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A large earthquake spawned a tsunami that killed nearly 200 people in the Samoan and Tongan islands in September 2009, and experts say that similar tectonic forces are at work in the Pacific Northwest. How likely is a tsunami to strike the Pacific Northwest? "It's inevitable," says Justin Sweet, a graduate student studying earthquakes at the University of Washington.

As was the case in Samoa, the earthquake will give only about 15 minutes of warning before the tsunami will strike. For some people, this is enough time to run to higher ground, but the elderly and people living in coastal lowlands need a more practical solution. Yumei Wang, a geotechnical engineer at the Oregon Department of Geology and Mineral Industries, presented plans for that solution, tsunami evacuation buildings, at the Geological Society of America meeting in October 2009. "Quite frankly, we're going to get hit, and it's going to be disastrous," Wang says. She also says the Northwest is not prepared for it. Many residents and tourists are not aware of the risk, even though scientists have understood the mechanisms of a tsunami for years.

At the Cascadia fault several hundred miles off the coast of Washington and Oregon, the Juan de Fuca tectonic plate is diving beneath the North American plate in a process known as subduction. The diving plate bends the edge of the upper plate downward slowly until it suddenly springs back up, like a diving board. An earthquake results from the spring, and in some cases, so does a tsunami. "The crucial ingredient of a tsunami is some sort of vertical motion," Sweet explains. The springing plate pushes ocean water upwards in waves, in the same way a diving board launches the diver. If the plate gives the waves enough energy, they will become a tsunami when they reach shallower water near shore.

"Everything gets piled up when those waves feel the bottom," says Dawn Wright, a professor of geography and oceanography at Oregon State University. The shape of the ocean floor, bathymetry, interacts with the bottom of the wave and makes it break. "Knowing the bathymetry will give you a fighting chance of understanding what these super-fast waves will do," Wright says. The bathymetry helps determine how far inland waves of a given height will go. Knowing that is crucial for planning evacuation routes and buildings. Wright is producing high-resolution bathymetric maps of the Southwestern Pacific. One map follows the path of the 2009 tsunami, from the Tonga Trench subduction zone to Samoa.

Cascadia earthquakes are more likely to spawn tsunamis than those in many other locations, including the Tonga Trench. Because the oceanic plate is diving shallowly under the North American plate, the earthquake loses less energy to the plate and more of it reaches the ocean to make the wave. The energy and high activity of the Cascadia fault makes the Northwest especially vulnerable to tsunamis. The most recent Northwest great earthquake struck in 1700.

Scientists estimate that the Cascadia fault will have a tsunami-causing earthquake every 300 to 500 years. So is the Northwest due for another one? It's impossible to forecast the earthquake, but by sensing it, scientists can predict the tsunami that will follow.

The National Oceanic and Atmospheric Administration (NOAA) monitors the Pacific Ocean for earthquake-generated water waves that could become tsunamis. NOAA continuously receives data from seismic networks, gauges, and its network of buoys (called Deep-ocean Assessment and Reporting of Tsunamis, or DART, stations). Wang points out, though, that DART was designed to detect far-field tsunamis, waves that originate from distant places such as Alaska or Japan. "The NOAA tsunami warning system will NOT help warn Cascadia coastal communities of a Cascadia near-field tsunami," she cautions. The DART buoys are in deep, offshore waters, about 20 to 30 minutes of tsunami travel time from the subduction zone. These buoys do play an important role in quickly determining the destructiveness of the tsunami in order to supply vital information on when it is safe to return to the evacuated areas.

However, a near-field tsunami will strike the coast even before the buoys can detect it, much less issue a warning. Our best bet for an early warning? If you feel the ground shake, run uphill. Then, wait for data from the buoys to tell you when it is safe to return.

The Pacific Northwest Seismic Network (PNSN) is developing the near-field tsunami warning system that the Northwest needs. The PNSN feeds NOAA live data on Cascadia earthquakes. With faster software and communications to come, NOAA and the PNSN together will be able to give a tsunami warning within three minutes of the Cascadia quake. "In a few years, we should be able to do a lot better than just ‘head for the hills'," promises John Vidale, director of the PNSN.

The Northwest will face major obstacles when a killer wave comes. Tsunami evacuation buildings, high-resolution bathymetry, and more sophisticated warning systems could help overcome these obstacles, and all three are on the horizon.

Kristin Poinar studies glaciology at the University of Washington.

Image: A high-resolution map of the ocean bottom at the Tonga Trench, the site of the September 2009 earthquake and tsunami in Samoa. The earthquake's epicenter is starred. The Samoan islands are on the high, red point north of the deep, purple trench. Image courtesy of Dawn Wright, Oregon State University