Fig. 5

An example of the hum signals, obtained from stacking of 15 global seismic stations. Peterson's (1993) low noise model, NLNM, is shown by dots in the top panel. A small box in the top panel is enlarged in the bottom panel. Vertical lines in the bottom panels show the eigenfrequencies of fundamental spheroidal modes of PREM. There are individual peaks of spheroidal modes between about 2 mHz and 8 mHz but the entire spectral bump from 2 to 15 mHz is the hum, mostly consisting of globally circulating Rayleigh waves