Question: how is a fault line created?

Tim Craig answered on 11 Jun 2018:

Even large faults grow slowly, out of smaller faults. Essentially, faults probably start as a whole set of small cracks in otherwise intact rocks. As these proto-faults slip in small earthquakes, they get slightly bigger each time. Eventually, they start of link up and lots of small cracks join into a larger fault. This process keeps happening, at a range of scales, with small cracks slipping, growing, and merging into larger and larger cracks, until the fault extends through the crust.