Tsunamis are typically caused by large, shallow earthquakes at subduction zones, where one tectonic plate pushes beneath another. When the seafloor shifts suddenly, it displaces a massive volume of water, creating waves that can travel across the ocean at speeds of 500 to 600 mph. The earthquake must be strong enough—usually above magnitude 7.6—and occur less than 100 kilometers deep to pose a serious tsunami threat. Coastal communities near the source often have only minutes to react.