How is Zazen different from Anapanasati?

As far as I am aware, in Zazen the hands are folded in a mudra, a larger emphasis is placed on holding the specific posture, the eyes are held open, and attention is placed underneath the navel. But other than this the basic instructions seem to remain the same: concentrate on your breath, and when other thoughts arise direct attention back to the breath. In Theravada Buddhism a large emphasis is placed on the *jhanas*. Is Zazen also supposed to lead to states of absorption? What are the differences between the practices?