This crack is on the same D-28 I mentioned in the other thread about the lifting bridge, and for what it's worth, there's a very good chance I won't attempt to fix this soundhole crack myself, but I do want to understand its cause and what would be a proper fix.
As you can see from the pictures. The crack is on the bass side of the soundhole, but it's not to the side of the fretboard. The crack goes under the board.
I know some of these cracks are inflicted by the neck block shifting inward, but I think this one was caused by dryness, for the following reasons: 1) the fret ends over in this area (fretboard extension) are sticking out, so that suggests the fretboard had shrunk there. 2) the crack doesn't continue all the way to the end block. 3) the treble side isn't cracked. I assume if the neck block was shifting, it would cause a crack on both sides of the fretboard. At the very least, it would happen on the treble side first as the higher strings have a greater pull. 4) Slackening the strings completely doesn't seem to cause the crack to align itself, nor does a gentle push on the headstock.
I couldn't find any loose braces in that area, and I wonder if it's possible for this crack to develop without any brace becoming loose. Any thoughts if the cause is indeed dryness or if it culprit is the neck block? EDIT: I forgot to mention I believe the neck is over set. I don't know if that can contribute to these cracks or not, though.
Now for the appropriate repair. If it's the neck block, then I know Frank has a page that shows the right technique. However, how can (and should) the crack be aligned if it's not a result of loose braces/shifting neck block? It seems to me that without relieving the fretboard the crack won't align. Perhaps only the tall soundhole brace and the popsicle brace should be removed to align and glue the crack?
Thanks. I'll do that. I forgot to mention it in the OP, but edited to reflect it: the neck is over set on this guitar. The saddle is a little too tall and tilted (yet the action is at factory standards). I don't know if that's a factor in figuring out this crack or not, though.