Donald W. Forsyth, ^* Spahr C. Webb, LeRoy M. Dorman, Yang Shen

The phase velocities of Rayleigh waves increase more rapidly with distance from the East Pacific Rise (EPR) axis than is predicted by models of conductive cooling of the lithosphere. Low velocities near the axis are probably caused by partial melt at depths of 20 to 70 kilometers in a zone several hundred kilometers wide. The lowest velocities are offset to the west of the EPR. Wave propagation is anisotropic; the fast direction is approximately perpendicular to the ridge, parallel to the spreading direction. Anisotropy increases from a minimum near the axis to 3 percent or more on the flanks.

D. W. Forsyth, Department of Geological Sciences, Brown University, Providence, RI 02912, USA.
S. C. Webb and L. M. Dorman, Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093, USA.
Y. Shen, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA.
^*   To whom correspondence should be addressed at donald_forsyth@brown.edu

Volume 280, Number 5367 Issue of 22 May 1998, pp. 1235 - 1238
©1998 by The American Association for the Advancement of Science.