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. After that tsunami deluged the island nation, scientists began to question Hawaii’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 Hawaii? Are our evacuation zones the correct size?”
The team used a wave model to determine how a tsunami would flood the Kauai coastline. The image below shows simulated major earthquakes (9.0-9.6) along the Aleutian-Alaska subduction zone. The model shows that a catastrophic earthquake hitting the right spot could push water up to 30 feet high into the Hawaiian coast.
Image credit: Rhett Butler
The chances of an earthquake producing a tsunami of this size is rare. They occur once every 1,000 years. That means during any specific year, there is about a 0.1% chance of it happening. Still, officials in Hawaii aren’t taking any chances. They are revising tsunami evacuation plans to take into account the potential for such a severe tsunami. Officials plan on having the new maps finished and in residents’ hands by the end of the year.
Top image credit: Tsunami impacts Hawaii in 1946/USGS