A harmonic tremor is a sustained release of seismic and infrasonic energy typically associated with the underground movement of magma, the venting of volcanic gases from magma, or both in volcanoes, and with repetitive stick-slip or other impulsive activity in non-volcanic systems. It is a long-duration release of seismic energy, often containing distinct spectral lines. Volcanic tremor often precedes or accompanies a volcanic eruption. Being a long-duration continuous signal from a temporally extended source tremor contrasts distinctly with transient and often impulsive sources of seismic radiation typically associated with earthquakes and explosions.