Jeff, this is not normal. A few hair cracks are sometimes normal, but you said the floor is settling down and cracks eminating from the main crack.
sounds like to me, that the home was built when the soil might have been dry and warm, and now the soil is frozen and moving up. This can definately be the culprit.
I personally think that this is just too excessive for a home of this age. I would consult your builder, take notes, take pictures, take down comments and phone calls. You might need this for litigation.