Why Japan issued its first-ever 'megaquake advisory' — and what that means

Scientifically speaking, is interesting that the Japanese meteorological department can suggest there's a heightened risk at the moment. Maybe dangerous construction or nuclear facilities could enact some precautions or delay some activities?

But I don't understand how that information is actionable. It looks like some beaches were closed where tsunamis could be particularly deadly.

But what are the people supposed to do? And for how long?

The article mentions that a large quake follows a 7.x quake maybe one out of several hundred times.

So is Japan going to issue these warnings hundreds of times before there's any result? That is kind of the definition of a warning that people ignore.