftop of a building.

Tsunami advisories, watches and warnings have also been issued in other countries around the Pacific, including the Philippines, Indonesia, Chile, Peru, Ecuador, Mexico, Panama and Taiwan.

With inputs from agencies