Tsunami warnings prompted gridlocked traffic as people tried to reach higher elevations in Hawaii, but the threat level was later downgraded.

"It's not one that's going to cause mass devastation. But it will cause coastal flooding and it will cause damage and it does put lives at risk if people don't move to high ground," an oceanography professor says.

ABC News foreign correspondent Britt Clennett says Japan is being extra cautious due to the country's history with earthquakes and tsunamis — referencing the 2011 Fukushima disaster.