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

In the classic microseismic band of 5-20 s 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 2-28 km, constraining the periods we analyzed to 0.25-2.5 s. 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 Lg, occurring in about 50% of the noise windows. Since Lg 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 (Pn/Pg at 6%), mantle bottoming P waves (14%), and core sensitive waves (PKP , 8%). This energy is mostly generated in deep water away from coastlines, with a region of the north Pacific centered at 165W and 40N being especially prolific. The remainder of the energy arriving in the noise consisted of Rg waves (28%), a large fraction of which may have a cultural origin. Hence, in contrast to the classic microseismic band of 5-20 s, at shorter periods fundamental-mode Rayleigh waves are the least significant component.