REVIEWS OF GEOPHYSICS, VOL. 25, NO. 6, PAGES 1177-1196, JULY 1987 U.S. NATIONAL REPORT TO INTERNATIONAL UNION OF GEODESY AND GEOPHYSICS 1983-1986 Structure of the Earth: Oceanic Crust and Uppermost Mantle JOHN A. ORCUTT Institute of Geophysics andPlanetary Physics, Scripps Institution of Oceanography, La Jolla, CA INTRODUCTION The past four years have witnessed the introduction of a variety of new instrumentsand 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 phenomenaand to a fuller understanding of the genesis and evolution of the oceanic lithosphere. Borehole seismicinstrumentation has become more widely employed; ocean bottom seismographs, while generally decreasing in number, have become significantly more reliable and useful; and multichannel seismic sys- tems have been employedin innovative experimentsrang- ing 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 understandingthe 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 uppermostmantle anisotropy ß Evolution of the oceaniccrust and uppermost mantle ß Arctic exploration ß Propagation of high frequency P,/S,/T phases: Reverberation or scattering ß Attenuation ß Seafloornoise and topographic scattering ß Seafloor and subseafloor receivers and sources ß Theoreticalseismology includingthe 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 man- tle 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 reportingperiod 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 sec- tions reconstructed through mapping of ophiolitesis fre- quently 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 extru- sion of mid-oceanridge basalts through the roof and the formation of the layered lower crust through differentiationand crystal settling. The structure of the oceanic crust is believed to be related both to the presence of a crustal magmachamber during crustal accretion and to subsequent hydrothermal alteration. Pallister and Hop- son [1981] found evidence for a large (> 10 km half- width) fluid magma chamber from mapping the Samail ophiolite.Morton andSleep [1985a] constructed a numeri- cal model with an approximate hydrothermal heat sink to demonstrate that a large magma chamber could exist beneath the fast-spreading EastPacific Rise. Lister[1983] argued for an intermittentmagmachamber basedon the anticipated cooling rate from seafloor hydrothermal circu- lation. Taylor [1983] found evidence for the penetration of hot water to at least 5 km depth based on oxygen iso- tope ratios. The high inferred water/rock ratio indicated that this deep circulation was probably confinedto fissures and would not lead to pervasive alteration in the mid- and lower-crust. Strongevidence for the presence of a magma chamber existed in the Pacific prior to the beginning of the reportperiod,although no seismic datacollected in the Atlantic supported this hypothesis [e.g.,Orcutt et al., 1975; Poehls, 1974]. Analyses of the Rivera Ocean Seismic Experiment (ROSE) data from 12øN on the East Pacific Rise have continuedduring the past four years. Bratt and Solomon [1984] used shear wave arrivals across the East Pacific Rise in this area to infer that any molten basalt chambers must be small and isolated. The shear and compressional wave arrivals which they observed were not attenuated significantly while propagating across the rise. A detailed seismic refraction experiment (MAGMA) using ocean bottom seismographs (OBSs) wascarried out in 1982 at 13øN. This is a site of vigoroushydrothermal activity and detailed mappinghad previouslybeen con- ducted by the Centre National pour l'Exploitation des Oceans [Francheteau and Ballard, 1983]. Orcutt et al. [1984] presented evidence from the MAGMA experiment, based on attenuation of refracted arrivals parallel to the rise crest, in favor of a crustalmagma chamberat most 1.5 km below the seafloor. The attenuation of arrivals beyond a range of 10-12 km was attributed to a shadow zone createdby the interactionof seismic waves with a crustal low velocity zone. McClain et al. [1985]examined 1177