Observations and modeling of seafloor microseisms

The seismic noise level on the deep seafloor has been essentially unknown at periods longer than 10 s and poorly known at shorter periods. We present data obtained with two new types of seafloor instrumentation: a differential pressure gauge and an antenna which measures a horizontal component of the electric field. The electric field is closely related to the horizontal ground motion. The observed spectra can be divided into three frequency bands. At periods longer than 40 s surface gravity waves produce velocity and pressure fluctuations which are felt at the deep seabed and dominate other sources. This signal is expected to be uniform throughout the ocean basins and will make detecting small seismic waves at periods longer than 40 s difficult. At periods between 10 and 40 s the spectrum is much quieter and may approach the background observed at quiet land stations. The pressure signal in this band may be at least partially caused by very low frequency acoustic waves in the atmosphere. The electric field spectrum is much noisier, suggesting that horizontal motions may be larger than vertical motions. At periods slightly shorter than 10 s the pressure spectrum rises sharply 45 dB into the microseism peak. Many studies of microseisms have established a clear relationship between the ocean surface gravity wave field and microseisms. We have found close agreement between the theory of microseism generation and observations obtained while a small storm moved over a seafloor instrument. The microseism spectrum evolved in concert with the changing wind wave field. We found that plausible estimates of the directionality of the wind wave field as a function of frequency could be derived from the microseism observations. The structure of the microseism peak in frequency is also related to the properties of surface waves trapped in the upper layers of sediment. Our calculations of the forcing of microseisms by a distributed pressure field at the surface elucidate this relationship and allow a partial determination of sediment structure at an instrument site from the microseism observations.

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