Check inside the fire-box to see if any grout has fallen. If so it may be dangerous to fire it up. Also problem may be result of the fireplace footing or piers under the fireplace heaving due to swelling soil. Check outside around the fireplace for ponding water. The crack inside appears to be...
If your home is build on expansive clay soil it is likely that moisture has encroached under the fire-place footing, or piers that were installed during repairs. Moisture expanding the soil would cause upward pressure on the fireplace causing upheaval resulting in the crack. Check outside around...
Depending on the soil in your area it looks like you may have some upheaval either under the fireplace footing or under the piers. Check around the outside for areas where water may pond next to the base of the fire-place. If piers were added, the soil may have compacted at the surface allowing...