An apparatus comprises a body, a shaft assembly, and an end effector. The shaft assembly extends distally from the body. The end effector is located at a distal end of the shaft assembly. The end effector comprises an ultrasonic blade and a clamp arm. The ultrasonic blade is configured to vibrate at an ultrasonic frequency. The clamp arm comprises a clamp pad. The clamp arm is movable toward the ultrasonic blade to compress tissue between the clamp pad and the ultrasonic blade in two stages. During the first stage, the clamp arm is configured to compress tissue with only a distal portion of the clamp pad. During the second stage, the clamp arm is configured to compress tissue with the distal portion and a proximal portion of the clamp pad. Thus, the tissue compression begins at the distal portion and subsequently progresses toward the proximal portion.