A half-graben is a geological structure bounded by a fault along one side of its boundaries, unlike a full graben where a depressed block of land is bordered by parallel faults.

How do half grabens form?

Half grabens form when the fault planes all dip in the same direction.

What do mean by graben?

: a depressed segment of the crust of the earth or a celestial body (such as the moon) bounded on at least two sides by faults.