Harmonic tremor describes a continuous rhythmic earthquakes in the Earth's upper lithosphere that can be detected by a seismograph and is often preceded or accompanied by a volcanic eruptions.

Harmonic tremor is a continuous release of seismic energy typically associated with the underground movement of magma. It contrasts distinctly with the sudden release and rapid decrease of seismic energy associated with the more common type of earthquake caused by slippage along a fault.