Example of Harmonic Tremor. Seismogram and spectrogram of a tremor event on February 23, 2001 recorded at broadband station E1S and short-period station CON. This event shows a monochromatic initation at 9 Hz with a slight downward gliding, followed by a very stable harmonic part with a fundamental frequency near 2.0 Hz. This audio record is sped up by a factor of 100 relative to the actual seismic signal.
Harmonic tremor showing strong amplitude variations recorded at broadband stations LEH and HOO on February 14, 2003. Frequencies remain stable despite strong source amplitude variations. Note the variability in relative amplitude between the two stations; if the source is stationary this indicates a strongly varying source radiation pattern during the tremor episode. This audio record is sped up by a factor of 100 relative to the actual seismic signal.