HILO, Hawai'i — Sixty years ago today, the earth shifted violently deep below the surface of the ocean on the north slope of the Aleutian Trench south of Alaska's Unimak Island. That distant jolt delivered a deadly tsunami to Hawai'i, killing 159 people throughout the Islands.

That day was seared into the memories of survivors, but experts worry that many of the rest of us have all but forgotten about the tsunami hazard.

That amnesia could be deadly. Tsunamis have killed more people in the past century in Hawai'i than all other types of natural disasters combined, and one is sure to strike again.

The undersea shift at 1:59 a.m. April 1, 1946, generated a tsunami that raced down the Pacific at an average speed of 490 mph. It crossed the 2,240 miles to Hawai'i in slightly more than 4 1/2 hours, with the first wave reaching Kaua'i at about 5:55 a.m. and reaching Hilo at 6:54 a.m.

Jeanne Branch Johnston was a 6-year-old staying at her grandparents' home at Keaukaha in Hilo that day. She noticed water and debris in the yard, and walked out in her pajamas to the mailbox with her 4-year-old brother, David, to look around.

The two went back into the house and into the kitchen on the second floor just before the second wave hit, a decision that probably saved their lives. The wave tore through the garage under the kitchen, submerging the family car. The backyard was under 5 feet of water that reached the top of the clothesline.

With the adults urging her to hurry, Johnston changed and grabbed her mother's jewelry. They fled out to the street to try to escape the next wave. They joined a neighbor family whose house had been destroyed. They all ran along the street with a whitewater wave coming at them.

They fled up a driveway to higher ground, helped by a neighbor who used a machete to hack through a tangle of hala trees to allow the children to cross old 'a'a lava to get them away from the shoreline .

"They kept yelling 'Run!' — which of course you couldn't do back there," said Johnston, who now lives in Honolulu.

BRIDGE PUSHED UPRIVER

At the other end of town, 15-year-old Bob Herkes was riding to school with his brother and looked out the car window as they crossed the Wainaku Street bridge over the Wailuku River to see a wall of water racing up the river channel.

They stopped the car and stared. The tsunami had ripped off a span of the railroad bridge that used to be where Hilo's "Singing Bridge" now stands, and was hurling the span up the river.

"We couldn't believe what we were seeing," said Herkes, who is now a state representative. "My grandfather designed that bridge, and he told me later that span weighed 100 tons. It just ripped it off and rolled it up the river."

Back in Keaukaha, Johnston's grandfather had refused to leave his home and was standing on the front porch as the third wave approached. He dived from the front porch inside the house as the wave picked up the entire house and sent it sailing into the backyard. He survived.

Eventually the adults who helped Johnston and the other children to escape decided it was safe to return. Johnston remembers walking back through her neighborhood and staring at a house turned upside-down and someone's arm protruding from under the debris.

TIMING WAS MERCIFUL

In Hilo alone, 96 people were killed. On the low-lying Laupahoehoe peninsula, 16 schoolchildren and five teachers died. Yet experts who studied the disaster afterward concluded that even more would have died if the tsunami had struck an hour earlier or an hour later.

If the waves had arrived earlier, more people would have been sleeping — and would have been swept from their beds. If the wave had arrived an hour later, Hilo's downtown business district would have been filled with workers and shoppers — and more would have died there.

Since then, tsunami inundation zones have been mapped and publicized, a better warning system was established, and civil defense response plans were drawn up and underwent drills. Still, experts worry that Hawai'i is unprepared for the day when another tsunami will arrive.

Johnston, who is now earthquake and tsunami program planner for state civil defense, wrote her master's thesis on a comparison of the 1946 and 1960 tsunamis, and her observations are not encouraging.

"Not only was there 13 hours of warning, there were sirens and police and fire and civil defense and radio and television," Johnston said. Even so, 61 lives were lost in Hilo in the 1960 tsunami. That death toll is "not a very good sign that awareness was increased by any means by the warning," she said.

Other tsunami survivors share her concern. Ka'u resident Bernard Waltjen survived the 1960 tsunami in Hilo by sprinting away from the Wailoa River area as the wave smashed through buildings behind him. "It sounded like 10 bulldozers going through a house," he said.

Waltjen believes Hawai'i is due for another huge wave and worries that by clearing much of the Hilo bayfront area battered by previous waves, authorities may have opened a path for the next wave to reach deeper inland.

Herkes shares some of those concerns. "I don't think we're at all prepared emotionally or any other way. ... We're building a courthouse in Hilo right at the edge of the tsunami zone and there's no shelter in it," he said.

George Curtis, tsunami adviser to the Big Island, said the state is "long overdue" for a damaging wave.

SHORTER INTERVALS

The last Pacific-wide tsunami hazardous to Hawai'i was in 1964, or 42 years ago. Previously, the longest known interval between such tsunamis affecting Hawai'i was the 27 years that ended in 1868, Curtis said.

Hawai'i did suffer damage and two deaths in an earthquake in 1975, but that was a locally generated tsunami, not Pacific-wide, Curtis said.

"The majority of the people in the state of Hawai'i either live in, work in or commute through the evacuation zones on a daily basis, so at any time day or night, it would be a huge impact," Johnston said. "Most of our critical infrastructure is in the evacuation zones."

She would like to see public schools include tsunami awareness and planning in the curriculum as they have included fire prevention. She said people should plan ahead because "when the siren goes off, that is not the time for critical thinking."

Johnston conceived and founded the Pacific Tsunami Museum in Hilo because she wanted to save the stories of the disaster and to help alert people that it could happen again.

"I realized we never talked about the tsunami. We just kind of cleaned up and went on with our lives, and nobody talked about it," she said.

Herkes said he believes there may be a "fatal attraction" that causes people to put themselves at risk because they want to see a tsunami roll in. Having seen one himself, Herkes doesn't share that impulse.

"I don't feel that at all," he said. "Let me see how high I can get, how far inland I can get."

Reach Kevin Dayton at kdayton@honoluluadvertiser.com.