Surfers love Hawaii’s waves, and many dream of catching “the big one.” For most people living in coastal areas vulnerable to tsunamis, though, “the big one” is a bad dream. We’ve seen many devastating events over the years, but our memory is not so long that Mother Nature can’t surprise us. The 2011 tsunami in Japan testified to that.

In 2001, sediment from a past tsunami was found in a sinkhole on the southeast side of the Hawaiian island of Kaua’i. The mouth of that sinkhole is about a hundred meters from the shoreline—and over seven meters above sea level. The largest tsunami measured in the area had been three meters, courtesy of Chile’s monstrous magnitude 9.55 earthquake in 1960. Could it be that an event was big enough to send tsunami waves over seven meters high to Hawaii in the past?

Researchers Rhett Butler, David Burney, and David Walsh simulated a variety of earthquakes around the Pacific to find out. They used a model that simulates the spread of tsunami waves, creating some virtual magnitude 9.0 to 9.6 earthquakes from Alaska to Kamchatka.

For earthquakes along Alaska’s mainland or those near Kamchatka, the worst of the tsunamis missed the Hawaiian islands. But along the eastern portion of the Aleutian islands extending west from the Alaska Peninsula, earthquakes put Hawaii directly in the line of fire. There, earthquakes magnitude 9.25 or larger, in which the fault slipped 35 meters or more, created tsunamis capable of flooding the sinkhole on Kaua’i. It is, in fact, possible to put tsunami debris there.

The Kaua’i sinkhole tsunami layer has been carbon dated to between 1430 and 1665 AD. Tsunami deposits have also been discovered on one island in the Aleutians, reaching as high as 18 meters above sea level. The most recent layer corresponds to an 8.6 earthquake nearby in 1957 (which created waves a little over two meters high in Hawaii), but the previous one has been dated to between 1530 and 1665. That could be the match for the Kaua’i sinkhole deposit.

In fact, deposits of a similar age have been found from British Columbia to Oregon. In the model, the 9.25 Aleutian earthquake produced tsunamis up to a meter in Japan and up to three to nine meters at locations along the Pacific Coast of the US and Canada.

The Kaua’i sinkhole opened up around 7,000 years ago, but there’s only one tsunami deposit inside, so this might have been a singular event. But if it occurred 350 to 600 years ago, enough plate tectonic motion has accumulated along the Aleutians for a similarly large earthquake to be possible in the not-too-distant future. Hawaii (and other regions) must plan for what’s possible, not what we can remember.

If this version of “the big one” happened again, Hawaii would have about 4.5 hours before the tsunami arrived. The researchers express concern for the reliability of the warning system, however, noting that there are only a few sensors near the Aleutians capable of detecting the resulting tsunami. It’s not uncommon for one of those sensors to be out of service for months before it can be repaired, and they point out that an extra pair of new sensors would be a very wise investment.

Geophysical Research Letters, 2014. DOI: 10.1002/2014GL061232 (About DOIs).