What distinguishes alternative power pop from plain old power pop? In the case of Great Divide, it might be the hip hometown (Minneapolis). Or the previous band (Trip Shakespeare). Or, most likely, the distortion and fuzzed vocals with which the trio Semisonic attempt to mask their happy, almost Beatlesque melodies. In other words, not much which doesn't lessen the enjoyment of these plump pop nuggets and ''na na na na'' harmonies.