Geologists offer new insights into frequency of devastating waves

  • ROB WITTER/USGS Geologists camped on Umnak Island in the Aleutians.

  • PETER HAEUSSLER/USGS Research Geologist Rob Witter

  • HOLLYN JOHNSON/Tribune-Herald

    The 1946 tsunami exhibit in the Pacific Tsunami Museum in downtown Hilo shows imagery of the destruction as well as stories from survivors. The tsunami, triggered by an earthquake in the Aleutian Islands, hit Hilo on April 1, 1946.

A paper from the U.S. Geological Survey suggests tsunamis like the one that devastated Hilo in 1946 might be predictable — and more frequent than previously thought.

In a November 2018 paper — titled, “Evidence for frequent, large tsunamis spanning locked and creeping parts of the Aleutian megathrust” — nine USGS research geologists conducted studies of a pair of coastal areas in the Aleutian Islands off the coast of Alaska and discovered the beaches had been inundated by the ocean several times in the past, likely by tsunamis.

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The tsunamis would have been caused by movement of the tectonic plates making up the Aleutian Trench. A similar earthquake originating near the trench caused the 1946 Hilo tsunami, which killed 173 people and demolished much of the community.

USGS research geologist Rob Witter, who led the team that conducted the studies, said evidence for the islands’ tsunami history was found in low-lying areas at Driftwood Bay on Umnak Island and Stardust Bay on Sedanka Island.

“The low-lying coastals are covered in peat,” Witter said. “And then, when the tsunami spreads inland, it carries sand with it into these peat bogs, and so you get this sort of peat-sand-peat sandwich.”

By using radiocarbon dating and further analysis of these sand layers, the geologists were able to conclude the sand came from oceanic sources and determined the approximate times when the tsunamis inundated the islands.

Based on their findings, the geologists concluded that the bays were inundated by large tsunamis at least eight times in the past 2,000 years. Witter said both sites appeared for the most part to be inundated at the same time by tsunamis triggered by the same fault shift.

“We really had no idea how often tsunamis were generated here (on the Aleutians),” Witter said, explaining that the findings challenge an existing theory that certain types of plate boundaries are less prone to hazardous seismic disruptions.

Witter said the tsunamis appeared to occur in an interval of between 164 and 257 years.

“If they recur fairly regularly, we do have some breathing room,” Witter said, explaining that the most recent tsunami at Driftwood Bay was in 1957. “But, there’s always a ‘but.’”

Even if movement of the Aleutian fault occurs regularly, Witter said seismic activity elsewhere can still impact the fault and cause additional tsunamis.

Witter said more information about the behavior of the Aleutian fault is extremely valuable for Pacific communities such as those on Hawaii Island to determine their tsunami preparedness plans.

“The way the fault is situated is that it can send these waves straight down to Hawaii,” Witter said.

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In particular, the 1946 tsunami that devastated Hilo originated from a disruption of the Aleutian Trench. That same quake generated 75-foot waves on the Pacific shores of Umnak Island.

Email Michael Brestovansky at mbrestovansky@hawaiitribune-herald.com.

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