Abstract

Vertical records of ocean‐bottom seismographs (OBSs) are usually noisy at low frequencies, and one important noise source is the varying ocean‐bottom pressure that results from ocean‐surface water waves. The relation between the ocean‐bottom pressure and the vertical seafloor motion, called the compliance pressure transfer function (PTF), can be derived using background seismic data. During an earthquake, earthquake signals also generate ocean‐bottom pressure fluctuations, and the relation between the ocean‐bottom pressure and the vertical seafloor motion is named the seismic PTF in this article. Conventionally, we use the whole pressure records and the compliance PTF to remove the compliance noise; the earthquake‐induced pressure and the seismic PTF are ignored, which may distort the original signals. In this article, we analyze the data from 24 OBSs with water depth ranging from 107 to 4462 m. We find that for most stations, the investigated frequency range (0.01–0.2 Hz) can be divided into four bands depending on the water depth. In band (I) of lowest frequencies (<0.11, <0.05, and <0.02  Hz for water depth of 107, 1109, and 2650 m, respectively), the vertical seafloor acceleration is composed mostly of pressure compliance noise, which can be removed using the compliance PTF. The compliance PTF is much smaller than the seismic PTF, so distortion of earthquake signals is negligible. In band (II) of higher frequencies (0.11–0.20, 0.05–0.11, and 0.02–0.05 Hz for water depth of 107, 1109, and 2650 m, respectively), the vertical acceleration and ocean‐bottom pressure are largely uncorrelated. In bands (III) and (IV) of even higher frequencies (>0.11 and >0.08  Hz for water depth of 1109 and 2650 m, respectively), the compliance noise is negligible, and the ocean‐bottom pressure is mostly caused by the seafloor motion. Thus, the compliance can be safely ignored in frequency band (I).

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