As data poured into the Pacific Warning Center of the National Oceanic and Atmospheric Administration (NOAA), scientists calculated how far the tsunami would travel and how powerful it would be when it arrived at the world's coastlines, ultimately issuing a warning for Hawaii and an advisory for the U.S. West Coast.
Hawaii's emergency officials sounded the islands' sirens, rousing the coastal residents and giving them at least 10 hours to evacuate before the most destructive waves would wash ashore.
"We were able to make pretty accurate forecasts," said NOAA Administrator Jane Lubchenco. "It provided the emergency managers and the public time to respond. People heeded the warning, sought higher ground, and we had no significant direct injuries or damages. That's a significant accomplishment."
The sensor, attached to a buoy, is among the newest tools deployed by the United States since the Dec. 26, 2004, Indian Ocean tsunami, which killed more than 200,000 people in Indonesia, Thailand and Sri Lanka. The killer tsunami, the most powerful in recent memory, prompted a new look at the world's warning systems.
"The Indian Ocean tsunami was a real wake-up call for people," Lubchenco said. After that, the United States accelerated plans for an international warning program by deploying new buoys and increasing staff at two NOAA warning centers, she said.
Tsunami waves occur when the ocean is disturbed, usually by a powerful earthquake. The waves radiate outward from the disturbance and can travel for thousands of miles. The United States established the Pacific Warning Center, its first warning system, in 1949 after a tsunami originating in the Aleutian Islands struck Hawaii, killing 150 people in 1946.
In 1967, the United States established a second center for the West Coast and Alaska. Nearly 60% of the world's tsunamis occur in the Pacific Ocean, NOAA said. After the 2004 tsunami, NOAA ramped up its staffing at the warning centers from eight hours a day to 24 hours, seven days a week, Lubchenco said. Scientists created 43 high-resolution computer models that predict how different tsunamis will behave under varying conditions, she said.
The United States increased the number of tsunami buoys it maintains from six to 39, she said. Australia contributes six buoys and Thailand and Indonesia each maintain one buoy. The Deep-ocean Assessment and Reporting of Tsunamis buoys monitor the ocean's activity by sending an acoustic signal between a transmitter anchored on the sea floor and a surface buoy.
As the tsunami travels, the sensor reports changes in water pressure that can signal an approaching tsunami and help scientists calculate the size of the waves. Satellites relay the data to warning centers.
On Saturday, the predictions for the tsunami's arrival in Hawaii were off by just an hour, although the waves were weaker than expected. "When you think of how fast it's moving and how much energy is in it, when you consider the tsunami is moving across the ocean at the speed of a jet airplane, this warning system was pretty effective," Lubchenco said.
Documents made public in June by Public Employees for Environmental Responsibility (PEER), a watchdog group in Washington, showed that 10 of the 39 deep-ocean stations operated by the United States were sending sporadic or unusable data, leaving uncovered several critical areas.
PEER Executive Director Jeff Ruch said NOAA "acted with some dispatch to reconnect the buoys."
"These buoys are critical. If they are working properly, it gives hours of advance notice to affected populations that a tsunami is on the way," Ruch said. "Those hours could mean the difference between life and death."
A few buoys were out of commission Saturday, Lubchenco said. A NOAA ship travels constantly among the buoys to make repairs, she said.