The period of a tsunami is long (say about 30 minutes), so the wavelength of that wave is about 30 miles, much longer than the deep ocean (about 3 km).

So tsunamis are shallow water waves, with a speed given by $v= \sqrt{g \lambda}$, even in the deep ocean.

The period of a tsunami is long (say about 30 minutes), so the wavelength of that wave is about 30 miles, much longer than the depth of the deep ocean (about 3 km).

And out on the ocean, speed is like that of a jet liner, about 800 km/h.

So tsunamis are shallow water waves, with a speed given by $v= \sqrt{g \lambda}$, even in the deep ocean.