The powerful earthquake that struck in Northern California on Thursday prompted a brief tsunami warning affecting about 5 million people along long stretch of the West Coast — from Northern California to Oregon — before being canceled. The National Weather Service says most tsunamis happen when there is a sudden displacement of the ocean, after tectonic plates collide and force one under the other. Tsunamis will radiate out in all directions from the point of origin and can move across ocean basins. When the waves reach land, they can cause catastrophic damage and coastal flooding. The National Weather Service counts tsunamis among the most powerful and destructive natural forces.