INTRODUCTIONThe past four years have witnessed the introduction of a variety of new instruments and methods for the study of the seismic structure of the oceanic crust and lithosphere. The application of these and existing tools has led to the discovery of a number of new phenomena and to a fuller understanding of the genesis and evolution of the oceanic lithosphere. Borehole seismic instrumentation has become more widely employed; ocean bottom seismographs, while generally decreasing in number, have become significantly more reliable and useful; and multichannel seismic systems have been employed in innovative experiments ranging from studies of fracture zones to the regular detection of magma chambers beneath rise axes. The techniques available for the analysis of seismic data have become more sophisticated. Waveforms collected in seismic experiments can now be used directly in constructing and evaluating seismic velocity models, and travel time data are regularly inverted directly for structure. Trial and error modeling has become increasingly unimportant. Marine seismologists are becoming increasingly involved in understanding the coupling between the ocean and the underlying oceanic lithosphere. This has led to a more complete understanding of seafloor noise processes and the partitioning of energy between acoustic and elastic waves. This review has been broken into several sections: ß Rise axis structure: magma chambers ß Fracture zone structure: thin oceanic crust ß Crustal and uppermost mantle anisotropy ß Evolution of the oceanic crust and uppermost mantle ß Arctic exploration ß Propagation of high frequency P,/S,/T phases: Reverberation or scattering ß Attenuation ß Seafloor noise and topographic scattering ß Seafloor and subseafloor receivers and sources ß Theoretical seismology including the inversion of data ß Seismicity ß Multichannel and reflection seismology ß Seismic refraction studies ß Epilogue RISE AXIS STRUCTURE: MAGMA CHAMBERS The structure of the oceanic crust and uppermost mantle has been proposed to result from the differentiation of a crustal magma chamber near the rise axis. Macdonald [1982] reviewed the general state of knowledge of rise axis processes at the beginning of this reporting period and dis-Copyright 1987 by the haerican Geophysical Onion. Paper number 7R0256. 8755-1209/87/007R-0256515. O0 cussed the growing literature linking ophiolites, observed on continents, with the structure of the oceanic crust. These allocthonous terranes are increasingly regarded as segments of oceanic crust and uppermost mantle which have been emplaced on land. The stratigraphy of the sections reconstructed through mapping of ophiolites is frequently taken as evidence for the presence of a large-scale magma body. This chamber would provide the source from which the crustal section forms through the extrusion of mid-ocean ridge basalts through the roof and the formation of the layered lower crust through differentiation and crystal settling. The structure of the oceanic crust is believed to be relat...
