UH Hilo tsunami

Tsunamis larger than the 1946 event have struck the island in the past, and may again. (credit: USGS)

A mass of marine debris discovered in a giant sinkhole in the Hawaiian islands provides evidence that at least one mammoth tsunami, larger than any in Hawaiʻi’s recorded history, has struck the islands, and that a similar disaster could happen again. Scientists, led by Rhett Butler, director of the Hawaiʻi Institute of Geophysics and Planetology at the University of Hawaiʻ at Mānoa, are reporting that a wall of water up to nine meters (30 feet) high surged onto Hawaiian shores about 500 years ago. A 9.0-magnitude earthquake off the coast of the Aleutian Islands triggered the mighty wave, which left behind up to nine shipping containers worth of ocean sediment in a sinkhole on the island of Kauaʻi.

The tsunami was at least three times the size of a 1946 tsunami that was the most destructive in Hawaiʻi’s recent history, according to the new study that examined deposits believed to have come from the extreme event. Tsunamis of this magnitude are rare events. 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.

“You’re going to have great earthquakes on planet Earth, and you’re going to have great tsunamis,” said Butler, lead author of the new study, “Paleotsunami evidence on Kauaʻi and numerical modeling of a great Aleutian tsunami,,” published online in Geophysical Research Letters, a journal of the American Geophysical Union. “People have to at least appreciate that the possibility is there.”

Examining Hawaiʻi’s past

Hawaiians have told stories about colossal tsunamis hitting the islands for generations, but possible evidence of these massive waves was only first detected in the late 1990s when David Burney, a paleoecologist at the National Tropical Botanical Garden in Kalaheo, was excavating the Makauwahi sinkhole, a collapsed limestone cave on the south shore of Kauaʻi.

Two meters (six and a half feet) below the surface he encountered a layer of sediment marked by coral fragments, mollusk shells and coarse beach sand that could only have come from the sea. But the mouth of the sinkhole was separated from the shore by 100 meters (328 feet) of land and seven-meter (23-foot) high walls. Burney speculated that the deposit could have been left by a massive tsunami, but he was unable to verify the claim.

The deposits remained a mystery until the Tohoku earthquake hit Japan in 2011. It caused water to surge inland like a rapidly rising tide, reaching heights up to 39 meters (128 feet) above the normal sea level.

Questioning the current evacuation maps

After that tsunami deluged the island nation, scientists began to question Hawaiʻi’s current tsunami evacuation maps. The maps are based largely upon the 1946 tsunami, which followed a magnitude 8.6 earthquake in the Aleutian Islands and caused water to rise only two and a half meters (8 feet) up the side of the Makauwahi sinkhole.

“[The Japan earthquake] was bigger than almost any seismologist thought possible,” said Butler. “Seeing [on live TV] the devastation it caused, I began to wonder, did we get it right in Hawaiʻi? Are our evacuation zones the correct size?”

The new research has prompted Honolulu officials to revise their tsunami evacuation maps to account for the possibility of an extreme tsunami hitting the county of nearly 1 million people. The new maps would more than double the area of evacuation in some locations, according to Fryer.

Read the UH Mānoa news release for more on the tsunami research.