An Experiment in Seismic Tomography: The Three-Dimensional Structure of the East Pacific Rise

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Understanding the processes that create new seafloor at mid- ocean ridges is a critical aspect of marine geological and geophysical research. Although it is now well accepted that seafloor spreading has created the present ocean crust during approximately the last 200 million years, the details by which new volcanic material is generated and emplaced at the ridges remains unresolved. An important unknown is the distribution and longevity of subsurface magma chambers in which partially melted mantle material is stored before being erupted to the seafloor. Recently acquired deep penetration seismic reflection data from the East Pacific Rise has revealed what appears to be the top of a fairly continuous magma chamber along the Rise crest. The objective of the present project is to use seismic refraction techniques to examine the magma chamber in more detail than can be obtained from the seismic reflection data. Specifically, the project will deploy a series of ocean bottom seismometer/hydrophones across the Rise crest and then use explosive and acoustic energy sources to record the horizontal transmission of energy through the crust. These data will be analyzed by recently developed tomographic techniques which allow three dimensional imaging using the entire data set. Specific questions to be addressed include the width, depth, and velocity structure of the subsurface magma chamber. This is a cooperative project between scientists at Woods Hole Oceanographic Institution and the Massachusetts Institute of Technology. The project will be funded at the following levels: Woods Hole -FY87=$30000, FY88=$297000, FY89=$87000 MIT -FY87=$30000, FY88=$163000, FY89=$133000