The objective of the Oblique Seismic Experiment was to determine the velocity structure of the upper oceanic basement beneath DSDP Site 485 in the mouth of the Gulf of California. To conduct the experiment, we clamped a vertical component borehole seismometer 224 meters below the seafloor in Hole 485A and fired a shooting pattern consisting of eight 12-km-long lines with the Glomar Challenger at its center. From resulting data, we then determined the velocitydepth structure for compressional waves in the upper 2 km of crust by slope-intercept interpretation of travel times, linear inversion of travel times, and synthetic seismogram modeling, but found no evidence for seismic anisotropy. The hole was not deep enough to determine either attenuation or the interval seismic velocity of the section penetrated by the hole. We studied lateral inhomogeneity by delay time analysis. It is clear from the data that lateral velocity variations in the shallow crustal structure near the site are not related to topography but to age. There is an apparent thinning or increase in velocity with age of Layer 2A-2B which occurs on a scale much smaller (less than 1 m.y.) than do similar phenomena observed by Houtz and Ewing (1976). At present, our observation stands alone and its significance is üfficult to estimate.
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