Japan has the most advanced early-warning and mass-alerting system on the planet. The sirens were sounding, but as you can see from the videos, the speed of the water was far higher than anyone could get away, excepting by air. This is not a scale that humans can manage. If you saw the video of the massive wave pushing a pile of flaming debris through farms and buildings, you'd see that there simply isn't any way to get away fast enough.

The quake was close to the shore, shallow, vast in size, and so powerful that even if you had 30 minutes warning, you would not have been able to get away.

You can't outrun this: