How can a tsunami go so fast?

Answer 1

The speed can be from winds, slopes, maybe because of the rate at which the ground moves in an earthquake, or the massive displacement from a landslide. These all are causes of why a tsunami can move so fast.

A little more...Tsunamis are not wind waves, and move much faster than wind waves, so the wind, slopes, etc. do not in fact make much difference in the overall Speed Over Ground (SOG) of a tsunami.

Instead, it's more related to wavelength. Tsunamis have a very small height or amplitude but a very long wavelength, and the math shows waves with long wavelengths travel faster.

The slippage of the earth, superfast asteroid or meteor strikes, localized, nuclear blasts, and the like can create tsunami waves. Wind doesn't.

In mid-ocean, where there's no friction against the bottom, a tsunami can travel roughly between 550-700MPH. As the bottom shallows, they slow down and pile up.

Answer 2

Shallow water.

The water tries to contain the same amount of energy,

as it gets to the shore there is less mass of water, so instead it goes higher.