The analogy "they are called harmonic tremors, perhaps because of their constant release of energy, like one long song played underground" suggests that the little earthquakes release energy in a continuous manner, much like how music can play constantly. This implies that the tremors happen in a steady and ongoing way, similar to background music that is often present but may not be noticed immediately. Therefore, the best response is:
Much like background music, little earthquakes tremor underground without much notice.