Time and time again we see risky areas that have buildings on them that are later deemed questionable. Why does that happen?  See this article in the NYT: No Easy Way to Restrict Construction in Risky Areas. From the intro:

After disasters like the Oso landslide in Washington State, a common question is why people are allowed to live in such dangerous places. On the website of Scientific American, for example, the blogger Dana Hunter wrote, “It infuriates me when officials know an area is unsafe, and allow people to build there anyway.”

But things are rarely simple when government power meets property rights. The government has broad authority to regulate safety in decisions about where and how to build, but it can count on trouble when it tries to restrict the right to build. “Often, it ends up in court,” said Lynn Highland, a geographer with the United States Geological Survey’s landslide program in Golden, Colo.

I got this citation from a posting on the topic by Phil Palin in the blog Homeland Security Watch.