The Risk Defined: New research reveals tsunami flood zones for Anchorage

A new report released Wednesday shows not only how a tsunami could reach the Upper Cook Inlet, including Anchorage, Girdwood, and Eagle River, but also just how far inland the water would go.
The Risk Defined: New research reveals tsunami flood zones for Anchorage
Published: Aug. 16, 2023 at 11:51 AM AKDT
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ANCHORAGE, Alaska (KTUU) - After decades of speculation, it is no longer a question of if, but when a tsunami floods portions of Anchorage. New research proves tsunami waves can reach Anchorage following a large mega-thrust earthquake.

A team of Alaska-based scientists released a new report Wednesday showing not only how a tsunami could reach the Upper Cook Inlet, including Anchorage, Girdwood, and Eagle River, but also just how far inland the water would go.

Southcentral Alaska sits on the edge of the world’s largest and deepest ocean and Alaska is the most seismically active region in the country. But until now, scientists never really knew how the water on the banks of Anchorage would respond to a major earthquake.

A team of scientists is now spreading the word that tsunamis are a real danger to communities in the Upper Cook Inlet, and anyone living or visiting the newly defined tsunami zones should take evacuation alerts seriously.

More than 50 Alaskan communities have detailed tsunami inundation maps, but until now, Anchorage was not one of the studied areas. Alaska Earthquake Center Tsunami Modeler Elena Suleimani said it’s time for that to change.

“Thinking about all these years when people thought that Cook Inlet is immune to tsunami hazards, and I couldn’t find the single good argument why it’s immune to tsunami impacts. And that’s why we had that study done,” Suleimani said.

NOAA awarded a grant to scientists at the Alaska Earthquake Center, the Alaska Division of Geological & Geophysical Surveys, and the Alaska Division of Homeland Security & Emergency Management to do the work to uncover the mystery of the tsunami threat to Anchorage.

The team modeled 16 hypothetical earthquakes and found several scenarios where a tsunami inundated sections of Anchorage. The first group of earthquakes is in between Kodiak Island and the Kenai Peninsula under the Kennedy and Stevenson entrances,” said Barrett Salisbury, the Earthquake and Tsunami Hazards program manager for the Alaska Division of Geological and Geophysical Surveys. “The second group of earthquakes occur southwest of Kodiak Island, and energy from those earthquakes travels through Shelikof Strait to get to Upper Cook Inlet.”

This map shows where the 16 hypothetical earthqakes would need to occur to create the tsunami...
This map shows where the 16 hypothetical earthqakes would need to occur to create the tsunami scenarios modeled in this report.(Alaska's News Source)

The models proved there are areas that could be underwater if a large quake happens in the right place at the right time. “Most of the inundation we see along the coastlines are where we have water bodies or rivers flowing into Turnagain or Knik arms,” Salisbury said.

The interactive map below shows the expected inundation zones for Anchorage, Knik Arm, and Turnagain Arm. Click on the map to zoom in to see a closer look at where the tsunami waves are expected to go, and how high the water is expected to be.

Suleimani also addressed a widespread misconception that suggested Cook Inlet is too shallow for a tsunami.

“The depth only affects the propagation speed, which means that in shallow water tsunamis propagate slower. It just means that it will reach Anchorage later rather than sooner. So, it will take several hours for that tsunami to get to the Upper Cook Inlet, but that doesn’t mean it’s not getting there,” Suleimani said. This means, should a tsunami be triggered by a large earthquake, areas in Upper Cook Inlet would have hours of warning time.

The areas just west and east of Anchorage have had real-life examples of mega-thrust earthquakes, including the 1964 Great Alaska Earthquake, which was the largest quake to ever hit the Northern Hemisphere. This quake generated the most destructive tsunami in North America, which killed more than 100 people, until now, it has been widely assumed that the tsunami did not hit Anchorage after the 1964 quake, but this research also proved that’s not the case.

“It came at 2 a.m., and it came on low tide, but it was pretty substantial,” Suleimani said. “It was about three meters, which is like 10 feet high. And if it happened at high tide during daytime, it would be noticeable, no doubt.”

With Anchorage’s extreme tidal ranges, timing is everything. Salisbury said when the next big earthquake hits, it will be crucial to look at whether the tide in Upper Cook Inlet will be high or low about four and a half hours after the quake.

Upper Cook Inlet has the highest tide change in the U.S. The tidal ranges average around 30 feet. That means a tsunami can enter with the tide, making the impacts far greater. Or it can enter against the tide, which would minimize how far the water will travel.

“We don’t know when it’s going to happen, but we need to get prepared because if it’s not going to happen during our lifetime, it will definitely happen during our children’s lifetime or grandchildren’s lifetime,” Suleimani said.

“We don’t want our grandchildren to run to the shore when the water goes away suddenly for no reason. We want them to know exactly what it means, that they need to go to high ground, they need to evacuate,” she said. “It doesn’t matter where you are, you need to be able to recognize the tsunami warning signs, as you need to know what to do exactly. So this is how we can save lives.”

The National Tsunami Warning Center based in Palmer monitors earthquakes worldwide around the clock. Should there be an earthquake that has the potential to trigger a tsunami, an alert would be issued within minutes of the quake occurring. Here is a closer look at how tsunamis are formed, and how scientists know if they are headed your way.

The Risk Defined: What to do when you get a tsunami warning — even in Anchorage

One area that could see significant impacts, should a tsunami hit Anchorage is the Port of Alaska, which sits near sea level and on the edge of the water. Here is a closer look at how the port is preparing now.

The Risk Defined: Port of Alaska inside tsunami inundation zone, new study says