The map, released close to the fifth anniversary of the 2004 Indian Ocean Tsunami, will be helpful in emergency response planning.

WEST COAST RISK: Some 350,000 California residents are at risk if a tsunami strikes there
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SAN FRANCISCO—Just days before the fifth anniversary of the 2004 Indian Ocean Tsunami, California officials on Thursday released a new map of the state's tsunami hazard, which details how an event could affect 350,000 people who live along the coast and cause tens of billions of dollars of damage.

A red line separates possible disaster from safety—the line past which you would have to run to be safe from an inundation. The map will help authorities plan emergency response plans, showing which areas may need to be evacuated. It covers the 50 percent of the California coastline that is significantly populated, and is "based on the largest anticipated tsunami event, plus a high tide," said State Geologist John Parrish of the California Geological Survey in a press conference at a meeting here of the American Geophysical Union.

Scientists preparing the map used computer simulations to model the tsunamis that happen once every 500 to 1,000 years—including those caused by remote earthquakes or underwater slides that could affect the California coast. The scientists heeded the lessons of the 2004 event, which showed how waves can still have deadly impacts after traveling thousands of miles. "Post-Sumatra, we started considering events that are all along the Pacific Ocean," said Costas Synolakis, director of the University of Southern California's Tsunami Research Center.

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