Tsunami Science: Reducing the Risk
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 information—and nothing else—the 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.
A DART buoy on the high seas.
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.
