Time domain analysis of Earth's long‐period background seismic radiation

A time domain detector of long‐period (200–400 s) surface‐wave energy is developed and used to investigate Earth's background seismic radiation. Several recent studies have presented convincing evidence for an incessant low‐level excitation of Earth's normal modes in the frequency band 2–7 mHz. The detection of these oscillations has been accomplished through analysis of low‐frequency amplitude spectra, which provides good resolution of the excited frequencies but poor resolution of temporal variations and signal amplitude. The new time‐domain method is based on the autocorrelation function. A seismogram is correlated with a copy of itself to which a reverse great‐circle dispersion operator has been applied. The resulting correlogram is labeled the “autogram”. If multiple‐orbit surface waves are present in the seismogram, their energy will be phase equalized and concentrated at zero lag time in the autogram, allowing for identification of coherent radiation. Analysis of vertical‐component seismograms recorded at quiet stations leads to nearly continuous detections of coherent seismic energy using correlation time windows of only a few hours duration. Stacking of autograms from several stations enhances the detections. Five years of vertical long‐period data from the Global Seismographic Network are analyzed. All detections with an amplitude equivalent to that of an earthquake of M w ≥6.0 can be correlated with known earthquakes; no evidence is found for the existence of anomalously slow or “silent” earthquakes in the period band 200–400 s. The background excitation level is remarkably constant, corresponding approximately to the maximum amplitude detection generated by a M w = 5.75 earthquake. Coherent Rayleigh wave energy is detected 94% of the time analyzed. A seasonal component with a period of 6 months is well resolved, with maxima in mid‐January and mid‐July, consistent with the hypothesis that atmospheric processes are the main cause of the phenomenon. Variations in the background signal level on timescales of several days to weeks are also documented.