At 2:46 P.M. Tokyo time one of the largest earthquakes of the past century hit just off the coast of Honshu, the main island of Japan. The 8.9 magnitude quake stirred up massive tsunami waves that battered coastal cities, especially along the east coast north of Tokyo.

The tsunami reached the coastline quickly; in some places the water had surged by more than three meters within about half an hour of the earthquake, according to data from the Japan Meteorological Agency. The death toll already exceeds 300, according to published reports, and is likely to climb higher as searches for missing persons continue.

But things could have been even worse had it not been for global networks of seismic and wave sensors that quickly register and disseminate information on possible tsunamis. By 2:55 P.M., nine minutes after the earthquake, the U.S. National Weather Service's Pacific Tsunami Warning Center (PTWC) in Hawaii issued an alert for the region based on a preliminary detection of the temblor. And as better data streamed in, the warnings grew more precise and more severe.

"I think from my perspective the system worked very well," says Stuart Weinstein, PTWC assistant director. The initial warning relied on preliminary seismic data that registered the earthquake as only magnitude 7.9, but it can take tens of minutes after the initial detection for long-period seismic waves to provide a fuller picture of the earthquake's energy. "People have to keep in mind that with great quakes it can take time to get good data," Weinstein says.

As the full extent of the temblor became clear, tsunami-monitoring buoys in the Pacific began confirming that the earthquake had generated tsunamis. At 3:19 P.M. a Deep-Ocean Assessment and Reporting of Tsunami (DART) buoy off the Japan coast registered a passing wave more than a meter high in the deep ocean. The National Oceanic and Atmospheric Administration maintains a network of 39 DART buoys, 32 of them in the Pacific, each of which comprises a seafloor pressure sensor and a floating buoy that beams wave measurements to satellites. At 3:43 P.M. the PTWC issued a new warning with the DART confirmation of a tsunami and a more accurate revised magnitude of 8.8 for the quake.

By that time, waves were already pounding Japan's shores. In such circumstances, where the time to act is measured in minutes, public awareness can play as large a role as early warning. "For an earthquake of this size people will certainly feel it," Weinstein says. "That's where public education comes in." Because of the country's turbulent geophysical setting, Japan's populace is highly earthquake-savvy, he adds: "They don't wait for a warning; if you feel the ground shake very violently and it keeps shaking, you know to head for higher ground." (Tsunami, in fact, is a Japanese word meaning "harbor wave.")

The massive Honshu quake immediately preceding the tsunami may have compounded the difficulty of escape, however. "Japan certainly issued their tsunami warnings and evacuations, but their infrastructure suffered terrible damage from the earthquake," Weinstein says, making communications and transportation difficult.

Other areas had more time to prepare for the arrival of the tsunami, and forecasters had more time and more data to construct their warnings. That was the case for Hawaii, which had several hours' warning before the tsunami reached its shores. Laura Furgione, deputy assistant administrator of the National Weather Service, noted in a March 11 teleconference with reporters that the tsunami warnings had accurately forecast the 3:00 A.M. local arrival time of waves in Hawaii. "As we continue to receive more information from our DART buoys that information is put into our models, and we update our models and advisories appropriately," she said. "Today we were able to get the information out to everyone that needed it." Hawaii did experience some flooding and wave damage, but the effects were not nearly as severe as in Japan, and no fatalities have been reported.

The tsunami carried on eastward, hitting Crescent City, Calif., around 8:45 A.M. Pacific Standard Time at a height of more than two meters, Furgione said. Craig Fugate, administrator of the Federal Emergency Management Agency, said in the teleconference that on the U.S. west coast the damage was primarily to boats, marinas and associated facilities. But news reports indicate that one man is missing after being swept to sea near Crescent City.

Forecasting tsunamis and issuing fast warnings is a continually evolving pursuit, one that depends on seismic models, available monitoring technology, and interagency communication. "After events like these we'll take a look and see what we could have done better," Weinstein says, adding that the system worked as it was designed to. "We issued timely information, we issued good information, and I do think a number of lives were saved today."