Seismic anisotropy of the shallow crust at the Juan de Fuca Ridge

Microearthquake data recorded on four ocean bottom seismometers are used to study shear-wave splitting on the Endeavour Segment of the Juan de Fuca Ri dge. The covariance matrix decomposition method is used to determine the se nsor orientation from explosive shot data and to estimate the anisotropy pa rameters for 238 earthquake records. At three of the four sites, the result s show a remarkably consistent fast direction parallel to the ridge axis. T he time delays between the fast and the slow waves range from 40 to 200 ms, with an average of 90 ms. They are not clearly related to earthquake range , focal depth or source-receiver azimuth. The splitting of the shear waves is interpreted as an effect of structural anisotropy due to the presence of ridge-parallel cracks in the shallow crust. If we assume that anisotropy i s concentrated in the upper 1-2 km, the splitting times require a high crac k density of similar to 0.1.