Introduction

The Monterey ocean bottom broadband station (MOBB) is a collaborative effort between the Monterey Bay Aquarium Research Institute (MBARI) and the Berkeley Seismological Laboratory (BSL). The MOBB is located 40 km offshore in the Monterey Bay, buried in the ocean floor at a water depth of 1000 m (Romanowicz et al., 2003).

Infragravity waves are ocean surface waves with periods longer than the wind-driven waves and the swell. Their wave amplitudes in the deep water are small ($<$ 1 cm) and they can be observed in the frequency band from 0.002 to 0.05 Hz. Infragravity waves have been identified as an important source of long-period noise at the ocean bottom (Webb et al., 1991). Recently, they have also been proposed as a source of the Earth's continuous free oscillations (Rhie and Romanowicz, 2004; Tanimoto, 2005).

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