Study shows tides and earthquakes could create tsunami inundating parts of Anchorage

There is a persistent belief that Anchorage, snug at the head of narrow Cook Inlet, is too far away from the open ocean to be at risk from tsunamis.

That is false, according to a newly released hazard-mapping report from the University of Alaska Fairbanks’ Alaska Earthquake Center and the state Division of Geological and Geophysical Surveys. The report provides the first-ever tsunami innundation analysis for Alaska’s largest city and the upper part of the inlet, a region where such hazards were previously overlooked.

“There is a rare but real tsunami threat to that area, and we describe why it is possible and all the hypothetical tsunami scenarios are outlined in our report,” said lead author Elena Suleimani, a tsunami modeler with the Alaska Earthquake Center, which is part of UAF’s Geophysical Institute.

The report, the product of years of study and modeling work, shows how a particular combination of high tides and earthquake conditions could send walls of water into certain low-lying areas in and around Alaska’s largest city.

There were some logical reasons for the past false sense of security, the study authors said in an online news conference.

Previously, the belief was “that when the wave comes, it will dissipate and it will just break apart and nothing will hit Anchorage,” said co-author Dmitry Nicolsky, also with the Alaska Earthquake Center. “We show in this study that this belief is not true.”

Additionally, Suleimani said, there is a mistaken assumption that Anchorage already endured the worst in the magnitude 9.2 earthquake of 1964. “I think there is that common kind of knowledge or common thinking that 1964 was the worst-case scenario. And it’s not, unfortunately,” she said.

Cook Inlet, a glacier-fed channel that leads to the Gulf of Alaska, has some of the highest tides in the world, with maximums over 30 feet. The modeling adds those tide conditions to potential earthquakes, with magnitudes ranging from 8.7 to 9.3, to map out expected tsunami flooding extents.

If a 1964-style earthquake ripped through the Gulf of Alaska off the Kenai Peninsula precisely when an extreme high tide was building in Cook Inlet, a tsunami could arrive four to five hours later, inundating low-lying areas from part of downtown Anchorage to sections of the Matanuska-Susitna Borough to communities along Turnagain Arm, the new study found.

Worst-case scenarios for Anchorage and Upper Cook Inlet, according to the new study, include flooding coastal Anchorage areas like Westchester Lagoon on the edge of the downtown area and along Knik Arm up to the mouths of the Matanuska and Susitna rivers, which lie past existing highway bridges. These scenarios also include up to 72 hours of dangerous waves and currents. Hotspots for flooding hazards include the vitally important Port of Alaska, the Campbell Lake area in South Anchorage and the sections of Girdwood and Hope that are closest to Turnagain Arm.

While the magnitude 9.2 quake that struck Alaska in 1964 was the second-most powerful on record in North America, analysis of seismic data collected over the past two decades shows that Southcentral Alaska has experienced even more powerful quakes in the last 1,500 years, according to the study.

The study uses a “worst-case kind of envelope” of scenarios, as is appropriate for tsunami preparedness, “because we cannot predict how big the next one is going to be, unfortunately,” Suleimani said.

Another revelation of the study is that Anchorage did experience a tsunami in 1964 – an event largely overlooked until this study. It went unnoticed because that wall of water, estimated at 10 feet high, arrived in Upper Cook Inlet at 2 a.m. and precisely when the tide was at its lowest, canceling out the effect.

Anchorage residents had other things to worry about at the time.  The massive damage sustained in the city from that event was from the shaking and ground and structure collapses. Of the total Alaska death toll of 115, nine deaths were in Anchorage. Most of the deaths from the 1964 quake were tsunami-caused, and there were fatalities as far away as California.

Tsunamis can be generated when earthquakes occur underwater. If the earthquake is close enough to the seafloor surface, it can deform it, causing parts to rise and other parts to fall and sending a mass of water inland.

Tsunamis were well-known hazards in Alaska even before 1964, when the earthquake ruptured a section where one of Earth’s tectonic plates, the Pacific Plate, slipped under another, the North American Plate.

The newly released study about Anchorage and Upper Cook Inlet hazards is part of a long-term project that has analyzed tsunami innundation risks for about 60 coastal communities in Alaska. That project started in the 1990s with communities, many of them tiny villages, that have tsunami experiences. Some, like Valdez, were completely or partially destroyed during the 1964 earthquake, and some were damaged by earlier tsunamis going back to the 18th century. In those communities, awareness is high, Suleimani said.

The situation in Anchorage is different, with more public education needed, as might be the case in a typical U.S. West Coast city, she said.

The impetus for studying tsunami risks in Anchorage and Upper Cook Inlet comes in part from the National Weather Service’s National Tsunami Hazardous Mitigation Program, which has a goal of addressing risks in all coastal communities, said study co-author Barrett Salisbury of the Alaska Division of Geological and Geophysical Surveys.

But part of the impetus was the experience in Anchorage on Nov. 30, 2018, when a magnitude-7.1 earthquake struck the city, he said. That triggered a brief tsunami warning — and “a lot of confusion,” Salisbury said. “There were no solid answers for us to say yes or no, there is a hazard, one way or the other.”

Some in Anchorage responded to that earthquake by heading to the mountains in their cars, resulting in heavy traffic all the way up to the Glen Alps parking lot of Chugach National Park, about 2,200 feet above sea level. It was an unnecessary step, as shown by the new study, which points out that most of the city would be beyond the reach of tsunami waters.

Now that the report is out, there will be more emergency planning, including possible mapping of evacuation routes that is common in other coastal cities, the authors said. Tsunami awareness in Anchorage could affect planning and design for development as well.

At the Port of Alaska, where officials were alerted last fall to the preliminary findings about Anchorage tsunami risks, the new information has already affected construction plans. Based on that new tsunami information, the port changed the site of equipment intended to boost the power system, said Jim Jager, director of business continuity and external affairs.

“We said, ‘You know, maybe it’s not such a good idea to put this resilience equipment down where it could be potentially taken out.’ So we’ve now located everything to the top of the hill on Government Hill,” he said in an interview earlier this summer, naming an Anchorage neighborhood.

Jager likened the tsunami-hazard information to avalanche-hazard information that is already used in urban and residential planning. The new information could affect availability of insurance or loans or have other consequences that should be considered, he said. “If you’re going to build a facility that’s going to be there for 50 years or 75 years, or for however long, let’s think about it,” he said.

Coming next for the tsunami-research group is an innundation study for the middle portion of Cook Inlet, territory that includes Kenai and Nikiski on the Kenai Peninsula and Tyonek on the west side of the inlet. That project is in the works and expected to be released in about a year.

A Lower Cook Inlet tsunami mapping study, encompassing Homer and Seldovia, was published in 2019. With the newly released study and the study to come, the full Cook Inlet region will soon be assessed, the authors said.

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