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 TÅhoku 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.
Oldest known stones date back to the early 1400s but there must have been older stones. Already those stones are mostly unreadable because of erosion and they are dated by secondary sources.
"The 2011 TÅhoku 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