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 (harmonic tremors) are continuous and consistent in their release of energy, similar to how a song flows.
Therefore, the best interpretation of the analogy is:
3. Much like background music, little earthquakes tremor underground without much notice.
This option captures the idea of the tremors occurring quietly and consistently, akin to background music that plays without drawing significant attention.