George Dikun
EF0
In addition to what Rob said, I'd like to point out that the difference between warning systems used in Japan and the United States aren't the systems themselves, but how people react to them. One can debate the merits of siren versus voice systems. But in the end, it is the responsibility of each individual to heed the advice given in official instructions.
Japanese society tends to be very orderly. When they receive a Tsunami alert with instructions to head to high ground, they generally do as they are instructed. In contrast, here in the United States, when an official tornado warning is issued and instructions given to seek shelter, all too often our reaction is to grab a Sony Handycam and head for the roof to get a better view. It is simply a difference in mindset between societies, rather than the warning systems used, that in the end makes the real difference.
The difference in reactions could also be due to the difference in calamities. If a tsunami warning is issued, a tsunami is very likely going to hit all/most of the areas under the warning. Though a tornado statistically affects very few people in the warned area. You could be "safely" filming a tornado right across the street. Often times, a tornado won't affect anyone (or it won't happen at all.)