The Big Island got the brunt of the tsunami that rolled through Hawaii early Friday.

Experts say that’s a function of the islands’ formation and the location of the earthquake that started the huge waves off of the coast of Northern Japan.

“It’s basically geography and a bit of bad luck,” said geophysicist Gerard Fryer of the Ewa-based Pacific Tsunami Warning Center, the part of the National Oceanic and Atmospheric Administration’s National Weather Service that monitored last week’s event.

“If there’s a tsunami coming from Japan, the Kona coast always seems to get it, and our numerical models seem to support that,” he said. “The Kona coast faces the wrong direction. While tsunamis can cause run-up on all sides of all islands, it’s usually the worst on the side facing the incoming tsunami.”

State Civil Defense spokeswoman Shelly Ichishita said that because you never know where an earthquake will happen, “you can never tell exactly how the energy is going to come at you.”

“With the wraparound effect, all islands are at risk,” she said.

For that reason, the state has not necessarily concentrated its resources in areas that have been historically hit harder by tsunamis.

“The majority of the sirens probably are on Oahu,” she said. “It’s because there are more people here and more people living in coastal communities.”

Kona got the worst of it last week, but other Big Island locations have been hit hard in the past. The history of tsunamis in Hawaii shows that some of the deadliest have hit Hilo.

Kahului, Maui, and Haleiwa, Oahu, also saw larger-than-average tidal shifts Friday.

“There are several locations that always trap tsunami energy,” said Kwok Fai Cheung, a professor in the University of Hawaii‘s Department of Ocean and Resources Engineering who designed the worst-case inundation models that have been used as the basis for evacuation maps.

He said that while the first wave emanating from the earthquake epicenter is the largest out in the open ocean, the second or third is often larger when they make landfall because “they are adding to the trapped energy of the trapped waves.”

“After the tsunami passes past the islands, we could not drop the warning mainly because the water around Maui was still bouncing up and down,” Cheung said.

Asked about the idea that coral reefs play a role in protecting some islands better than others, Fryer said, “Reefs do a very good job of protecting the shoreline, but we don’t have them in Hawaii” — with the exception of the south shore of Molokai and Kaneohe Bay in Windward Oahu.

“If you have a coral reef offshore, that cuts down the amplitude tremendously,” he said. “Everywhere else in the tropical Pacific, the tsunami hazard is amazingly low.”

That’s hardly comforting to those in wave-battered Kona.

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