in places where earthquakes are a threat, what are the odds a reinforced "bomb shelter" type structure would help? even in the basement for instance, a steel cage type room that could withstand the weight of a crumbled house? with all the usual provisioning of course, this could serve as a good holding area until help arrived. is there even a chance of making it into the cage successfully?
The problem with earthquake shelters is that you don't know to seek shelter before the initial shock, and if you survive that it's probably better to shelter in the open in anticipation of aftershocks than it is to go into a structure with gas lines, electrical lines, uncertain structural integrity, etc.
It might make more sense to put resources into storing camping gear in an externally-accessible bin rather than trying to build in internal shelter: try to arrange things so that you don't go to go throughout the house to find all the parts of the tent, sleeping bags, etc. What makes sense depends on the layout of the house. A small shed out back for camping gear would be much preferred rather than going back in that house.
(the popular view seems to be that you can't predict earthquakes, but you _can_ anticipate that there will be aftershocks, so speaking of preparing for an earthquake makes some sense)
Disclaimer - I've never live in an earthquake area, only in hurricane country.