Crack-induced seismic anisotropy in the oceanic crust across the East Paci¢c Rise (9330PN)

The seismic anisotropy of the shallow oceanic crust across the East Pacific Rise (9330PN) is studied with P-wave refraction data collected during a controlled-source, three-dimensional tomography experiment. The anisotropy is indicated by a cos(2a) pattern of travel time residuals, where a is the receiver to shot azimuth. The travel time data are best fit by a model containing 4% anisotropy from 500 m to 1 km depth below the sea floor, 2% from 1 to 2 km, and 0% below 2 km depth. The upper 500 m of crust is not independently constrained by the data since ray paths at these depths are oriented near vertically. The fast direction of P-wave propagation is aligned along a trend oriented 6 þ 103 counterclockwise from the rise axis. The results are consistent with the presence of vertically aligned cracks that form on or very near the rise axis, are oriented nearly parallel to the rise trend, penetrate to less than 2 km depth, and provide 6 10% of the total pore space in the shallow crust. In comparison with similar studies across slowand intermediatespreading ridges, there is no apparent spreading-rate dependence of the magnitude of anisotropy. ß 2001 Published by Elsevier Science B.V.