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Fear the Future Tsunami?

Hilo Bay faces the Pacific Ocean on the northeastern shore of the island of Hawaii. It is 4,000 kilometers due south of Alaska’s Aleutian Islands, home to one of the most active subduction fault lines in the world. A tsunami launched from the Alaska-Aleutian subduction zone would take just three hours to reach the city of Hilo. From 1900 to 1964, a tsunami with a run-up of at least a meter (inundating all areas below one meter above sea level) struck Hilo an average of once every five years.

On May 7, 1986, warning of a magnitude 7.7 Aleutian earthquake reached the Pacific Tsunami Warning Center in Honolulu. Not all earthquakes produce tsunamis, but based on this seismological informationand nothing elsethe state of Hawaii decided to evacuate Hilo and the surrounding area. “It was an absolutely horrendous situation,” says Christian Meinig, head of engineering at NOAA’s Pacific Marine Environmental Laboratory (PMEL) in Seattle, Washington, “because the wave that arrived was very small and not damaging at all.” The false alarm triggered panic, phenomenal traffic jams, and cost the state both credibility and tens of millions of dollars in expenses and lost productivity.

DART

A DART buoy on the high seas.

NOAA/PMEL


On November 17, 2003, a magnitude 7.8 earthquake rumbled through the Aleutians. This time, the warning center didn’t fear a false alarm. For the first time in history, they could track the wave directly as it traveled. Three special monitoring buoys designed by Meinig’s lab floated near the Aleutian fault, and they detected and measured the wave height as it passed. The trio relayed their data via satellite to the warning center in Honolulu. “The wave was about 2 centimeters high,” says Meinig. “Based on that information we knew it was not going to be a damaging event. The warning was called off within 90 minutes of the earthquake.”

The system, called DART, for Deep-ocean Assessment and Reporting of Tsunamis, was developed by PMEL in the mid-1990’s in response to the relentless threat posed by Aleutian-born tsunamis. It has been fully operational only in recent years. Right now, 11 DART systems reside in the Pacific Ocean: Four float near the Cascadia subduction zone along the Pacific coast, four follow the Aleutians, two are mid-Pacific, and one sidelines Chile. After last winter’s devastating tsunami in the Indian Ocean, extending the DART system is a critical aspect of global tsunami monitoring.

Whither the Water

Each DART buoy is built for accuracy and speed. The two-meter-high buoys are tethered with nylon and wire rope to a 3,107-kilogram anchor on the ocean floor 4,000 to 6,000 meters below. A device that records pressure, called a “tsunameter,” is anchored nearby.

Normally, one psi (pound per square inch) of seawater equals 670 millimeters of water height. If the tsunameter detects a sudden higher-than-normal pressure of the column of water above the tsunameter, and therefore an increased sea-level height, a giant wave is certainly passing. In the open ocean, tsunami wave heights can be anywhere from millimeters to meters, which DART can measure to within a quarter-millimeter accuracy.

The tsunameters measure natural tidal cycles, but these effects are predictable and easily subtracted out. The tsunameters do not register minor surface-wind waves, however. This is because of their wavelength. Wind waves affect the water column as deep as half their wavelength, which can be up to a few hundred meters. Tsunamis, however, have wavelengths approaching 200 kilometers. These monster waves affect the water column clear to the seafloor and the awaiting tsunameters. “We’re measuring pressure in a normally very stable, very cold, very unchanging environment in the deep ocean,” says Meinig. “This gives us a pure energy measurement of the tsunami.”

The Old Wave

Monitoring tsunamis with DART is an astounding improvement over conventional methods, which were used in Sumatra in December 2004 and are still in use in much of the world. As happened during the 1986 Hilo event, scientists sometimes predict tsunamis based only on seismometer readings of an earthquake. Why every subduction-zone earthquake doesn’t cause a tsunami is still unclear. Meinig says, “Seismic information alone isn’t useful, because it’s not a direct measurement of the wave.” He points to a second Sumatran earthquake that followed three months after the 9.0 quake. Although it was a magnitude 8.7, and occurred just 150 kilometers southeast along the fault line from the December 26 epicenter, it produced a much weaker (though still damaging) tsunami. 

Another conventional tsunami-measuring technique is through tide gauges, plastic tubes installed vertically near shorelines. As water fills the tube, a computerized sensor records the sea level. But tide gauges are problematic. Many must be physically accessed to retrieve the data. Also, the topography of shorelines can reduce or amplify a tsunami’s wave height, leaving researchers unable to predict how that same wave may affect coasts far away. Tide-gauge data oftentimes come too late to be useful, and the tsunami can wash away the tubes.

“When you measure offshore, deep-ocean pressures with a tsunameter, you can see what the energy means for specific communities,” says Meinig. “Our goal is to have site-specific forecasts just like we’re able to do for a hurricane. But the challenge is that we don’t have three days to do it. We have three hours.”

The New Wave

After the tsunameter measures the wave, it relays the digital data acoustically to the floating buoy. The buoy transmits the information to a satellite, which beams it to ground-based scientists who incorporate the data into community-specific computer model forecasts. The modelers then send their predictions to local emergency managers. All these steps can be completed within 15 minutes.

Improving tsunami monitoring and warning in the Indian Ocean means installing better seismometers, tsunami-appropriate tide gauges, and of course, DART systems. PMEL is now working to adapt the DART technology for the challenges of the tropical climate. “Alaska has freezing rain and monster waves,” says Meinig. “The tropics have instead tremendous bioproductivity. The buoys will be covered with things like mussels and gooseneck barnacles.” Also, the Indian Ocean is among the most-pirated seas in the world. Vandalism, boat tie-offs, and even gunshots will threaten any buoys placed there.

NOAA is also working on increasing the U.S. monitoring system to 39 buoys by mid-2007. As technology improves and scientific understanding of tsunamis matures, monitoring them will become more akin to weather forecasting, predicts Meinig. “We’re getting better and better weather forecasts all the time,” he says. “We’ll also get better and better tsunami forecasts to benefit people all over the world.”

Related Links

Pacific Marine Environmental Laboratory DART program
http://www.pmel.noaa.gov/tsunami/Dart/dart_home.html

The Physics of Tsunamis
http://www.geophys.washington.edu/tsunami/general/physics/physics.html

National Data Buoy Center real-time data
http://www.ndbc.noaa.gov/dart.shtml

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