[lloeki]

My crude understanding and associated mental imagery is that tsunami waves are more like tide than "usual" waves: your typical 3m wave is mostly "surface-ish" in comparison, while 3m tsunami has both greater wavelength and "depth", which is what makes the "usual" wave break quickly when it reaches shore, whereas tsunami waves continue pushing real hard for real long because of all the length * undersurface water you don't see.

Gently shaking a glass of water sideways or blowing upon it won't make much water spill out, but jolt it vertically and most of it goes out real quick.

This is all probably very incorrect physics but it helps drive my intuitive understanding that 3m wave and 3m tsunami are quite different, the former being scary but the latter being muuuuch scarier.