Seismic anisotropy of the shallow crust at the Juan de

2000 
Microearthquake data recorded on four ocean bottom seismometers are usedto study shear-wave splitting on the Endeavour Segment of the Juan de Fuca Ridge. The covariance matrix decomposition method is used to deter- mine the sensor orientation from explosive shot data and to estimate the anisotropy parameters for 238 earthquake records. At three of the four sites, the results show a re- markably consistent fast direction parallel to the ridge axis. The 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 in- terpreted as an eect of structural anisotropy due to the presence of ridge-parallel cracks in the shallow crust. If we assumethatanisotropy isconcentratedintheupper1-2km, the splitting times require a high crack density of0.1.
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