How do junior faculty in the U.S. normally address senior faculty in emails? In the United States, what is the common way junior faculty address senior faculty in the same department or different department? 


Hi First or Last name
Hi Dr ___
Dear Dr __
Dear Doctor ___
Dear Prof. ___
Dear Professor ___
No greetings, start the topic directly.
In my experience in the United States, most professors invite even their graduate students to call them by their given (first) name.  Junior faculty would be no less invited to familiar address.  Whether you say "Hi, Name" or just dive into communication is often a matter of personal style and communication medium---for example, emailing from one's phone often has much terser communication, and this is generally understood and not taken amiss.

Just because you are addressing somebody by their given name, however, doesn't mean you are actually an intimate or a peer.  Many senior faculty in the United States will still expect deference by junior faculty; they will just expect it to be shown by the tone and actions of interaction, rather than merely the formality of address.