Crack in crawl space

I am purchasing a house in Chicago area. The house is about 6 years old. It has a full basement and a crawl space. During the home inspcetion, I noticed there is a vertical crack in the interior wall of the crawl space. It was fairly thin and not very long (I can barely stick a piece of paper in it). The inspector said it is a minor problem and quiet normal. A month later, I had the final walk-through. I found the crack became longer and wider. The widest part is about 0.15", and the crack goes down to the ground. Why does the crack grow so fast? Does it mean there is something else wrong with the house? If yes, how serious it can be?
Any advice/comment is welcome! Thanks in advance!
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A crack which is "no problem" should not be changing visibly in a short period of time. It is, I suppose, possible that it seemed smaller to you earlier and with the inspector's comment you more or less ignored it, and now that it is closer to the move-in date it is looking more worrisome, but assuming that you know what you are talking about and it really has changed, something is going on that is not good. Vertical cracks are usually due to movement of the footing (settling), and this could be caused by unusual water conditions or unusual drying out. But footings should not settle this fast. That much movement in a month would be disastrous in its implications, or possibly the area had an earthquake in the past month that you didn't hear about.

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