Hawaii’s civil defense sirens began blaring at 9 p.m. that Friday.

The first waves arrived less than two hours later — a 1-foot surge reported by the Coast Guard at Nawiliwili, Kauai.

Soon, an 11-foot wave rolled into Kahului Harbor, Maui, where damage would be tallied at $50,000.

Waves topping 12 feet hit Hilo, flooding restaurants at the head of Reeds Bay and breaking up a sidewalk near the west end of Waiakea Bridge.

By 1 a.m., the state issued a collective sigh of relief as the all-clear sounded.

One of the planet’s most powerful paroxysms occurred exactly 50 years ago.

The great Alaska earthquake of 1964, with a magnitude of 9.2, was and remains the largest quake ever recorded in the United States and the second largest anywhere, beaten only by the 9.5 quake in Chile in 1960.

A 30-block section of downtown Anchorage was nearly leveled. Sections of the Seward Highway split open down the center and buckled, and railroad tracks were bent into crazy curves.

Fishing boats were washed high and dry in Kodiak.

This megaquake not only devastated a vast swath of south-central Alaska, killing 131 people and causing $2.3 billion in damage (in today’s dollars), it shook up scientific notions about how great quakes are generated, affirming the then-still-novel theory of plate tectonics.

Hawaii largely was spared by the seismic sea waves that were generated— there were no casualties and damage was light. That’s because most of the tsunami energy was directed at the West Coast; 11 people died in Crescent City, Calif.

But the anniversary of the Alaskan carnage comes as scientists are reassessing what the proper response should be if a magnitude-9.2 shock happened not in Prince William Sound as in 1964 but in the central Aleutian Islands. That energy would be directed right at Hawaii.

"Certainly we will see more inundation if we have an extreme event from the Aleutian Islands," said Kwok Fai Cheung, a professor of ocean engineering at the University of Hawaii at Manoa and a tsunami modeler. "We are currently working on such a scenario to see what it might do to Hawaii."

Bottom line: The current coastal evacuation plans would not be sufficient for a 9.2 quake somewhere between Adak and Unimak.

But researchers are proceeding cautiously because the new evacuation lines would bring considerable disruption, said Gerard Fryer, a scientist with the Pacific Tsunami Warning Center.

"If we are going to come up with a new evacuation plan that turns everybody’s lives upside down, we are going to have to justify it with hard evidence," he said. "And the only evidence we have now is a sinkhole on Kauai."

The Makauwahi Sinkhole on the southeast shore of Kauai was discovered by David Burney, the former director of the National Tropical Botanical Garden on Kauai, in 1992.

Inside its limestone walls is evidence of a massive tsunami that struck sometime between 1540 and 1660, says the interim director of the Hawaii Institute of Geophysics and Planetology, a seismologist with the unlikely name of Rhett Butler.

The hole, 24 feet above sea level, contains the equivalent of nine shipping containers full of basaltic boulders, corals and shells — material that could have been deposited there only by an extreme tsunami, likely from an earthquake in the Aleutians, Butler says.

Butler, who visited the site last year with Fryer, gave a report on the sinkhole at the annual meeting of the American Geophysical Union in San Francisco in December.

"You have to model it in a way to get the water to go that far and push a lot of coral and basalt and shell rubble into it and you can date it," he said. "I’ve been working with the U.S. Geological Survey and they have dated a tsunami deposit in the Aleutians with the same date range. So we have a self-consistent picture."

Current tsunami inundation maps don’t account for the amount of water that flooded the sinkhole, Butler said.

One vulnerable spot in particular is the Kahe Power Plant, about 22 feet above sea level. Butler said 49-foot runups could be expected there.

The five decades since the Aleutian quake have seen a revolution in the way seismic waves can be modeled and described. Engineers also have studied the 1964 quake phenomenon known as "soil liquefaction," when the ground gets to the consistency of soup.

There is also a very efficient tsunami warning system in place, and Hawaii’s residents are generally well aware of the hazards, Fryer said.

"We’re ready for a tsunami now," he said. "We’re as ready as we’re ever going to get because we have had a lot of practice recently."

That includes the magnitude-8.8 quake in Chile in February 2010, the 9.0 quake and tsunami in Japan in March 2011, and the 7.8 quake in western Canada in October 2012.

"Unfortunately, these things happen on a time scale such that the next one comes when everybody has forgotten about the previous one," Fryer said.

"The real problem in 1964 was that there was no warning system that could respond fast enough for Alaska, and that actually remains a problem," he added.

Most of the deaths were from locally generated tsunamis from landslides, he said.

"We always say that the closer you are to the source, the more you are on your own, and that was certainly true in that case."

Landslides on the submarine slopes of Hawaii island could produce similarly dangerous waves, scientists have long warned.