Tsunami warnings caused panic. How close to disaster did we come?

USA Today

3 days ago

The earthquake that hit a Russian island was among the most powerful ever recorded and forecasters immediately feared it could spur cataclysmic walls of water thousands of miles away in Hawaii, California and Alaska.
But hours later, evacuation warnings were lifted for nearly all of the United States. Damage was determined to be minimal ‒ at least in the United States so far, with Honolulu's dangerous traffic gridlock the main impact.
Earthquakes roughly the size of the 8.8 magnitude quake off Russia's coast have triggered deadly tsunamis before, including the one that struck Japan in 2011, killing 18,000 people, and one in the Indian Ocean in 2004 that killed more than 200,000. The giant waves haven't materialized this time, although some parts of Alaska's Aleutian Islands remain under a less urgent advisory.
Many factors go into whether a tsunami will devastate communities or become mostly a false alarm. AccuWeather Meteorologist Peyton Simmers said forecasters didn't expect the Russian earthquake to generate waves in the United States as large as Japan saw in 2011. Still, Simmers said it could have been extremely perilous.
"It could have been a far worse situation, that's for sure," Simmers told USA TODAY. "It could have caused a lot more damage."
Authorities said those returning to their homes should exercise caution and keep an eye out for damage. In Alaska, forecasters said tsunami waves could continue their impact for days and make currents dangerous for anyone at the beach.
How close to disaster did we come?
Experts didn't expect a catastrophe akin to the tsunamis caused by some of the other largest earthquakes recorded – such as the 1960 Chilean quake of 9.5 magnitude that sent a tsunami to the United States, killing 61 in Hawaii and two along the West Coast or the Japan tsunami from a 9.0 magnitude earthquake – Simmers said.
That's probably what many were worried about when they heeded evacuation orders and moved to higher ground or higher levels of buildings the afternoon of July 29. Forecasters predicted waves between 3 and 9 feet with a projected maximum of 9.8 feet to hit Hawaii.
Instead, the highest wave amplitudes recorded were just about 6 feet, Simmers said. By way of comparison, Japan's tsunami generated waves that reached about 130 feet in the prefecture of Iwate.
Part of the reason it wasn't worse here, Simmers said, was the distance. The tsunami wave generated by the Russian earthquake caused some damage there but had to travel thousands of miles before it could reach Hawaii, which is in a uniquely vulnerable position in the middle of the Pacific Ocean, where it can be reached by tsunamis originating from all directions, he said.
Earthquakes cause tsunamis when they're big enough and close enough to the ocean floor, causing their energy to displace the ocean floor. When the ocean floor suddenly rises or falls, so does the water above it, creating a tsunami.
A distant tsunami is generally only a threat when an earthquake's magnitude is at least 8.0, according to the National Oceanic and Atmospheric Administration. But as the tsunami travels, it loses energy, Simmers said.
Other factors that impact how serious a tsunami is include the shape of the coastline and the shape and size of a continental shelf, Simmers said.
In this case, the size of the tectonic plate rupture and subsequent sea-level rise was not as large as comparably sized earthquakes that have caused much more damaging tsunamis, Simmers said.
The relationship between earthquake size and tsunami size isn't quite straightforward, either, according to Brandon Shuck, an assistant professor of geology and geophysics at Louisiana State University.
Add that to the near impossibility of knowing just how much an earthquake has caused the sea floor to shift, and it makes predicting a tsunami's size and impact difficult, Shuck said.
"Nevertheless, the earthquake did produce a trans-Pacific tsunami ‒ a tsunami that can be detected from traveling all the way across the Pacific Ocean. This is still a huge and notable event, placing it firmly in the top 10 largest earthquakes ever recorded, and the tsunami warnings were all warranted," Shuck said.
Could a catastrophic tsunami happen in the US?
Hawaii, Alaska and some areas of the West Coast have a long history with tsunamis. Hawaii is one of the most susceptible spots in the world for tsunamis, and, on average, it experiences a destructive tsunami once every 11 years.
It's not a matter of if, but when, another one happens there, Simmers said. Hawaii sits in a zone infamous for seismic activity known as the Ring of Fire. Huge, destructive earthquakes are rare, but it only takes one to trigger a tsunami that could have extreme devastation in Hawaii or the West Coast, Simmers said.
"There's always that chance," he said. "In theory, it can, and probably will one day. When that is: who knows."
In 1946, an 8.6-magnitude earthquake in the Aleutian subduction zone generated a tsunami in the Pacific that reached as far south as Antarctica, according to the U.S. Geological Survey. The April 1 tsunami that year devastated the town of Hilo, Hawaii, on the Big Island. More than 150 people were killed when the tsunami, with runup wave heights between 33 and 55 feet, struck. Most of the deaths were in Hilo.
A formal tsunami warning system wasn't established until after the 1946 catastrophe, but the Coast Guard issued warnings to many communities. In some communities, though, the warnings had the reverse effect, drawing onlookers to the coast to observe the phenomenon, according to a 1993 account from the NOAA.
"This event happened on April 1, April Fool's Day, and some mistook the warning and reports of a tsunami as a hoax," the NOAA report reads.
Contributing: Dinah Voyles Pulver