Scientists have known for some time about the 700-mile-long fault off the coast of Washington, Oregon and California, where a wayward slab of the earth's crust known as the Juan de Fuca plate is trying to slide under continental North America. What they didn't appreciate until quite recently was that the juncture where the two plates are locked together can snap violently like a giant spring, unleashing a tsunami as large and terrifying as the one that pummeled South Asia.
U.S. Geological Survey researcher Brian Atwater led the detective work that nailed down the tsunami-rich history of the Cascadia Subduction Zone, finding such clues as Native-American fire pits buried under a layer of tsunami sand three centuries ago and linking them to Japanese records of what appears to be the same tsunami striking villages on Honshu in January 1700.
Cascadia has been relatively quiet ever since--which can be interpreted as good news or bad, given a geological record showing that the time between Cascadian tsunamis ranges from 200 to 1,000 years. Canadian geophysicists are still puzzling over a series of rhythmic tremors they identified a couple of years ago beneath the floor of Puget Sound. They don't know what caused them, but they think the tremors may be associated with rising stress along the fault. A bit of subterranean rustling doesn't mean that a great earthquake is imminent, of course, but the tsunami warning signs on local beaches remind us that those who live and play along Cascadia's jagged coast do so at their risk.
AN EARTHQUAKE FACTORY The Cascadia Subduction Zone???where the Juan de Fuca Plate meets the North American Plate???is remarkably similar to the subterranean system that triggered the tsunami in the Indian Ocean and is capable of generating equally powerful earthquakes and equally destructive waves Some key differences: a tsunami???warning system, better housing construction and a more rugged and less populous coastline. The death toll from a tsunami in the Pacific Northwest might be in the hundreds but not in the tens of thousands
Whole lotta shakin??? Hundreds of minor quakes have been recorded along the Juan de Fuca and Explorer plates over the past 25 years
A Seattle tsunami? If a Cascadia quake were large enough, it could drive a wall of water toward Seattle and Vancouver. The Puget Basin has its own network of faults fully capable of generating large earthquakes and tsunamis
Danger from afar A tsunami triggered by an Alaskan earthquake in 1964 swept four children off a beach in Newport, Ore. It also claimed 12 lives in Northern California
Tsunami central Before last month's disaster, only three earthquakes in the past 100 years were magnitude 9.0 or higher: Kamchatka in 1952 (9.0), Chile in '60 (9.5) and Alaska in '64 (9.2). ??Each came out of the Pacific's Ring of Fire, and each produced oceanwide tsunamis
Rising magma forces sea floor apart where plates meet
Rupture Zone When plates moving past each other get stuck, pressure builds until it???s eventually released in an earthquake. The more pressure, the bigger the quake. An earthquake of magnitude 9.0 radiates as much energy in a few minutes as the entire U.S. uses in a month
Plate begins to melt as it???s driven downward