An American Tsunami?

By J. MADELEINE NASH

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

Dense magma

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

Magma fuels??a string of volcanoes in the Cascade Range, including Mount St. Helens

Sources: Brian Atwater, U.S. Geological Survey; Kenji Satake, AIST, Geological Survey of Japan; Vasily Titov, National Oceanic and Atmospheric Administration; Natural Resources Canada; LandScan/UT-Battelle

SOUNDING THE ALARM For residents of the Pacific Northwest coast, the jolt of an earthquake would be the first signal that they should head for higher ground People too far away to feel the quake would be alerted by sirens or broadcast warnings, thanks to??a 26-nation network of seismic, tidal and sea-level monitors

Warning system The workhorses of the Deep-ocean Assessment and Reporting of Tsunamis are a series of buoys tethered to the ocean floor and linked to the outside world by satellite. The Pacific is tsunami ready, but not the Atlantic or Indian oceans

Bottom-pressure recorder Can detect minute changes in water pressure caused by a passing tsunami as small as 0.4 in. (1 cm)

Computer Sends signal to buoy

Communications buoy Receives data from ocean floor along with readings from surface weather instruments, and relays to a satellite

IT HAPPENED BEFORE Geological and historical evidence points to a giant Cascadia earthquake that sent a tsunami across the Pacific in January 1700

Hour 1 Waves strike North America Hour 5 Tsunami front passes Hawaii Hour 10 First waves reach Japan Hur 15 Philippines, New Zealand hit Hour 20 Tsunami spans Pacific ocean