On the near-hourly hidden periodicity of earthquakes

Abstract

The so-called hour-mark effect, which reflects a response of the lithosphere to anthropogenic forcing, was initially detected when processing the earthquake catalogues by the method of synchronous detection. When attempting to reveal this effect by spectral analysis, we encountered an interesting feature of global seismicity. Namely, the spectrum of seismic activity indeed contains a peak at a frequency of 0.277 mHz, and this peak has a clearly anthropogenic origin (the hour-mark effect). At the same time, the spectrum also contains a stronger peak at a frequency of 0.309 mHz, which corresponds to a period of 54 min. We have independently detected this period in the aftershock sequences in the epicentral zones of large earthquakes and in the variations of seismicity in the antipodal zones. The 54-min periodicity coincides with the fundamental mode 0S2 of the free oscillations of the Earth. It is suggested that oscillations of the Earth as a whole result in a weak but detectable modulation of seismic activity.