Multiple Alaskan Tsunamis Over Thousands of Years


This century's deadly tsunamis kicked off an intense search for buried clues to prehistoric killer waves along Alaska's southern shores. The coastal geology there has unleashed some of the biggest tsunamis in the Pacific Ocean, but the historical record of past earthquakes and waves is sparse.

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Now, new evidence uncovered at several spots along the scenic coastline reveals that many tsunamis have flooded Alaska's islands and fjords in the past several thousand years, according to research presented at the Seismological Society of America's annual meeting in Anchorage, Alaska, last week (April 30-May 2).

The findings will help researchers fill the gap in Alaska's earthquake record and improve tsunami prediction models in the Pacific Ocean. Earthquake hazard models, which forecast future shaking, rely on an accurate understanding of the size and location of past earthquakes and tsunamis. The U.S. Geological Survey (USGS) is currently updating its seismic hazard map for Alaska.

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"There seems to be more frequent tsunami inundation than the models presume, but that's because we have no data," said Rich Briggs, a USGS researcher who was involved in the studies.

Mega-earthquakes along Alaska's subduction zone send deadly waves throughout the Pacific Rim (countries bordering the Pacific Ocean), primarily affecting Hawaii and the west coast of North America. Alaska's subduction zone, where the Pacific and North America tectonic plates collide, unleashed the second most powerful earthquake ever recorded, in 1964. (Gallery: The 1964 Great Alaska Earthquake)

"Ultimately, we hope our research helps Alaskans and coastal communities around the Pacific Ocean prepare for future tsunamis and thereby reduce the tragic losses like those witnessed in recent disasters," said Rob Witter, a research geologist at the U.S. Geological Survey's Alaska Science Center, who was a study co-author.

Witter and his colleagues presented evidence for several past tsunamis in the so-called Unalaska seismic gap. A "seismic gap" is jargon for a portion of a fault that has not produced an earthquake in recent history, even though surrounding sections have unzipped.

Nearly all of the Alaska-Aleutian megathrust, the name for the subduction zone fault, has ruptured in the past 100 years, producing earthquakes stronger than magnitude 8, but the Unalaska seismic gap has not done so.

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