Picked up the paper this morning and this was on the front page: http://www.latimes.com/local/earthquakes/la-me-adv-fault-map-20140506-story.html#page=1

In a nutshell, the piece sketches some of the forces enabling denial of potential damage from earthquakes. When you have millions invested in a property, of course there is no danger from faults - faults, what faults? I paid a gemologist good money to tell me there are no faults under my building.....

Unfortunately, many faults are "hidden" until, say, some seismic activity reveals their presence. Take the LA basin, for instance. There are many new faults discovered in the past couple decades. For example, geologists think the Puente Hills Fault can cause significant damage to the LA area but it was only discovered in 1999.

The rapid expansion of fracking has also revealed many new faults and fault systems that were unknown before, too.

There is a two fold problem. The first issue is, as you note, simply finding and mapping in detail the faults. That there was probably a significant fault somewhere along the southern side of the eastern Santa Monica Mountains was recognized by geologists as early as 1908. Mapping the fault in sufficient detail to be a useful guide about where or where not to build is even tougher. In areas with a cover of soil and vegetation it can be extemely difficult to find faults at all, let along map them in detail. Areas that have been built up are even tougher, since many subtle topographic hints of faulting will have been bulldozed and paved over.

The second problem is determining a likelihood that the fault is still active. Since the repeat time of quakes can be many decades or even many centuries, the fact that no historical quake has been known to occur does not mean the fault is inactive. The methods of Paleoseismology have only been developed since the '70s, and can only work in certain environoments. And the results are often approximate at best. In onshore areas this typically requires trenching, and even then may not be definitive. And perusading someone to dig a deep trench in an area of high real estate values can be even more challenging!

It is the same issue that comes up in any discussion of potential risks, and expensive mitigation. It is not really any different than telling people they shouldn't build in river flood zones, below landslides, on barrier islands subject to hurricanes and sea level rise, etc etc. The annoying part is that all too often, the people who deny there is any risk, or actively suppress any effort to publicize those risks, are the same people who after a quake/flood/slide demand that the public help pay for them to rebuild in those same areas.

I hope your "gemologist" bought some nice jewellery with the money you paid him?