Well, it's not as if anyone ever said, "Hey, I've got a great idea; let's build a large metropolis in this utterly disaster-prone location." Presumably, the town originally started there because, as the intersection between the continent's longest river and the ocean, it looked like a great trading spot. And then it grew because they were right. At what point do you say "Nope, too many people here, too dangerous, please stop growing"? Or, better yet, "Sorry, we've re-assessed the dangers, please 500,000 of you move 80 miles north now". Um, yeah.

So, I'm not saying it's a good idea. But I haven't the foggiest clue how you'd ever prevent such things from developing.