More than a decade ago, investigation of a collapsed limestone cave on the south shore of the Hawaiian island of Kaua’i revealed an unusual find—a layer of coral fragments, mollusk shells and beach sand six feet below the surface. It wasn't the contents that were so surprising, though; it was their location.
That debris from the sea had somehow washed to the sinkhole over more than 300 feet of land and 23-foot-high walls. David Burney, a paleoecologist at the National Tropical Botanical Garden in Kalaheo, wondered if the deposit was the traces of a massive tsunami. But he couldn’t be sure.
Then, the 2011 tsunami that hit Japan sent a water surging as much as 128 feet high in some places.
Watching the coverage of the 2011 tsunami and the destruction it caused spurred Rhett Butler, a geophysicist at the University of Hawaii at Manoa, to a different question: "[Did] we get it right in Hawaii? Are our evacuation zones the correct size?"
To answer that question, Butler began scouring the Pacific for signs of massive historical tsunamis that may have struck Hawaii. The team dated marine deposits around the western rim of the Pacific and the sinkhole on Kaua’i that Burney had already wondered about. They also modeled the action of waves created by enormous earthquakes, with magnitudes ranging from 9.0 to 9.6 and origins in the 2,113-mile-long ocean trench along the Alaska coast and Aleutian Island chain, where the Pacific tectonic plate dives under the North American plate.
The team's efforts confirmed that about 500 years ago, a 30-foot tsunami caused by a 9.0-magnitude quake in the Aleutian Islands slammed into Hawaii. It left the approximately nine shipping containers worth of ocean debris in the sinkhole.
An earthquake in the eastern Aleutian Trench big enough to generate a massive tsunami like the one in the study is expected to occur once every thousand years, meaning that there is a 0.1 percent chance of it happening in any given year – the same probability as the 2011 Tohoku earthquake that struck Japan, according to Gerard Fryer, a geophysicist at the Pacific Tsunami Warning Center in Ewa Beach, Hawaii.
Hawaii has quickly moved to update its tsunami evacuation plans, which were previously based on the 1946 tsunami. That event was the worst in recent history. But Japan’s experience has indicated that planners need to look farther back.
“You’re going to have great earthquakes on planet Earth, and you’re going to have great tsunamis,” Butler said in the AGU statement. “People have to at least appreciate that the possibility is there.”