Great question. Tsunamis are generally caused by large shallow earthquakes beneath the ocean. So earthquakes are a warning sign for tsunamis. Geologists use sesimometers to detect small movements within the earth that indicate an earthquake. The tricky part is this:
"When officials detect a large, shallow earthquake under the ocean, they issue a warning. But this method is plagued by false alarms, since not every earthquake necessarily triggers a tsunami. For example, since Hawaii's Pacific Tsunami Warning Center was established in 1948, about 75 percent of warnings that resulted in costly evacuations turned out to be false alarms.
To overcome this, tsunami watchdogs have turned to sensors that sit on the ocean floor and detect tsunamis. Japan has installed a cable of sensors along its coastline that detects any incoming tsunamis. The US is now starting to do this too in critical places.
Tsuanmis are never days away, there is not enough ocean for them to be days away. A tsunami starts in the ocean and once it crashes onto land it comes to a halt. Tsunamis can be hours away like the one in Thailand and India in December 2004 that traveled up to 500 miles per hour. The earthquake that started the tsunami originated in Sumatra and it took hours to hit land. Tsunamis can be minutes away if they happen closer to shore.
Even minutes can be valuable in people getting quickly to higher elevation and being out of the reach of the earthquake.
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