Title

Analysis and observations of spatially coherent seafloor microseisms

Date of Award

1990

Availability

Article

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Applied Marine Physics/Ocean Engineering

First Committee Member

Tokuo Yamamoto, Committee Chair

Abstract

Analysis of spatially coherent seafloor motion is conducted using data from two ocean bottom seismometer arrays deployed by the University of Miami's Geo-Acoustic Laboratory. Data from Atlantic Generating Station 6-point (1987) and 4-point (1989) arrays are used to measure the directional spectra of a seafloor microseisms as well as the contemporaneous ocean gravity wave directional spectra. Directional spectra phase velocities are found to be consistent with Scholte wave phase velocities calculated for the study area. The propagation directions of Scholte wave microseisms measured, are in between the direction of propagation of the opposing ocean gravity wave spectrum. In particular, during a time of strong ocean gravity wave spectrum from approximately east (110$\sp\circ$) and west (250$\sp\circ$) directions, microseisms are found to propagate from the south (180$\sp\circ$). During a period of a non-opposing sea state, microseisms do not exist. These observation are consistent with the Longuet-Higgins's (1950) theory for the origin of microseisms--that they are a result of nonlinear interactions of opposing ocean gravity waves, and that the microseisms travel in a direction represented by the sum of the wavenumbers of the opposing ocean gravity waves.The directional spectra of infragravity waves (35-85 s) determined with the pressure array indicate these waves are strongly uni-directional and are shown to be coming from the cast in contrast to previously believed theories that suggest infragravity waves exist only as a longshore wave.The propagation characteristic of microseisms, Rayleigh type motion to that of Love wave motion, is tested using a newly developed spectral technique that examines 3-component seismic data from a single instrument. The preliminary results indicate that Rayleigh type motion dominates Love wave motion.The relative importance of microseisms as an energy sink for ocean gravity waves is also investigated. Using the seismic structure estimated for the study area, Scholte wave microseism displacements and stress profiles, based on measured seafloor boundary conditions, are used to estimate energy fluxes. Analysis indicates that the Scholte microseismic wave energy fluxes are an unimportant energy sink in the attenuation of ocean gravity waves in shallow water.

Keywords

Geophysics

Link to Full Text

http://access.library.miami.edu/login?url=http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:9104429