Can you only predict a tsunami 50% of the time on the beach?

Question

I am quite sure my professor claimed, that you can’t always predict a tsunami incoming on the beach by looking at the water pulling into the ocean.

She said that’s only possible 50% of the time, depending on where the tsunami is in it’s wave(positive amplitude or negative)

But I can’t really find something about this online or understand why this should be true. Please help me.

Ps. If nobody knows, I’ll go knock on her office and update here.

Answer 1

It's definitely not true for megathrust tsunamis, like those of 2004 amd 2011. These create troughs on both sides of the fault involved. Both plates subside due to the mutual release of compressive stress.