RUI: Using the Earth as a Gravitational Wave Detector

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One of the important challenges facing physicists and astronomers in the next decade is the direct detection of gravitation waves (GW's). Most of the experimental effort in the last twenty years has been directed at developing detectors which are sensitive to the millisecond bursts of radiation expected to accompany the gravitational collapse of stellar objects. A useful benchmark with which to compare limits on a stochastic background is the closure density of the universe. The existence of a background with an energy density much larger than this would conflict with current limits on the deceleration parameter. The limits implied by pulsar timing and microwave background isotropy are both several orders of magnitude below closure density per octave. Limits on a millihertz stochastic background derived from terrestrial seismic observations are much higher, several times closure density per octave; however, using the earth as a GW detector has the advantage of providing discrimination against intrinsic fluctuations in the detector. This is virtually impossible for the above two methods. Under a previous NSF grant Prof. Boughn analyzed more than 100 station-years of data collected by the Project IDA global network of seismometers in order to look for anomalous excitation of the low frequency quadrupole modes of the earth by a stochastic background of gravitational radiation. The absence of an average excitation of the oS2 and 4S2 modes allowed him to set upper limits on a background of gravitational waves at 0.31 mHz and 1.72 mHz of a few times the critical density of the universe per octave which is several orders of magnitude lower than previous limits. Under this award, he would search for pulse excitation of the oS2 modes by analyzing the data of the quietest IDA stations during periods of low seismic background noise. The sensitivity limit which is imposed by background seismic noise corresponds to a dimensionless strain amplitude of h 10-13. In order to evaluate the ultimate usefulness of the earth as a gravitational wave detector he will compute the expected excitation of the oS2 mode by terrestrial sources.