Volume 16 - 2006

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Tsunamis

Through Time

By Sara LaJeunesse

Weak from starvation, their strength waned like the ebbing tide. The ocean, which once provided them with an abundance of whale meat and oil, was now empty … except for Killer Whale. For weeks the vile creature had prowled the frigid waters near their home and slaughtered the other whales. One day, mighty Thunderbird soared down from its mountain home, and when Killer Whale surfaced for air, the great bird plunged its sharp talons into the exposed, wet skin. As the two beasts struggled, the earth shook, and the sea rose up in a wall of water that flooded the land and demolished the village.



Versions of this story have been passed down through generations of Native American groups in the Pacific Northwest. But the legend tells of more than the struggle between good and evil; it serves as a rich oral history, recounting a genuine catastrophic event.

Harry Williams, associate professor of geography at the University of North Texas, thinks that stories like this one point to powerful tsunamis that hit the northwest coastline during the last 2,500 years. By digging down through layers of sediments in the tidal marshes near Discovery Bay, Wash., Williams has found evidence suggesting that up to nine tsunamis occurred during this time. He hopes that by understanding their patterns, he can contribute to improved tsunami-preparedness in the future.

"Tsunamis impact the Pacific Northwest, on average, about every 300 years," says Williams. "Although you can't predict the exact timing of future tsunamis, you can carry out mitigation measures, such as planning evacuation routes and informing and educating the public, if you know which coastal areas are at risk."


Why the Pacific Northwest?

Although the concept of a tsunami striking the United States may seem strange, the possibility is far from fiction. Because of its position along the Cascadia subduction zone - the area where the undersea Juan de Fuca plate is being shoved, sometimes violently, under the continental North America plate - the Pacific Northwest coast is highly susceptible to submarine earthquakes, the primary source of tsunamis.

Often incorrectly referred to as a tidal wave, a tsunami is actually a sequence of waves - called a wave train - that resembles a massive, onrushing tide. It was this rapid surge of water that devastated Native American groups in the Pacific Northwest, inspiring the Thunderbird and Whale stories.

Geographer Harry Williams who studies sediments to uncover the patterns of ancient tsunamis

Geographer Harry Williams, who studies sediments to uncover the patterns of ancient tsunamis, now plans to examine storm surge deposits left by hurricanes. His research may contribute to improved preparedness in the future.

Geological data suggest that the last major tsunami in the region occurred in 1700 A.D., a year that matches up with many of the stories. Generated by a powerful earthquake (estimated at magnitude 9.0), the tsunami caused flooding and damage as far away as Japan.

"The event was probably similar in size to the one that hit South Asia in 2004," says Williams. "And," he adds, "there's a good chance it will happen again."


Digging into the Past

To better predict future tsunami events, Williams looks to the past. Using a hollow, metal cylinder called a soil corer, he tunnels down several feet through the sediments of the tidal marshes near Discovery Bay, going farther backward in time the deeper he digs. The retrieved column of soil serves as a chronological record of events, with layers of sand indicating the incidence and approximate time of a tsunami.

"A tsunami can pick up sand from the lower tidal zone, or even offshore, and carry it into the high marsh," says Williams. "The result is an anomalous sand layer that becomes buried in the marsh."

Gathering this type of data is no easy task. Williams takes hundreds of soil cores from marshes in northwest Washington and on southwest Vancouver Island in British Columbia.

"For every site where I've found tsunami sand layers, I've probably dug into 10 other marshes and found nothing," he says. "But the effort is worth the excitement of discovering a tsunami sand layer that no one else has found before."

Williams determined that the tsunamis that hit the Discovery Bay area in the past 2,500 years had multiple origins. Sources included subduction zone earthquakes generated by the Cascadia subduction zone and, to a lesser extent, crustal earthquakes originating from shallow faults and submarine landslides.


The 6-foot Mark

Williams also found that the shape of Discovery Bay - wide near the open ocean and tapering to a point further inland - causes tsunamis to become amplified. As water from the ocean is funneled into the bay, it becomes increasingly confined, as the same volume of water is forced into a smaller space. The result is a wave train with taller waves and a greater potential to do damage.

While he admits it's difficult to know exactly how high the ancient tsunamis' waves were, Williams guesses they reached at least 6 feet and might even have achieved heights of 20 feet.

"Clearly, if I'm right," he says, "a house within 6 feet of high-tide level is at risk."

Fortunately, most of the homes in the area today are perched on hillsides, out of harm's way. But a few stores along Highway 101, beyond the marsh, are below the 6-foot high-tide level.

"There are some big cities in the Pacific Northwest, but not many people live that close to the coast, so the loss of life would be a lot less than it was during the recent tsunami in South Asia," says Williams. "But," he adds, "even one life lost is too many."


Tsunami's Evil Stepsister

The tsunami isn't the only member of the natural disaster family that may leave behind clues in coastal marsh sediments.

"Much of my expertise on tsunami deposits can be adapted to studying hurricane deposits," explains Williams, who says that last year's hurricane season inspired him to study hurricanes.

"I think hurricanes leave a geologic record in coastal marshes, so the potential exists to study them with similar applications used to study tsunamis," he says.

With funding from the National Science Foundation, Williams plans to examine storm surge deposits left by Hurricane Rita in the coastal marshes of Louisiana.

He will identify the source and composition of the deposits and then will resurvey the area 18 months later to find out how much of the deposits were incorporated into the marsh. The work will contribute to an understanding of marsh growth.

Williams hopes that results of this study, together with investigations he is planning of ancient hurricane storm surge deposits in Louisiana, will help determine which areas are at high risk for damage from hurricane sediment deposits.



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Harry Williams discusses evidence of ancient tsunamis at Discovery Bay, Wash.

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