Upending previous understanding of Anchorage’s tsunami vulnerability, researchers said Wednesday that a “rare but real risk” exists that a confluence of conditions could lead an earthquake-produced tsunami to inundate parts of coastal Anchorage, including the Port of Alaska and much of Ship Creek.
The findings, presented by researchers from the Alaska Earthquake Center and the state Division of Geological and Geophysical Surveys, are the result of a first-time effort to model earthquake scenarios’ potential tsunami impact on Anchorage.
“A rare combination of earthquake magnitude, location, and timing must be satisfied for tsunami wave energy to reach upper Cook Inlet coincident with a natural high tide,” the study found.
Anchorage and state emergency management officials said they are weighing what to do about the new revelation about Anchorage’s tsunami vulnerability.
“We are thankful for the work the state has done to provide a definitive data that shows the Upper Cook Inlet has a rare but real tsunami hazard,” said Amanda Loach, director of the Anchorage Office of Emergency Management. The city and state will work together to develop a plan to address the risk, she said, starting with a series of community meetings as soon as October.
Researchers have long been modeling tsunami impacts for coastal Alaska communities, first focusing on places that had seen damage in the past, said Barrett Salisbury, one of the authors of the research and the Alaska Earthquake and Tsunami Hazards program manager for the state. It wasn’t until 2018, after a magnitude 7.1 earthquake shook Anchorage and caused widespread confusion about whether the city was or was not susceptible to tsunamis that the researchers began modeling for Alaska’s largest city, Salisbury said.
In the past, researchers have said that the shallow waters of Upper Cook Inlet would work to snuff out the power of a tsunami wave generated by an earthquake, though smaller, landslide-triggered earthquakes would still be possible. But that understanding was not based on scientific modeling, said Elena Suleimani, an author of the report and a tsunami modeler with the Alaska Earthquake Center.
“Up until now, our understanding of the risk or level of hazard exposure was just anecdotal,” Suleimani said.
Part of the reasoning that Anchorage was “immune” to tsunami destruction, the researchers said, was that during the 1964 earthquake — with a magnitude of 9.2 — there was no observation of a tsunami in Anchorage. But the earthquake, researchers found through modeling, did produce a 10-foot tsunami — which went unnoticed because it arrived at 2 a.m. during a minus-16-foot low tide, which meant the water level stayed below normal high tide levels.
The modeling of a potential Anchorage tsunami looks at what would happen in hypothetical scenarios involving a severe quake above 8.5 in magnitude. The researchers mapped hazards but don’t address the probability of such a quake occurring. But Southcentral Alaska is earthquake country: “Despite this relatively recent 9.2 earthquake, the region still has high potential for future large earthquakes,” the researchers found.
A potential worst-case scenario tsunami in Anchorage would affect some waterfront homes but largely parkland and infrastructure — like the port, Loach said. And the dynamics of the Upper Cook Inlet mean a destructive wave would probably be hours away, so people could be warned in advance, she said.
“We’re looking at mostly park areas and then secondary effects that might affect critical infrastructure. So we’re not so much thinking about life safety issues, so much longer-term effect,” she said.
Loach, the Anchorage emergency manager, said residents shouldn’t be unduly alarmed by the new information but should think about preparedness. The newly discovered risk means Anchorage will need to transition from being a city that doesn’t think of itself as susceptible to tsunami to one that does, she said.