Understanding Tsunami Alert Levels: What Actions Should You Take if Notified?

HONOLULU — A powerful earthquake off the coast of Russia prompted tsunami warnings and advisories for large areas of the Pacific, including Alaska, Hawaii, and the U.S. West Coast.

The earthquake had a magnitude of 8.8 and was situated off the coast of the Kamchatka Peninsula, as reported by the U.S. Geological Survey. The quake occurred early Wednesday local time, which was still Tuesday in the U.S.

Dave Snider, the tsunami warning coordinator at the National Tsunami Warning Center in Alaska, described the earthquake as “absolutely notable” and “a significant earth event.”

Here’s what to know about tsunamis and their warnings:

Tsunamis result from earthquakes, underwater volcanic eruptions, and submarine landslides. When an underwater earthquake occurs, the seafloor rises and falls, causing the water above to lift and drop. This energy transfers into the water, creating waves.

Many people think of tsunamis as one wave. But they are typically multiple waves that rush ashore like a fast-rising tide.

“Tsunamis move across the ocean at hundreds of miles an hour — as fast as a jet airplane — when in deep water,” Snider explained. “However, as they approach shore, they slow down and begin to build up. That’s when the risk of inundation becomes more probable.”

Some tsunamis are minor and cause no harm, while others can lead to extensive destruction. For instance, a 9.1 magnitude earthquake in 2004 struck off the coast of Indonesia, creating waves that devastated remote villages, ports, and tourist destinations around the Indian Ocean in Southeast and South Asia, resulting in about 230,000 deaths.

In Hawaii, emergency authorities blast alerts to people’s cellphones, on TV and radio and sound a network of sirens.

In Alaska, some communities have sirens and information is also available on weather radio or public radio broadcasts. Official accounts on social media and cellphone push alerts spread the word. In some places, local officials have relayed warnings door to door.

Earlier this month, after another earthquake off Alaska’s Aleutian Islands, public safety departments in King Cove and Unalaska sent alerts urging those in coastal areas or those that could see inundation to seek higher ground.

Authorities urge people to move to higher ground when they have indications a tsunami will arrive.

In Hawaii, the state’s emergency management agency directs people to check maps and to evacuate if they are in a tsunami hazard zone. It also tells people to stay at least 100 feet (30 meters) away from inland waterways and marinas connected to the ocean due to the possibility of wave surges and flooding.

Some communities have buildings designated on higher ground as meetup points during tsunami warnings, such as a school, while others might simply urge residents to retreat up a hillside.

For those in an evacuation zone, experts recommend you take your “go bag” and evacuate outside the tsunami zone or evacuate vertically to at least the fourth floor of any building at least 10 stories tall.

In the U.S., the National Weather Service has different levels of alerts:

— A warning means a tsunami that may cause widespread flooding is expected or occurring. Evacuation is recommend and people should move to high ground or inland.

— An advisory means a tsunami with potential for strong currents or dangerous waves is expected or occurring and people should stay out of the water and away from beaches and waterways.

— A watch means that a tsunami is possible and to be prepared.

It depends on how far the epicenter of the earthquake is from a coastal area. It could take just minutes for waves to hit land next to the site of a major quake. It could take hours for tsunamis to cross the Pacific Ocean.

The speed of tsunami waves also depends on ocean depth. They travel faster over deep water and slow down in shallow water.

Waves less than a foot (under 30 centimeters) above tide levels were observed in the Alaskan communities of Amchitka and Adak, Snider said.

Waves washed up to the shoreline on Japan’s Hokkaido in the north and Ibaraki and Chiba, just east of Tokyo, in footage aired on Japan’s NHK public television. A tsunami of 50 centimeters (1.6 feet) was detected at the Ishinomaki port in northern Japan, according to the Japan Meteorological Agency.

The National Tsunami Warning Center in Alaska said some places could still be feeling impacts from the tsunami for hours — such as in Adak, a community of about 70 people in the Aleutian Islands — or perhaps more than a day.

A tsunami warning was issued for Hawaii, with the first waves expected to arrive after 7 p.m. local time. The Pacific Tsunami Warning Center in Honolulu said the tsunami could cause damage along the coastlines of all the Hawaiian islands.

Much of the Pacific coast of North America spanning from British Columbia in Canada to down the U.S. West Coast and into Mexico was under a tsunami advisory.

Waves up to 5.7 feet (1.7 meters) were also possible in Northern California’s Crescent City, which was under a tsunami warning and turned on its warning sirens.

The Oregon Department of Emergency Management said on Facebook that small waves were expected to reach parts of the state’s coastline starting around 11:40 p.m. local time, with wave heights between 1 to 3 feet (30 to 91 centimeters).

It urged people to stay away from beaches, harbors and marinas and to remain in a safe location away from the coast until the advisory was lifted.

“This is not a major tsunami, but dangerous currents and strong waves may pose a risk to those near the water,” the department said.

Mexico’s navy warned that waves between 1 to 3.3 feet (30 to 100 centimeters) were possible on its coast.

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