Waves emanate in widening circles from the disturbance, eventually hitting the shore.
Tsunamis are no different. A cosmic body crashing into an ocean can trigger a tsunami, so can volcanoes erupting under the sea or landslides. Usually, however, an underwater earthquake starts the event. The seafloor buckles where drifting plates that make up the Earth's outer shell slowly, over millions of years, collide. The heavier oceanic plate slides beneath the continental plate but not always smoothly.