I'm looking to replace the flooring in my rec room on my first floor. While doing that, I found a crack in my concrete subfloor, and I'm pretty concerned... Online, I've read that hairline cracks are normal, since concrete shrinks when it dries but this is no hairline crack. The floor on either side of the crack is flat, but it does stretch all the way across the room. It looks to be 4mm large at the largest point, and it stretches in one direction. At the deepest point I was able to measure, the crack was 4cm deep. From the outside, right outside the door, there is no visible crack in the foundation. The room is about 10ft by 22ft. I don't see any other cracks. (so the crack stretches about 10ft) The townhouse is about 20 years old, 3 stories high, and an earthquake has rocked the region maybe 5 or 6 years ago. The crack might be from that. Is the crack a cause for concern? Does this need to be repaired? Sealed? Thanks for any advice you can give.