Generally speaking, when there is an earthquake happens in the seabed it does not necessarily cause a tsunami. But when the earthquake is shallow and its magnitude is big enough, it can cause a change to the seabed, such as vertical displacement, oceanic trench avalanche and volcanic eruption, etc., and thus causes the sea level to form waves with a long wavelength, which is called a tsunami. Because its wavelength is long, the distance between the two neighboring wave crests may reach 500 to 650 kilometers. For more information on this situation, please see the next question below.