Hydrostatic pressure can be quite high if your water table is very high in relation to the foundation. If the foundation is really as bad as you suggest, it probably should be replaced, not just because of earthquakes, but because water in the ground around your foundation tends to change over time, the ground becomes more or less supportive, and the foundation begins to experience great amounts of stress. A weak foundation, as you are describing it, tends to develop problems such as leaning out of square, cracking significantly, or falling apart altogether. It doesn't need an earthquake, just time.
In either case, have the home inspected, have 2 or 3 foundation contractor professionals come out and give you an educated professional opinion that we cannot give you without seeing it first hand, and then make some decisions.
All I can do from here is tell you who to call, because I can't actually see what you are talking about to trully give you a professional assesment.