The two largest earthquakes to occur in southern California this century occurred between 4 and 5 in the morning. Earthquakes can happen at any time and we should plan for them at home, work and school. When thrust-faulting earthquakes happen under the ocean, the earthquake can push large blocks of ocean floor up. When the ocean floor moves up, the water that was in that spot has to go somewhere else. That somewhere else is into a large wave called a tsunami.
Tsunamis can travel across the Pacific.
Tsunamis generated by earthquakes in Chile have killed people in Japan. Very few of the offshore faults in southern California are thrust faults so while tsunamis are theoretically possible here, they are very rare.