Variations of period and Q of free oscillations due to mode overlap

Summary Line broadening of the spectrum of free oscillations of the Earth as a consequence of intrinsic attenuation, can generate shifts of the spectral peaks and alter spectral line widths due to mode overlapping effects. We find that the frequency shifts are dependent on the distance from the earthquake, on the relative excitation amplitudes of the spectral lines, and on the relative phase at some reference time such as the origin time, or times of windowing. With regard to the distance dependence, the frequency shift of a spectral line is maximized near modal nodal lines and is minimized near modal antinodes. The frequency shifts have a zero average in unbiased global observations. Earlier eigenperiod data used in global inversions of free mode data probably contain systematic errors because of the non‐uniform distribution of seismographs and seismic sources, and were derived from non‐tapered records. The bias has been corrected in recent work. A synthetic seismogram consisting of fundamental toroidal modes only, yields Q values that are systematically biased toward low values and show no tendency to have zero average errors even for a uniform sampling over the Earth.