Crack-induced seismic anisotropy in the oceanic crust across the East Pacific Rise (9 degrees 30'N)

The seismic anisotropy of the shallow oceanic crust across the East Pacific Rise (9 degrees 30 'N) is studied with P-wave refraction data collected du ring a controlled-source, three-dimensional tomography experiment. The anis otropy is indicated by a cos(2 theta) pattern of travel time residuals, whe re a is the receiver to shot azimuth. The travel time data are best fit by a model containing 40% anisotropy from 500 m to 1 km depth below the sea fl oor, 2% from 1 to 2 km, and 0% below 2 km depth. The upper 500 m of crust i s 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 +/- 10 degrees counterclockwise from the r ise axis. The results are consistent with the presence of vertically aligne d cracks that form on or very near the rise axis, are oriented nearly paral lel to the rise trend, penetrate to less than 2 km depth, and provide < 10% of the total pore space in the shallow crust. In comparison with similar s tudies across slow- and intermediate-spreading ridges, there is no apparent spreading-rate dependence of the magnitude of anisotropy. (C) 2001 Elsevie r Science B.V. All rights reserved.