A tsunami can travel as fast as a jet in deep ocean—typically about 500–550 mph (800–890 km/h), slowing to roughly 20–30 mph (30–50 km/h) as it reaches shallow coastal waters. Its speed mainly depends on ocean depth: in the deepest basins, it can even exceed 700 mph (over 1100 km/h), but it always slows dramatically near shore while growing much higher and more destructive.