I have determined that this is not a foundation problem (the cracks have existed for at least two years without getting worse, other than in the winter they open up just a little. No other foundation issues,such as, exterior or interior wall cracks, wood flooring or tile cracks, etc). Crack1 runs about 4' somewhat diagonally from the corner of walls 1a and 1b; i.e. the drywall sheet is cracked. Crack2 appears about 6" from the corner of walls 2a and 2b and runs about 6' perpendicular to wall 2a, along the seem of two drywall sheets. I read an article some time ago (that I now cannot find) that discussed how to brace/structurally support the corner of 2 interior walls and adjacent ceiling joists to prevent drywall ceiling cracks at or near the corner of the 2 walls. Any master carpenter or drywall pro out there who can advise me would be greatly appreciated.
Could be a problem with the structure in the ceiling, or a problem with how the drywall was attached to structure. An imperfect but easy first attempt might be to add some screws into structure near the cracks, then patch with compound and tape. If cracks appear again after a couple of years, you have a structural issue and probably need to rip out the existing drywall to address that. If not, the likely cause was either some initial settling of the house.