Tsunamis More Likely to Hit U.S. Than Asia

Stefan Lovgren
for National Geographic News
January 3, 2005
Even as the death and destruction toll from the tsunami that struck much of Southeast Asia a week ago continues to mount, attention is also focusing on the Pacific Ocean.

Scientists say that huge tsunamis are much more likely to develop in the Pacific Ocean than in the Indian Ocean.

That's because the Pacific Ocean has many more subduction zones, which produce the most powerful earthquakes and tsunamis.

Knowing the danger, Pacific Ocean countries have set up a sophisticated warning system. They hope the system will keep the death toll from a potential giant tsunami far lower in the Pacific Ocean than it was in the Indian Ocean, where at least 140,000 people were killed by the recent tsunami.

Residents along North America's Pacific coast would have very little time to get to higher ground.

"If a magnitude 9 earthquake were to strike in the Pacific Northwest and generate a tsunami, we'd have less than 15 minutes warning [before it hit the shore]," said Robert Yeats, a professor emeritus of geosciences at Oregon State University in Corvallis.

Cascadia Danger

A tsunami is a series of great sea waves. While tsunamis can be triggered by landslides or volcanic eruptions, most tsunamis originate as a result of underwater earthquakes.

Tsunamis are primarily generated by earthquakes in subduction zones. In these zones one section of the Earth's crust, called a tectonic plate, moves over or under another. A giant rupture causes the seafloor to warp, displacing a vast amount of seawater. This raises the sea level and sets off the tsunami.

(By contrast, an earthquake along a strike-slip fault, where the tectonic plates grind against each other sideways, will not lift or drop the ocean floor.)

A 9.2 magnitude earthquake on the Aleutian Trench fault off Alaska in 1964 triggered a huge tsunami that killed more than 100 people, including 11 in Crescent City, California, where the tsunami crashed onto the shores hours after it originated.

Perhaps the biggest threat to the U.S. Pacific coast, however, is the Cascadia subduction zone, which extends from southern British Columbia to northern California.

Scientists recently determined that a 9.0 magnitude earthquake struck along the Cascadia subduction zone in 1700, creating a tsunami that hit the Pacific Northwest while also sending 16-foot-high (5-meter-high) waves slamming into Japan 10 to 20 hours later.

"The closer the land is to the event that generated the tsunami, the sooner it will be struck," said Brian Atwater, a geologist of the U.S. Geological Survey, based at the University of Washington in Seattle. "Thats what happened in Banda Aceh [in Indonesia] during the Asian tsunami."

A common misconception is that a tsunami is generated from the epicenter of the earthquake. In fact, it originates from the entire length of the rupture.

In a great earthquake, a longer segment of the fault line will rupture than in an earthquake of lesser magnitude, potentially producing a wider tsunami. Where the tsunami will travel depends on which direction the fault line faces.

The Cascadia fault runs parallel to the Pacific Northwest.

Deep-Ocean Monitors

There were 405 tsunamis in the Pacific Basin from 1900 to 1983, most of them very small. Pacific tsunamis are known to have killed 140,000 people since the Japanese began recording them more than 1,300 years ago.

A 1946 Aleutian quake, which generated a tsunami that struck Hawaii, and the 1964 Alaska event led the United States to establish a federal tsunami-monitoring system, managed by the National Oceanic and Atmospheric Administration (NOAA).

Today six deep-ocean monitors, located from Alaska's Aleutian Islands to the middle of the Pacific Ocean, watch for patterns that could hint at an earthquake or landslide capable of generating a tsunami. A recorder's readings are beamed up to a buoy and relayed to NOAA's network of weather satellites.

The satellite data is analyzed at NOAA's tsunami warning centers in Hawaii and Alaska, which issue alerts to emergency officials and the U.S. military in the event of a tsunami.

The Indian Ocean, on the other hand, has no such warning system. Experts say the death toll from the Asian tsunami would probably have been far lower if a warning system had been in place.

U.S. seismologists recorded the Asian earthquake but first thought it measured 8.1, too small to generate a giant tsunami. The figure was not revised upward to 9.0 until hours later, after most of the tsunami had already struck land in Asia.

Crash Course

Since 1997 state and local authorities in the Pacific Northwest have drawn up tsunami emergency plans, and some counties have set up high-tech tsunami warning sirens on their beaches. Schools in Seaside, Oregon, have had tsunami evacuation drills.

New federal tsunami-preparedness plans could recommend constructing "safe havens" for residents of U.S. coastal cities vulnerable to tsunamis.

Under construction standards adopted since 1996, engineers must design buildings to withstand giant earthquakes in the northwestern United States and southwestern Canada. But these buildings are not necessarily designed to withstand tsunamis.

At the Tsunami Wave Basin Laboratory at Oregon State University, scientists from around the country conduct experiments in an Olympic-size pool. Their goal is to learn how tsunamis behave in different types of ocean terrain and what effects they might have once they reach land.

"The actual destructive power of a tsunami when it impacts a building is still an open research question," said Dan Cox, an engineering professor who directs the 4.8-million-U.S.-dollar facility, which is funded mainly by the National Science Foundation.

Researchers use a wavemaker at one end of the pool to push water onto a simulated coastline at the other end. The experiments have enabled scientists to learn more about what happens when the wave approaches the coast.

"As the water depth decreases, a wave will slow down and wave heights will increase, adding to the energy density or power of the wave at a particular location," Cox said. "You have certain hot spots along the coast where wave heights are much higher and the tsunami would be much more intense."

Crescent City and Seaside, Oregon, for example, had a particular offshore topography that made them more susceptible to the 1964 tsunami from Alaska. The risk to other coastal cities for future tsunamis depends on the source location, scientists warn.

For now, the most efficient tsunami warning sign for people living along the Pacific coast is a big earthquake preceding it.

"If you're on the coast and it starts to shake like crazy, don't wait for the sirens to go off," Yeats said. "Go uphill."

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