Infragravity waves can be observed at KEBB in the period band from 30 to 400 sec. A comparison of the power spectral density (PSD) at KEBB, MOBB (ocean bottom station in Monterey Bay, CA; Romanowicz et al., 2006), and YBH (land station in northern CA, one of the quietest BDSN stations) is shown in Figure 2.23. KEBB shows a noise hump for periods longer than 30 sec and MOBB for periods longer than 20 sec. The longer short-period cutoff value of the noise hump at KEBB is due to hydrodynamic filtering since KEBB is in deeper water (2376 m) than MOBB (1000 m). The two peaks observed at KEBB between 20 and 30 sec are due to instrumental noise and are present throughout the deployment. The long-period noise on the horizontal component is stronger at KEBB than at MOBB.
When compared to the energy of the short-period ocean waves recorded at local buoys, the low-frequency seismic noise is found to be mainly generated when the short-period ocean waves reach the coast, and not when the storm passes directly above KEBB (Figure 2.24).
Two types of modulation of the infragravity signal are observed at KEBB (Dolenc et al., 2007b). A long-period modulation is best correlated with the energy of the 14-16 sec period ocean waves. The entire infragravity band signal is also modulated in phase with the tides.
Berkeley Seismological Laboratory
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