On the Composition of Earth's Short-Period Seismic Noise Field

In the classic microseismic band of 5-20 sec, seismic noise consists mainly of fundamental mode Rayleigh and Love waves; however, at shorter periods seismic noise also contains a significant amount of body-wave energy and higher mode surface waves. In this study we perform a global survey of Earth's short-period seismic noise field with the goal of quantifying the relative contributions of these propagation modes. We examined a year's worth of vertical component data from 18 seismic arrays of the International Monitoring System that were sited in a variety of geologic environments. The apertures of the arrays varied from 2 to 28 km, constraining the periods we analyzed to 0.25-2.5 sec. Using frequency-wavenumber analysis we identified the apparent velocity for each sample of noise and classified its mode of propagation. The dominant component was found to be L-g, occurring in about 50% of the noise windows. Because L-g does not propagate across ocean-continent boundaries, this energy is most likely created in shallow water areas near coastlines. The next most common component was P-wave energy, which accounted for about 28% of the noise windows. These were split between regional P waves (P-n/P-g at 6%), mantle bottoming P waves (14%), and core-sensitive waves (PKP at 8%). This energy is mostly generated in deep water away from coastlines, with a region of the North Pacific centered at 165 degrees Wand 40 degrees N being especially prolific. The remainder of the energy arriving in the noise consisted of R-g waves (28%), a large fraction of which may have a cultural origin. Hence, in contrast to the classic microseismic band of 5-20 sec, at shorter periods fundamental mode Rayleigh waves are the least significant component.