The link that Steve Ornellas gave to you is excellent. I actually submitted some of the photos for it as well as another very informative site, http://www.scec.org/earthquakes/.
We've been disaster response general and manufactured home contractors since 1986 and have worked with FEMA and/or independently in the after math of every major EQ during that period.
But I'll focus on the January 17, 1994, 6.7 magnitude Northridge earthquake just to cut to the chase. Some of the most vulnerable homes in that event were built circa '50's and before; Especially homes built circa '30's and earlier.
Many if not most homes built in that era were not bolted to the foundation. In fact many homes in LA Ventura and Santa Barbara area were built on piles of rocks with a 2x6 redwood plate just lying on top not supported or connected to anything. I saw and worked on some pretty incredible projects in the aftermath of that event and couldn't believe just how dangerous some of these homes were if subjected to a strong seismic event.
Needless to say these homes didn't stand much of a chance in that strong of an EQ because they lacked adequate if any lateral resistance and simply shook off their foundation if you want to call it that. Homes that go through that are pretty much a total loss as every structural member in the home suffers some level of damage if not total destruction.
Another huge problem, albeit there were several, was the lack of shear paneling on cripple walls under the home. Cripple walls are very common in most homes of that circa due to their architectural genre which called for a higher profile.
Or they were built on slopes and the only way to make the home level in those days and even today depending on the architectural and engineering specs was to either cut and fill the slope in order to level it or simply and far more cost effective built the structure on a cripple wall. So we kept pretty busy for a few years after that event retrofitting hundreds of foundations and cripple walls.
Another huge problem in the aftermath of a strong EQ is with older Trailers and Mobile Homes that were set up on cinder blocks or steel piers. Literally thousands of them fell off their supports or "PANCAKED" as they say in that industry. It was like someone pulling the rug out from under you be it a home, trailer or Mobile Home.
That particular EQ changed the engineering calcs completely and by mid 1996 there was a whole new ball game mandated by the UBC and other national and international building regulators. Homes built since '96 are more EQ resistant than any other homes built before them. In fact as we continue to research after every event we continue to see improvements in building technology.
Now, all this stated, don't be afraid to purchase an older home. Just make sure that you have it thoroughly inspected by an EQ retrofitting licensed and experienced contractor who knows exactly what to look for and how to estimate the cost. if that's done correctly it will be almost as safe as homes built according to todays standards. But it can be costly so if you're not ready for that you'd be better off purchasing a home built circa '96 and newer.