About five minutes after a 7.0-magnitude earthquake struck off the coast of Northern California on Thursday morning, forecasters issued a tsunami warning at the highest of four levels.
The warning, sent shortly before 11 a.m. Pacific Time, advised people to evacuate because of the potential for waves that could “injure or kill people and weaken or destroy buildings.” It was part of the National Tsunami Warning Center’s protocol in response to an earthquake, according to Dave Snider, a tsunami warning coordinator at the National Oceanic and Atmospheric Administration. The warning was canceled about an hour later, but Mr. Snider said it was typical for a broad alert to be issued initially before it was refined or rolled back.
In the first five minutes of an earthquake, the warning center acts quickly to decide whether to issue an alert and at which level, based on the magnitude and the location “because that’s all we have” at that point, Mr. Snider said.
Then, about 20 minutes after a quake strikes, the center receives more details that can inform whether a warning should remain active.
The center continued monitoring data from deep ocean sensors and coastal tide stations on Thursday, watching for “any very small and local problems to develop from underwater landslides,” Mr. Snider said. After determining that no tsunami was occurring, the center canceled the warning at 11:54 a.m.
The warning center takes care with its decision to issue a tsunami warning, Mr. Snider said. “That’s a big deal.”
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