Bokelmann, G.H.R., Baisch, S., 1999, Nature of narrow-band signals at 2.083
Hz, Bulletin of the Seismological Society of America, 89, 1, 156-164
We study the nature of extremely narrowband signals which appear in typical
long-time spectra of seismic recordings.
At the German Experimental Seismic System (GERESS) array we observe several of these time-continuous
spectral lines.
The most prominent one has a frequency near 2.083 Hz. The close agreement of
this frequency with one of the feasible rotation frequencies of synchronous
machines (50 Hz /24) suggests an industrial origin of the signal.
An observed signal gap of 11 hours duration is thus interpreted as a temporary
machine shut-down. Interestingly, the signal vanishes (or is
at least strongly reduced in amplitude) in the whole eastern part of the
German Regional Seismic Network (GRSN) suggesting 1) that a single source
dominates the signal in that region and 2) that the narrowband signal propagates
to distances of more than 300 km.
On the other hand, we study the character of the wave by performing a suitably
adapted array analysis. Estimates of the propagation direction suggest a source
location near the German-Czech border.
We obtain an estimate of the apparent velocity near 4 km/sec. Comparison with
transient arrivals from regional earthquakes suggests that the narrowband energy
propagates as a regional phases of Lg and/or Sg-type.
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