Seven years ago, seismologists discovered 1–3 that Earth is vibrating in a discrete set of frequencies between 2 and 7 mHz. The peak frequencies coincide with the spheroidal fundamental modes predicted by a model of Earth. To find these fundamental normal modes, which are masked by earthquakes, seismologists had to look exclusively at data from quiet sites. They also needed the broadband, low-noise seismometers available since the 1980s.
Decades ago, seismologists had seen the so-called microseismic noise at frequencies that peak near 0.2 Hz. Microseisms are known to be caused by ocean-wave interactions, which generate pressure fields that do not wane with ocean depth. The same mechanism cannot explain the generation of the much lower frequency normal modes. Some researchers had speculated that these fundamental oscillations were instead caused by atmospheric turbulence interacting with the solid Earth. Recently, Junkee Rhie and Barbara Romanowicz of the University of California, Berkeley,...
