Alaska researchers say most of Anchorage is safe from the threat of a tsunami, but they warn that such a wave could affect Girdwood, Hope and some other coastal areas, including the Port of Alaska, under certain conditions.
That’s according to first-of-its-kind tsunami hazard modeling of Upper Cook Inlet in a report out Wednesday from the Alaska Earthquake Center, the state Division of Geological and Geophysical Surveys and the state Division of Homeland Security and Emergency Management.
The risk of a tsunami hitting Anchorage – Alaska’s largest city, at the head of Cook Inlet – has been the subject of debate for years. After several nearby earthquakes in the past decade, there’s been uncertainty around alerts warning residents that they are in danger. In the past, geologists have said the majority of Anchorage residents are safe from a tsunami, but there had never been a thorough study until the one released Wednesday, which includes detailed and updated inundation maps.
“One major thing that this report does is dispel the myth that there is zero chance a tsunami could reach Anchorage. We know that that’s not true,” state Earthquake and Tsunami Hazards Program Manager Barrett Salisbury said. “There are low-lying coastal areas that will potentially be inundated above high tide. But thankfully the majority of homeowners in Anchorage and people will not need to worry about their homes or their properties.”
According to the report, if a big enough earthquake hit in the right location at the right time – specifically, when there is a high tide in Upper Cook Inlet – a tsunami could overrun parts of the coast in the Anchorage area, including at the Port of Alaska and along Ship, Chester and Fish creeks. And the report notes that an earthquake could trigger a localized landslide in Cook Inlet, which itself could cause a fast-moving tsunami.
The study authors said in a press conference Wednesday that the potential tsunami impacts to the port are unclear and would require more research. About 75% of all water-bourne freight to Alaska enters the state through the Port of Alaska, in Anchorage.
People near any coastline when a big earthquake hits should always be concerned about a potential tsunami, the researchers said.
Still, in Anchorage, there would likely be plenty of time to warn people about an earthquake-generated tsunami, because, according to the report, Cook Inlet’s shallow water would make for a slow-moving wave.
The report authors modeled Alaska’s magnitude 9.2 1964 Good Friday Earthquake – the second-largest earthquake ever recorded – and found that a 10-foot wave likely hit the city’s coastline more than eight hours after the earthquake. But that tsunami went undetected, the report says, because it came in the middle of the night and coincided with an outgoing tide, which lessened the tsunami’s effect.
In a hypothetical, worst-case scenario described in the report, a large earthquake could strike at the entrance to Cook Inlet as the tide is coming in, causing strong currents and high water. The new inundation maps show the most acute impacts would be flooding in the Turnagain Arm communities of Girdwood and Hope and across the Knik Arm from Anchorage in the Point MacKenzie area.
Despite ongoing and widespread confusion from tsunami warnings buzzing residents’ phones – caused by “overalerting,” as one researcher put it – large earthquakes in the region will continue to trigger alerts for the entire Anchorage area, for the time being.
Dave Snider, warning coordinator with the National Tsunami Warning Center in Alaska, said his agency is working to solve that problem.
“So today, we’re limited by the ability to specifically warn very targeted parts of our geography,” Snider said. “And in the future, we’ll be able to warn very specific parts of our coastline, including the Anchorage coastline. But right now, the limitations that we have will likely alert a lot more people than actually need to move away from the coast.”