Long-period seafloor seismology and deformation under ocean waves

The deformation of the seafloor under loading by long-period ocean waves ra ises vertical component noise levels at the deep seafloor by 20 to 30 dB ab ove noise levels at good continental sites in the band from 0.001 to 0.04 H z. This noise substantially limits the detection threshold and signal-to-no ise ratio for long-period phases of earthquakes observed by seafloor seismo meters. Borehole installation significantly improves the signal-to-noise ra tio only if the sensor is installed at more than 1 km below the seafloor be cause the deformation signal decays slowly with depth. However, the vertica l-component deformation signal can be predicted and suppressed using seaflo or measurements of pressure fluctuations observed by differential pressure gauges. The pressure observations of ocean waves are combined with measurem ents of the transfer function between vertical acceleration and pressure to predict the vertical component deformation signal. Subtracting the predict ed deformation signal from pressure observations can reduce vertical compon ent noise levels near 0.01 Hz by more than 25 dB, significantly improving s ignal-to-noise ratios for long-period phases. There is also a horizontal-co mponent deformation signal but it is smaller than the vertical-component si gnal and only significant in shallow water (<1-km deep). The amplitude of t he deformation signal depends both on the long-period ocean-wave spectrum a nd the elastic-wave velocities in the oceanic crust. It is largest at sedim ented sites and in shallow water.