On Remote Aleutian Island, Surprise Evidence of Tsunamis
IN 1998, ON A TRIP to the remote Sedanka Island in Alaska, then-Humboldt State Geology Professor Gary Carver came across driftwood in an unlikely place.
About 15 years later, Carver’s seemingly insignificant find has led to evidence of prehistoric tsunamis on the island. It also raises the possibility that this section of the Aleutian subduction zone could generate a large earthquake and a powerful tsunami that could inundate Hawaii and the west coast in the United States.
U.S. Geological Survey researchers—Rob Witter, Richard Briggs, SeanPaul La Selle, and Guy Gelfenbaum—in addition to Carver, HSU alumnus Rich Koehler, and HSU Geology scholar Eileen Hemphill-Haley published their findings in a January issue of Geophysical Research Letters.
The 1,200-mile-long chain of Aleutian Islands extends along the Aleutian Trench, a subduction zone where the Pacific Plate pushes below the North American Plate.
Sedanka is part of the Aleutian archipelago’s eastern zone, which scientists previously believed wouldn’t generate quakes large enough to create tsunamis because the plates are “creeping.” Unlike locked plates, which build pressure that leads to a quake, creeping plates should, in theory, relieve tectonic stress.
What Carver found on Sedanka decades ago suggests otherwise.
While exploring the island, he spotted something unusual. “I stumbled across an old moss-covered log on an island that doesn’t have and never has had trees,” he says.
He started digging and found sand sheets nearly 50 feet above sea level. “The only source for the beach sand was the beach, which is a half-mile away,” he says. “It’s just mind bending to be so far from the ocean and standing on a driftwood log that was carried by a tsunami.”
In 2012, he and other researchers returned to the same spot on Sedanka and dug deeper into the hole. They found six to seven layers of sand between layers of peat. Witter sent a core sample to longtime colleague Hemphill-Haley back at HSU.
She found distinctive ocean diatoms, a type of algal microfossil found in freshwater and marine environments, leading her to believe some of the sand was likely from the shallow marine shelf offshore.
The team concluded tsunamis pushed up and over the beach, flooded the lowland and surrounding hillside, leaving a swath of sand in their wakes. The oldest sand layer dates back 1,700 years, and it appears tsunamis have occurred approximately every 300-400 years. The team estimates the most recent sand sheet is from a tsunami generated by the 8.6-magnitude earthquake in 1957.