Ominous empty harbors: Why water recedes right before a tsunami
Weather gauges at Port San Luis, on the central coast of California, showed 'a rapid and damaging surge' of ocean water, moving from low tide to high tide within minutes, the weather service in Los Angeles said around 5 a.m. ET July 30.
Powerful waves 'could return multiple times through the next 24 hours' in areas where the tide had pulled back, it said.
More: Tsunami warnings, advisories issued on West Coast; What to do in threatened areas
Tsunami waves arrived on the shores of Hawaii, California, Oregon, Washington and Alaska several hours after the July 29 earthquake, according to the weather service. No injuries and little damage were immediately reported.
The rapid withdrawal of water from a shoreline is a common warning sign of an impending tsunami. It happens when the lowest part of the tsunami wave, known as the trough, reaches the coast first, according to the Scripps Institution of Oceanography at the University of California San Diego.
As the water is pulled offshore, a larger section of the sea floor than is typical during low tide may become visible. Waves may rush in minutes later.
Not all tsunamis act in the same way, the National Oceanic and Atmospheric Administration advises. Tsunamis may arrive as a series of high-speed and receding waves. They can barrel onto the shore, like a wall of water, or appear as a fast-moving flood.
The Weather Service advises those under a tsunami advisory to stay out of the water and away from beaches and waterways.
Thousands of people on the coast of Thailand died during the 2004 Indian Ocean tsunami after they wandered to look at the receding waves, only to be swept away as the water surged back. A 10-year-old British girl saved more than 100 people during that disaster when she alerted fellow beachgoers after she saw the waves begin to pull away, according to NOAA.
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