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Cummins, P, Wahr JM, Agnew DC, Tamura Y.  1991.  Constraining Core Undertones Using Stacked IDA Gravity Records. Geophysical Journal International. 106:189-198. AbstractWebsite

A search is made for oscillatory gravity signals, possibly associated with core undertones, in the frequency range between the diurnal and semidiurnal tidal bands. IDA tidal data recorded during 6 month intervals following four large earthquakes were used, and the data from different stations were stacked according to spherical harmonic surface amplitude patterns with angular order l less-than-or-equal-to 4. The detection level of a signal in the diurnal-semidiurnal frequency band was roughly 10-20 ngal, but the only signal detected was a 9 ngal signal of 9.54 hr period whose origin remains unexplained. Two of the earthquakes were chosen because of claims that core undertones were excited following these events, and it is established that signals of the required amplitude, if they exist, can only be associated with a few spherical harmonic patterns for which the Brussels instrument is near an antinode. We also used NCAR barometric data to correct the gravity recordings for the effects of atmospheric pressure variations, which reduces the noise level in the diurnal-semidiurnal frequency band by 2-3 dB.