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Waves of Destruction: Tsunami
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Remembrance of Waves Past

Remembrance of Waves Past

by Daniel Pendick

Tsunamis are watery creatures, but they leave footprints on the land as clear as any. Along the rivers and marshes of the Pacific Northwest coast of the United States, geologists have used those footprints to reconstruct a violent tale of the time nearly 300 years ago when the earth shook and tsunamis swept in from the sea. Some of the most telling clues in this scientific detective story are found in the mud and peat bordering coastal waterways from Vancouver, British Columbia, to Northern California. Three centuries ago, seismographs were not available, but, like a tape recorder, the coastline has preserved a record of an earthquake.

In 1700, the offshore Cascadia Fault gave way and caused large portions of the coastline to sink as tension was released from the ground. According to the latest evidence, virtually the entire coast -- from Vancouver Island to Northern California -- may have been rocked by the quake, estimated to have measured magnitude 9.0 or greater. Some of the casualties of the event tell the tale: Geologists have found the stumps of Sitka spruce and red cedar that were poisoned when the land sank and seawater invaded ground that was formerly above the high tide level. Study of tree rings in the killed trees show that they grew their final layer of wood during the 1699-1700 season.

Tree ring

Geologist Brian Atwater with cross-section of cedar killed by the Cascadia earthquake of 1700.

Then the tsunamis came. Slicing into the peat with shovels and chain saws, geologists found a layer cake of peat, sand, and mud laid down by the inrushing tsunamis. The waves flattened plants and then entombed them in a layer of sand. The parts of the coast that ended up below the tide mark because of the earthquake were then covered by muddy sediment, and were colonized by salt-tolerant plants.

The story doesn't stop on the Pacific Northwest coast. Across the ocean, in Japan, meticulous government records speak of flooding in several coastal towns on the evening of January 27/28, 1700. In northern Japan, 20 homes were damaged in the town of Miyako by waves up to 10 feet high. Farther south, rice paddies and storehouses were flooded. Using a computer simulation to work backwards, scientists figured out that to make a six-foot tsunami in Japan, the Cascadia earthquake of 1700 had to be magnitude 9.0 or larger. Stories passed down for centuries by Native Americans in the region speak of shaking and flooding that came on a winter's night, consistent with the historical evidence from Japan.

The geologists have also found clues to the size of the tsunamis that attacked the Pacific Northwest. The thickness of the sand deposits they left behind is one kind of clue. Thicker deposits mean larger waves carrying more sediment scoured up from the seafloor. By comparing the historical deposits to those left behind by more recent events -- for example, the tsunami that attacked the California coast after the great Alaska quake of 1964 -- geologists can estimate the size of the past waves. Another clue comes from topography: By mapping the location of tsunami sands, it's possible to estimate the minimum height of the waves. The presence of a tsunami sand deposit east of a 10-foot-high ridge, for instance, means the tsunami that carried the sand there must have been at least 10 feet high. This and other evidence suggests that the waves were up to 30 feet or higher -- high enough to devastate low-lying coastal communities.

Article: A Deadly Force | Sidebar One: Catching a Tsunami in the Act | Sidebar Two: Remembrance of Waves Past | ANIMATION
Hell's Crust: Our Everchanging Planet  |  The Restless Planet: Earthquakes
Out of the Inferno: Volcanoes  | Waves of Destruction: Tsunamis


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