Observation of spectrograms of harmonic tremor from Montserrat, West
Indies found the phenomena of gliding spectral lines. These lines are seen
in harmonic tremor which follows the build up of repeated hybrid events.
These hybrids appear to get so close together that they are no longer distinguishable
as separate events.
Repeated events in the time domain produce spectral peaks in the frequency
domain, with the relationship:
δt=1/δω
with δt = time interval and δω = frequency interval.
These peaks are unaffected by external factors, unless the noise levels get extremely high.
The accuracy of the time separation of the events must be less than or equal to 2%, to allow the resolution of all spikes in the frequency range of 0.5-10 Hz. For peaks spaced at 0.5 seconds, this is an error of only 1/100 second, thus δt must be very precise.
Gliding spectral lines can be recreated by gradual reduction of the time gaps between the events. The gliding spectral lines found in the real data can be directly reproduced by changing the time gap from 1 second to 1/3 second over a period of 12 minutes.