That is the PERFECT description of your foundation problem, which is basically that your concrete contractor did not have enough cement and fine particles to fill the voids in the mix, so all you have is aggregate. A good earthquake and boom, you're in the basement, alive if you're lucky.
It sounds like to me that the foundation needs to be replaced, but this is not an easy proposition, given the fact that you happen to have a house sitting on it. The fix, in my opinion, involves putting two steel beams through the foundation, jacking the house up, and placing timbers under the beams as the house is raised. Once the house is sufficiently clear of the foundation with enough room to work, the existing foundation is demolished, and a new, structurally sound foundation is poured under the house. What a terrific opportunity to create really usable living space!
This whole project costs a pretty penny and definately is NOT for the do it yourselfer.
Contact www.ImproveNet.com for 4 referrals to prequalified concrete foundation contractors.