Cybernation,

Your best bet is to remove and replace. If you hire this out, make sure they put an inch or three of 3/8" crushed "road base" rock and hand compact it before pouring the new deck.

To explain --

Road base rock has a specific amount of various grades of fine rock chips and sand in the mix as opposed to "clean" which is 85% same size material.

You want the "fines" in the mix for your application, to help with the stability. The fine components will lock it all together, and aid in the compaction process.

Have you had an unusual amount of rain this year? From your pics, it appears that the underlying dirt/clay has expanded/swelled from high amounts of water in a short period of time.