Abstract
Data from a seismic-refraction experiment on the East Pacific Rise at the mouth of the Gulf of California indicate that the axial valley in this area is underlain by low-velocity material (possibly a magma chamber) and, in addition, that the crust thickens rapidly away from the axis. The crust apparently thickens very rapidly with age by at least 2 km within 200,000 years. Inversion of P and S travel times on crust adjacent to the axis gives a model in which the crustal velocity changes gradationally with depth and has a Poisson's ratio of 0.28 in the lower crust. As part of an IPOD site survey, ocean-bottom seismometers (OBS) were placed in a sediment pond 12 km east of the East Pacific Rise at the mouth of the Gulf of California. A bottom-shot experiment suggests that P-wave velocity of the sediment is about 1.5 km/sec. Using reflection profiles and P—S conversions we can infer a shear-wave velocity and sediment thickness of 0.14 km/sec and 140 m, respectively. The P-wave velocities for the upper crust beneath the site increase rapidly from 4.6 km/sec to the 6.9 km/sec observed for the lower crust. The Poisson's ratio for the lower crust is about 0.28.
Published Version
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