What Can Superconducting Gravimeters Contribute to Normal-Mode Seismology?

The seismic free oscillations of the Earth can be observed in the frequency band from 0.3 to 20 mHz, and estimates of their frequencies constitute the principal constraints for spherically symmetric Earth models such as the Preliminary Reference Earth Model (PREM). While the bulk of the mode observations rely on recordings of the spring gravimeters deployed in the International Deployment of Accelerometers (IDA) network and more recently on the Streckiesen STS-1 seismometers deployed in the global seismic network (GSN), we show here that the most recent generation of superconducting gravimeters (SGs) can achieve lower noise levels than either one of the aforementioned sensors at frequencies lower than ~0.8 mHz. While the splitting of modes above 1 mHz is largely due to structural heterogeneities in P- and S-wave velocities, the modes below 1 mHz are unique for two reasons: (1) the destabilizing effect of self-gravitation leads to a high sensitivity to density heterogeneities and (2) the vicinity of these modes to the frequency of the Earthʹs rotation leads to pronounced Zeeman splitting, which in turn depends on spherically averaged density structure. Thus it is argued that SGs can make a significant contribution to the illumination of long-wavelength density heterogeneities in the Earthʹs mantle. At frequencies above 1 mHz, current SGs exhibit higher noise levels than the quietest seismometers deployed in the GSN. Furthermore we show that above 3 mHz, even the Streckeisen STS-2 seismometers compare favorably against the SGs if the former are installed with elaborate shielding from environmental effects.