Northern tsunamis aimed at Hawaii

Four simulated giant quakes from the Alaska-Aleutian area would send a huge tsunami south. One from the Eastern Aleutians would take dead aim at the Hawaiian Islands, circled in red. Color bar shows wave energy. / Credit: Courtesy Rhett Butler

Hawaii officials are busy redrawing their tsunami warning maps, enlarging the danger zone further inland and higher. That is because geologists have found evidence that the idyllic, peaceful Hawaiian island of Kauai was clobbered by a 30-foot wall of water about 500 years ago. That is three times the size of what used to be considered the biggest wave to hit the islands, a 1946 tsunami. And the East Aleutian seafloor is easily capable of unleashing this size of calamity again, scientists say. The giant wave would also run up the shore of the Big Island, if it was triggered by a quake as strong as the one that devastated Japan in 2011.

The new upper limit of tsunami danger is marked by a pile of ocean debris more than 300 feet inland from the shore, left by the paleo-tsunami, according to research published this week in Geophysical Research Letters. The distance and height above average sea level--30 feet--have prompted officials to draw a much larger evacuation zone around the edges of the islands. "The revisions to maps are extensive, in some cases twice as far inland," says Rhett Butler, a geophysicist at the University of Hawaii in Manoa and lead author of the new study.

Tsunami researchers gathered in Vancouver this week, at the meeting of the Geological Society of America, said the caution was well-advised. "It's not surprising to see a run-up that high, because past waves from quakes in the Aleutians have approached that," says Breanyn MacInnes, a geologist from Central Washington University who studies tsunamis in the Eastern Aleutian area, part of a long ocean trench stretching across the Pacific just south of Alaska, where two techtonic plates are colliding.

Geologists have only been able to do surveys in this region in the past several years, says MacInnes; a decade ago they would not have known the trench was capable of unleashing waves of such power.

Butler and his colleagues had found a pile of coral fragments, mollusk shells, coarse ocean bottom sand, and other marine debris in a sinkhole on the Kauai coast several years ago. It looked like material carried hundreds of feet onshore and 30 feet up a slope by a tsunami. But the previous record was from the 1946 wave, which came up only eight feet, so tripling that seemed outlandish.

Then the 2011 Tohoku earthquake, magnitude 9.0, hit Japan, with its devastating tsunami. It was bigger than almost any seismologist had predicted for that part of Japan, and it made Butler wonder if researchers had also been underestimating the dangers in Hawaii. The Tohoku quake was a once-per-thousand year event. But that year came up for Japan and it could for Hawaii too, he thought.

With computer simulations, Butler and his colleagues created models of four 9.0 magnitude quakes along the Aleutian-Alaskan plate collision zone, along with the tsunamis such quakes would produce. They learned the geometry of the East Aleutian area would funnel water right at Kauai and the Big Island, with enough energy to climb 30 feet once it hit shore. That would put it right at the sinkhole, whose material geologists think is between 350 and 500 years old.

The recent surveys in the East Aleutian have tied ancient 8.6 magnitude quakes to tsunamis reaching nearly that height, says MacInnes, so Butler's conclusion seems very much within geologic reality.

Hence the enlarged evacuation zone. Tsunami researchers in Vancouver at the geology meeting very much endorsed the idea of abundant caution when planning for these waves. Hannah Baranes, a master's student from the University of Massachusetts working on tsunami run-up in southern Japan, noted that in her region, geological observations pointed to waves that were much higher than the ones generated by computer models based on theory. And this region holds a nuclear power plant on the coast. "An important question to answer," says Baranes, "is what's that worst-case scenario?"