The analogy "they are called harmonic tremors perhaps because of their constant release of energy like one long song played underground" compares harmonic tremors in geology to a continuous piece of music. In this context, harmonic tremors refer to a specific type of seismic activity that involves the steady release of energy from underground sources, much like a song that has a consistent melody.
The use of the phrase "constant release of energy" emphasizes the ongoing nature of harmonic tremors, suggesting that they have a rhythmic, persistent quality similar to a song that plays without interruption. Overall, the analogy helps to convey the idea that these tremors produce a regular, predictable pattern of energy release, which can be thought of as a kind of underground symphony of seismic activity.