[mytailorisrich]

You can put warnings everytime there is a tsunami, which is "often" in Japan, but the issue is that a massive one like the 2011 earthquake and tsunami is a once in a millenium event so would indeed need to rely on very old warnings:

"The 2011 Tōhoku earthquake occurred in exactly the same area as the 869 earthquake, fulfilling the earlier prediction and causing major flooding in the Sendai area. [1]

Modern society is not good at this sort of very long term consideration and planning.

[1] https://en.wikipedia.org/wiki/869_J%C5%8Dgan_earthquake